• Spiral, gram-negative rods; may appear coccoid in older cultures.
• Generally heat-resistant, and are cultured at 42 degrees Celsius (note that Campylobacter upsaliensis is an exception).
• Grow in microaerobic conditions with relatively low oxygen levels and elevated carbon dioxide.
• Oxidase positive.
• Tiny, so they can be filtered from stool samples for diagnostic purposes.
• Vulnerable to stomach acids; thus, illness is most common in individuals with reduced stomach acid production.
• Zoonotic infections; that is, illness occurs after interactions with colonized animals.

Virulence factors:

• Lipo-oligosaccharide endotoxin.
• Polysaccharide capsule.
• Flagella facilitate motility and invasion of host cells.
• Cytolethal distending toxin is thought to cause cell death and IL-8 secretion. IL-8 is a pro-inflammatory cytokine that, when produced in excess, damages host tissues.

Infections:

Campylobacter species are the number one cause of bacterial gastroenteritis in the United States.

• Gastroenteritis is characterized by watery, sometimes bloody, diarrhea with blood cells present in the stools; some individuals have fever.
• Fortunately, the diarrhea is usually self-limited; illness lasts approximately six days.
• In severe cases, or where complications arise, erythromycin or azithromycin can be administered.

Campylobacter jejuni

• Primary pathogenic species; infection is associated with consumption of contaminated poultry and cow products.
• Damages the mucosa of the jejunum, ileum, and colon.
  – Villus ulceration (can become bloody)
  – Neutrophils, eosinophils, and mononuclear cells invade the lamina propria
  – Abscesses form in the glands and crypts.
• Infection can also cause bacteremia and cardiovascular dysfunction.

Post-infection complications:

• Guillan-Barré syndrome is an autoimmune disorder that damages the myelin of the peripheral nervous system (hence it’s a demyelinating disorder).
• Individuals typically experience symmetrical weakness that begins in the lower extremities; weakness may ascend, and, in severe cases, can impair cardiovascular and respiratory functioning due to autonomic system involvement.
  – Blood pressure can swing wildly, so clinicians must be careful not to react too quickly to spikes in blood pressure or they can inadvertently cause devastating low blood pressures.
  – Indicate that early treatment with IVIG or plasmapheresis is important in management but most important is good supportive care while the illness runs its course, meaning early intubation, if respiratory distress is identified, and good nursing care to avoid superimposed infections.
• Keys to diagnosing the disorder are:
  – Hyporeflexia (from the peripheral neuropathy).
  – CSF findings of cytoalbuminologic dissociation (meaning normal WBC but elevated protein)
  – EMG findings that may be normal early on but later show prolonged F Waves, conduction block, and eventually the demyelination (prolonged distal latencies and conduction velocities).
• Campylobacter jejuni infection, itself, is associated with a particularly aggressive form of Guillain-Barre, called Acute Motor Axonal Neuropathy (AMAN) (it’s most common in China and Japan).
  – This variant actually affects the axons, themselves, rather than simply disruption of the myelin-coating of the nerve – and thus will demonstrate as an axonopathy on EMG.

• Reactive arthritis is typically characterized by joint swelling and pain in the hands, knees, and ankles.

Campylobacter coli infection is associated with consumption of contaminated pork and poultry products.

• It primarily causes gastroenteritis, but is also associated with some extra-intestinal infections.

Campylobacter upsaliensis is an emerging pathogen associated with cats and dogs; thus, outbreaks have been traced to pet stores.

• It primarily causes gastroenteritis.
• Has been associated with Guillain-Barré syndrome.

Campylobacter fetus

• Infection primarily occurs in immunocompromised individuals.
• Resistant to serum killing by antibodies and complement, thanks to the presence of S protein, which prevents C3b binding.
• Bacteremia and septic thrombophlebitis (venous thrombosis with bacteremia)
• Endocarditis
• Meningoencephalitis
• Gastroenteritis

Be aware that this species was originally named Vibrio fetus.

VIBRIO

• Gram-negative rods.
• Facultatively anaerobic
• Oxidase-positive
• Polar flagella facilitate rapid motility.
• Found in water, especially estuaries and coastal waters, because salt is required for growth.
• Upon ingestion, pathogenic strains cause gastrointestinal disease.

VIBRIO CHOLERAE

Causative agent of cholera.

• Ingested as free-living cells, micro-colonies, or as biofilms in contaminated foods or water.
• Sensitive to stomach acids
  – Infection typically requires exposure to a large quantity.
  – However, individuals with impaired stomach acid production are vulnerable to lower infectious doses.
• Gastroenteritis caused by Vibrio cholerae causes mild to severe vomiting and watery diarrhea
  – In severe cases, the profuse stool has a characteristic milky-white “rice water” appearance.
• Proper sanitation and thorough cooking of food can prevent cholera epidemics.
  – Vaccination can help prevent worsening conditions in areas where cholera is endemic.
• Infected patients should be given antibiotics, such as tetracycline, to avoid dehydration and death.

Cholera epidemics: Vibrio cholerae O1 and O139

• Vibrio cholerae O1 is further subdivided into biotypes and serotypes.
  – Biotypes: Classical and El tor
  – Serotypes: Ogawa and Inaba; Hikojima is thought to be a hybrid transitional state.
• Severe fluid loss leads to dehydration, which can result in metabolic acidosis, hypokalemia, hypovolemic shock, cardiac arrhythmia, and renal failure.
• Cholera is endemic in some parts of the world; asymptomatic carriers contribute to its maintained presence.
• Cholera outbreaks occur in areas where humanitarian and/or environmental crises lead to overcrowding and poor sanitation.

We show how Vibrio cholerae O1 and O139 acquire the virulence factors that promote severe gastroenteritis.
– Horizontal gene transfer is key to this process.

• First, we draw a couple of small intestine epithelial cells and indicate the intestinal lumen.
• Then, we draw a non-pathogenic Vibrio cholerae bacterium.
  – Its chromosomal DNA has already acquired Vibrio Pathogenicity Island -1 (VPI-1), which carries genes for Toxin co-regulated pili.
• Toxin co-regulated pilus is a type of bundle-forming pilus that promotes microcolony formation, which is important for Vibrio cholerae colonization.
• This pilus is also a receptor for bacteriophage CTXφ, which injects DNA into the V. cholerae microbe.
  – Without the pathogenicity island and toxin co-regulated pilus, the bacteriophage would not be able to attach to the microbe and transfer DNA.
• Next, we show that the CTX prophage has been integrated into the chromosomal DNA.
• The CTX prophage triggers production of cholera toxin,
  – Cholera toxin interacts with binding sites on the small intestine epithelial cells.
• Cholera toxin increases cyclic AMP, which leads to water and electrolyte secretion into the lumen.
  – Profuse watery diarrhea ensues.
• The CTXφ prophage also carries genes for two additional toxins:
  – Accessory cholera enterotoxin (ACE) contributes to water and ion secretion; some authors report that this enterotoxin, alone, can induce gastroenteritis.
  – Zona occludens toxin (ZOT) disassembles epithelial tight junctions, which increases intestinal permeability.
• Neuraminidase increases the availability of cholera toxin binding sites on host cells
  – The nanH gene that codes for this enzyme is carried separately.

Non-O1 and non-O139 Vibrio cholerae strains can cause mild diarrhea.

Virulence factors of these strains vary.

• Non-O1 strains have polysaccharide capsules that facilitate spread beyond the intestine.
• Various toxins, including heat-stable enterotoxin, induce diarrheal symptoms.

VIBRIO PARAHAEMOLYTICUS & VIBRIO VULNIFICUS

Associated with gastroenteritis, septicemia, and wound infections.

Vibrio parahaemolyticus

• Lives as free cells in contaminated food and water;
• Halophilic
  – “Salt-loving;” growth on most media requires the addition of sodium chloride.
• Virulence factors:
  – Type three secretion systems inject protein effectors into host cells.
  – Thermostable Direct Hemolysin (TDH) and TDH-Related Hemolysin (TRH) are enterotoxins that increase intestinal fluid secretion; they also act as cytotoxins that affect other host cells.

Thermostable direct hemolysins produce beta hemolytic halos when grown on Wagastuma blood agar; this is called the Kanagawa phenomenon. However, be aware that strains carrying only the TDH-related hemolysin (TRH) gene are Kanagawa phenomenon-negative, but can still cause gastroenteritis. Thus, the absence of beta hemolysis does not necessarily mean that the strain is non-pathogenic.

• Plastic motility:
  – With a single flagellum, it moves as a rapid swimmer cell
  – In more viscous environments, the microbe produces multiple lateral flagella and moves as a swarmer cell.
• Capsule synthesis is also up- or down-regulated in response to environmental changes.

Vibrio vulnificus

• Associated with warm saltwater.
• Halophilic.
• Ferments lactose, which can aid in its identification.
• Strains can be further classified into three biotypes.
• More likely to cause infections in males; it has been suggested that estrogen has protective effects.
• Individuals with elevated free iron levels are also more susceptible to infection, likely because Vibrio vulnificus thrives in iron-rich environments.
• Overall, Vibrio vulnificus is responsible for most sea-food related deaths in the U.S.
• Virulence factors:
  – A polysaccharide capsule protects from host immune responses
  – Proteases break down host tissues
  – Hemolysins release iron from host storage
  – Cytolysins cause cell death
  – Endotoxin comprises LPS; triggers cytokine release.

V. parahaemolyticus & V. vulnificus Infections

Infection is more common in individuals with immunodeficiencies and/or liver disease, which is associated with decreased neutrophil activity.

Self-limiting Gastroenteritis is associated with consumption of raw oysters that are contaminated with Vibrio parahaemolyticus.

• Vibrio parahaemolyticus is associated with more than half of all cases of seafood-borne bacterial gastroenteritis.
• Results from consumption of shellfish, especially raw oysters.
• Symptoms include watery diarrhea, abdominal cramps, nausea, vomiting, headache, and fever.
• Fortunately, gastroenteritis is preventable by cooking, which kills the bacteria.
• In most cases, gastroenteritis is self-limiting.

Septicemia is associated with consumption of raw oysters that are contaminated with Vibrio vulnificus

• In the bloodstream, Vibrio vulnificus triggers a systemic inflammatory response; the bacteria are protected their polysaccharide capsules, but massive release of pro-inflammatory cytokines damages the host.
• Gastrointestinal symptoms followed by chills, fever, and septic shock are associated with septicemia.
• The mortality rate is high; in some reports, more than half of infected patients die.
• Antibiotics

Wound infections

• Occur after exposure to contaminated water.
• Mild infections can lead to cellulitis; this is more common when Vibrio parahaemolyticus is the causative agent.
• Severe cases can lead to necrotizing fasciitis; this is more common when the wound is infected by Vibrio vulnificus.
• Treatment includes antibiotic administration; if necrotic tissue is present, surgical debridement is also necessary.

Helicobacter pylori

• Spiral rod-shaped bacterium; it can appear coccoid in older samples.
• Very common pathogen; present in about half the world’s population.
• Colonization is life-long; introduction often occurs during childhood, though symptoms of infection occur later in adulthood.
• Bacterial carcinogen.
  Pathology
• Gastritis
  – Inflammation of the stomach lining.
  – Inflammation can be localized or widespread, and can affect the duodenum.
  – Neutrophils infiltrate the mucosa.
• Peptic ulcers
  – Bacteria erode the mucosal lining of the stomach and/or duodenal.
• Gastric adenocarcinoma
  – Associated with chronic, widespread inflammation.
  – Mucosal tissue is replaced by fibrous tissue and intestinal–type epithelium.
• Gastric mucosa-associated lymphoid tissue B-cell lymphomas
  – B-cells migrate to the stomach mucosa and establish lymphomas.

Campylobacter

• Common causes of bacterial gastroenteritis.
  – Also cause extra-intestinal infections.
• Spiral rod-shaped but may appear coccoid in older specimens.
• Campylobacter jejuni
  – Distinct histopathology, with ulcerated and bloody villi, white blood cell infiltration, and the formation of abscesses in the intestinal glands and crypts.
• In some individuals, Campylobacter jejuni infection triggers immune-mediated disorders.
  – Guillain-Barré syndrome occurs when the immune system targets the peripheral nervous system.
  Patients experience tingling and ascending weakness; ultimately, impairment of inspiratory muscles can lead to respiratory failure.
  – Reactive arthritis is characterized by joint pain and swelling.

Vibrio cholerae and Vibrio parahaemolyticus

Vibrio parahaemolyticus is also associated with wound infections.

• Consumed in contaminated food or water.
• Gastroenteritis caused by Vibrio species is characterized by watery diarrhea, cramps, vomiting, and nausea.
• Choleara
  – Two strains of Vibrio cholerae are associated with cholera, which is a severe gastroenteritis that produces profuse watery diarrhea (sometimes referred to as “rice water” diarrhea).
  – If untreated, Cholera can lead to fatal dehydration.

BARTONELLA

• Transmitted to humans via insect vectors.

Bartonella bacilliformis causes Carrión disease, primarily in the Andes mountains.

• 1st phase of infection: Oroya fever
  – Acute hemolytic bacteremia with fever and anemia.
  – Can be fatal.
• 2nd phase: Chronic verruga peruana (Peruvian warts)
  – Blood-filled cutaneous nodules.
• Treat with chloramphenicol or ciprofloxacin.

Bartonella quintana is transmitted in louse feces;

• Trench fever, which is also referred to as “5-day fever” because of recurring fever with headache, weakness, and pain in the long bones, particularly the tibia.
• Immunocompromised individuals can develop bacteremia with myalgia, malaise, weight loss, fever, and headache.
  – Bacteremia can lead to subacute endocarditis or bacillary angiomatosis, which is characterized by neovascular proliferation and formation of blood-filled nodules in the skin, subcutaneous tissues, and bones.

Bartonella henselae is transmitted by cats and their fleas.

• Cat-scratch disease typically presents as lymphadenopathy; however, infection can become disseminated.
• Bacillary angiomatosis of the skin, lymph nodes, spleen, and liver.
• Subacute endocarditis

BORDETELLA

Causes pertussis, aka, whooping cough.

• Childhood vaccine DTaP (Diphtheria, Tetanus, and Pertussis) and “booster” vaccines prevent pertussis.
  Bordetella pertussis and Bordetella parapertussis; Bordetella pertussis causes the more severe form of illness.
• Pertussis is most severe in infants, in whom it can lead to respiratory failure.
• Cases of adolescent and adult pertussis are on the rise; these populations are reservoirs for the bacteria.
• Stages of Pertussis
  – 7-10-day incubation period
  – Catarrhal stage: 1-2 weeks of symptoms that resemble the common cold.
  – Paroxysmal stage: 1-10 weeks of forceful coughing fits followed by inspiratory gasps – the “whooping” sound. Post-tussive vomiting is common.
  – Convalescence stage: Coughing is reduced, but serious complications may occur. Pertussis is associated with the development of pneumonia, seizures, apnea, and encephalopathy.

Virulence factors of Bordetella pertussis

Facilitate invasion and destruction of the respiratory tract.

• Adhesins, including filamentous hemagglutinin and fimbriae.
• Adenylate cyclase toxin blocks phagocytosis and prevents T-cellactivation.
• Dermonecrotic toxin forms necrotic lesions in the tissues.
• Tracheal cytotoxin destroys ciliated cells of the respiratory tract.
• Pertussis toxin increases cyclic AMP, promotes leukocytosis, and suppresses the host immune response
  – Secreted via Type IV secretion systems.

BRUCELLA

The number one cause of bacterial zoonotic infections worldwide.

• Brucella bacteria are associated with livestock; epidemics among farm animals lead to economic devastation.

Brucella melitensis, Brucella abortus and Brucella suis

Brucella melitensis is associated with most human infections.

• Intracellular pathogens that cause chronic infections with granuloma formation, particularly in the reticuloendothelial system.
• Treatment includes doxycline and streptomycin or rifampin; relapse is common.
• Brucellosis mimics other infectious diseases; because of this, diagnosis is often delayed.

Brucellosis

• Early infection is characterized by undulant fever with chills, sweating, coughing, vomiting, and weakness.
• Advanced infection is systemic with multi-organ effects and increased risk of abortion.

FRANCISELLA TULARENSIS

• Associated with rabbits, ticks, and flies.
• Intracellular pathogen.

Tularensis

• Ulceroglandular tularemia is most common manifestation.
  – Characterized by localized ulcers and glandular swellings.
• Glandular tularemia produces glandular swellings.
• Oculoglandular tularemia
• Oropharyngeal infections
• Pneumonic tularemia can be fatal.
• Treatment is gentamicin.

LEGIONELLA PNEUMONIA

• Found in water, including natural bodies of water, spas, air conditioners, and water misters.

Legionnaires disease

• Severe pneumonia with consolidation, as well as multi-organ damage.
• Treatment involves Macrolides

Pontaic fever

• Less severe than Legionnaires disease
• Fu-like symptoms, including fever, chills, myalgia, malaise, and headache.
• Antibiotic treatment is unnecessary in most cases, because the illness is self-limiting.

General characteristics:

• Gram-negative, straight or slightly curved rods.
• They do not catabolize glucose, so they are called “non-fermenters”
• Aerobes
  – However, in the lungs of patients with cystic fibrosis, research indicates that formation of biofilms may facilitate anaerobic respiration.
• Ubiquitous in the environment: they are often found in soil and water.
• Flagellar and pili twitching motility
• Oxidase-positive (aids in laboratory identification)
• Fruity, grape-like aroma
• Survive on trace nutrients – for example, they’ve even been found in distilled water.

Pseudomonas aeruginosa
Causes opportunistic infections; as we’ll see, it is especially well-adapted for survival in patients with cystic fibrosis.

• Produces two pigments that produce a distinctive appearance in culture:
  – Pyocyanin, which is a bluish color
  – Pyoverdine, which is a yellowish-green color (occaisionally spelled “pyoverdin”)

Virulence Factors

Some virulence factors are regulated by quorum sensing, and some factors work together in disease pathogenesis.

• Adhesins and pili facilitate adherence to host cells.
• Biofilm production facilitates immune system evasion and, as mentioned earlier, plays an important role in infection of cystic fibrosis patients.
• Endotoxin, which comprises lipopolysaccharide; as in other Gram-negative bacteria, endotoxin produces symptoms of sepsis and shock.
• Polysaccharide capsule with alginate.
  – Capsule is anti-phagocytic and prevents clearance by antibodies, and is upregulated in patients with cystic fibrosis.
  Exotoxins and enzymes are injected into host cells by Type III secretion systems, or into the tissues by Type II secretion systems.
• Exotoxin A inhibits protein synthesis and contributes to tissue necrosis.
• Pyoverdine, which, as described above, lends the bacteria a yellow-green hue, regulates secretion of Exotoxin A.
  – Also acts as a siderophore to “steal” iron from the host.
• Pyocyanin, the blue pigment, increases intracellular levels of cytotoxic superoxide and hydrogen peroxide
  – Also promotes apoptosis of neutrophils, which inhibits the innate immune response.
  – Interferes with respiratory cilia and damages mucosal cells, which are otherwise important mechanisms of microbe clearance.
• Alkaline protease inhibits complement and contributes to tissue destruction.
• Exoenzymes S, T, U, and Y
  – Exoenzymes S and T disrupt host cell actin cytoskeletons and promote cell death.
  – Exoenzyme U is cytotoxic, particularly to alveolar epithelial cells and macrophages.
  – Exoenzyme Y causes edema.
• Elastases* degrade complement and elastin, which is an important component of lung tissue.
  – LasA and LasB.
  – Host produces anti-elastase antibodies that form immune complexes; their deposition in tissues contributes to damage and malfunction.

Opportunistic infections

Infections are associated with hospital settings, especially in wet or moist areas.
Innate immune deficiencies or trauma, especially burn wounds, promote Pseudomonas infections.

Pulmonary infections range from mild tracheobroncitis to severe pneumonia with necrosis.
– Patients with cystic fibrosis are particularly vulnerable.
– Patients who rely on mechanical ventilation are also at a higher risk of infection.

Skin and soft tissue infections, particularly from burn wounds, are common.
– Pseudomonas aeruginosa can also cause folliculitis and nail infections; cases have been linked to contaminated water in hot tubs, spas, and salons.

Urinary tract infections, particularly in patients with in-dwelling catheters.

Osteochondritis can develop after puncture wound infection; This is common in foot wounds caused by stepping on contaminated nails or other sharp objects.
– Be aware that osteochondritis can also occur when infection spreads from other sites.

Otitis externa ranges from mild cases, aka, Swimmer’s ear, to more severe cases, which are associated with diabetics and elderly people.

Corneal infections, which can produce ulcers, can occur after trauma; for example, corneal scratches caused by contact lenses.

Bacteremia with a high mortality rate, often because of multi-drug resistant strains and the fact that infected patients tend to be immunocompromised.
– Ecthyma gangrenosum is characterized by necrotic and hemorrhagic skin lesions.

Antibiotic resistance:

– Intrinsic properties of their cell walls.
– Acquired resistance via horizontal gene transfer.
– Adaptive resistance; for example, environmental triggers in the lungs of cystic fibrosis patients induces upregulation of resistant mechanisms.

Closely related to Pseudomonas

Many of these were once considered members of Pseudomonas until recently.
Burkholderia cepacia Complex comprises multiple species that are associated with Pulmonary infections, urinary tract infections, and bacteremia; they are generally susceptible to treatment with Trimethoprim-sulfamethoxazole (TMP-SMX).

Burkhoderia pseudomallei causes melioidosis, aka, Whitmore’s disease; cutaneous and pulmonary infections can be treated with TMP-SMX.

Stenotrophomonas maltophilia causes pneumonia and bacteremia, and can also be treated with TMP-SMX. (formerly known as Xanthomonas maltophilia)

Acinetobacter species are associated with infections of the respiratory tract, urinary tract, and wounds; unfortunately, these species are resistant to many antibiotics.

Moraxella catahrralis is associated with Bronchial infections, sinusitis, and otitis; it is penicillin-resistant, but susceptible to other types of antibiotics.

HAEMOPHILUS

• Pleiomorphic small rods or coccobacilli
• Facultative anaerobes
• Growth of most species requires addition of X factor and/or V factor on chocolate agar.
• Pili and non-pilus adhesins that attach to host cells are key to pathogenesis.

Haemophilus influenzae

this species derives its name from early suspicions that it was the causative agent of influenza, which we now know is caused by a virus.

• Spread in air droplets and respiratory secretions.
• Endotoxin comprises lipo-oligosaccharide, which impairs ciliary function and damages respiratory epithelium.
  – Allows the bacteria to evade ciliary clearance and to invade pharyngeal tissues to reach the blood.
• IgA 1 protease inhibits humoral immunity, which would otherwise activate complement and destroy the microbes.
• Pili are specifically adapted to adhere to mucus, epithelial cells, and extracellular matrix proteins.

Haeomophilus influenzae Non-Encapsulated

• Aka, Non-typeable
• Common colonizers of the upper respiratory tract
• Opportunistic pathogens that invade epithelial cells and macrophages, which facilitates infection.
• Associated with sinusitis, otitis, bronchitis, and pneumonia.
  – These infections are more likely in individuals with adaptive immune system deficiencies; in healthy individuals, serum is bactericidal and prevents microbial spread.

Haemophilus influenzae Encapsulated

• Further classified into serotypes a – f based on the antigens of their polysaccharide capsules
  – These strains are more often associated with invasive disease.
• Prior to the development of an effective vaccine, Haemophilus influenzae type b was a significant cause of pediatric bacterial infections; in regions where immunization programs are not well-established, it remains a major source of childhood illness.
• Meningitis particularly in children 3-18 months of age
• Arthritis
• Cellulitis around the cheeks and periorbital area.
• Epiglottitis is a life-threatening disease that primarily affects children 2-4 years old
  – It is characterized by pharyngitis, fever, and, difficulty breathing that progresses as swelling obstructs the airways.
• Be aware that non-type b encapsulated Haemophlius influenzae infections may be on the rise.

Haemophilus aegyptius occasionally causes acute purulent conjunctivitis.

• Conjunctivitis is more commonly associated with Gram-positive bacteria (specifically, Staphylococcus and Streptococcus).

Be aware that some authors call Haemophilus aegyptius a biogroup of Haemophilus influenzae.

Haemophilus ducreyi

• Causes chancroid, a sexually transmitted infection that is uncommon in the United States.
  Be aware that some authors call Haemophilus aegyptius a biogroup of Haemophilus influenzae.
  – Infection causes genital ulcers that can be treated with erythromycin.

PASTEURELLA

• Pasteurella multocida commonly colonizes the mouths of cats and dogs.
• Pasteurella canis colonizes the mouths of dogs.
  – Pasteurella canis is less commonly associated with human infection.
• Coccobacilli
• Facultative anaerobes
• Polysaccharide capsules that comprise hyaluronic acid.
• Infection usually results from animal bites or scratches.

Pastuerella multocida

• Systemic infection particularly in individuals with immune or liver dysfunctions.
• Exacerbation of underlying chronic respiratory disease.

Pastuerella multocida and Pasteruella canis

• Localized cellulitis and lymphadenitis.

ACTINOBACILLUS

• Colonize the oropharynx of humans and animals; infections are relatively rare.
• Actinobacillus equuli and Actinobacillus ligniereslii can cause bite wound infections, especially bites by farm animals.
• Actinobacillus ureae can cause septicemia, meningitis, and pneumonia.
• Actinobacillus hominis can cause septicemia and pneumonia.

AGGREGATIBACTER

• This genus comprises species that were formerly classified as Actinobacillus or Haemophilus
  – Be aware that some authors continue to refer to these species by their former genus names.
• Aggregatibacter actinomycetemcomitans is often found with Actinomyces in mixed infections, hence its name.
  – Colonies are described as star-shaped or like cigars crossing.
  – It is associated with subacute endocarditis of damaged valves.
  – Some strains cause localized aggressive periodontitis; toxins, including leukotoxins, cause significant damage to the tissues of the oral cavity.
• Aggregatibacter aphrophilus is also associated with endocarditis of damaged valves.
  – This bacteria has also been found in brain abscesses in children with congenital heart disease.
• Endocarditis caused by Aggregatibacter species can be treated with cephalosporins
• Periodontitis treatment includes biofilm debridement and administration of doxycycline or other antibiotics.
• Brain abscesses caused by Aggregatibacter aphrophilusinfection can be treated with meropenem.

RESPIRATORY ILLNESSES CAUSED BY GRAM-NEGATIVE RODS

Haemophilus influenzae

• Transmitted via respiratory droplets.
• Two key types of Haemophilus influenzae: Nonencapsulated and Encapsulated.
• Haemophilus influenzae non-encapsulated, aka, Non-typeable colonizes the upper respiratory tract of many people.
  – The bacteria can migrate to other parts of the respiratory tract and cause:
  Sinusitis and otitis
  Bronchitis and pneumonia; patients can develop bronchial inflammation and/or pneumonia with consolidation. Patients with chronic obstructive pulmonary disease (COPD) are at especially high risk of pneumonia from Haemophilus influenzaemigration.
• Encapsulated Haemophilus influenzae, especially type B, causes serious, life-threatening infections in unvaccinated children.
  Meningitis and septic arthritis
  Epiglottitis is characterized by a swollen epiglottis that can obstruct the airways and esophagus. Patients have a sore throat, and may drool due to the inability to swallow saliva; may be life-threatening.

Bordetella pertussis and Bordetella parapertussis

• Transmitted via respiratory droplets.
• These bacteria cause pertussis, aka, whooping cough.
• Infection is highly contagious, and is most severe in infants, who experience respiratory failure.
• Clinicians are seeing an increasing number of cases in adolescents and adults.
• DTaP vaccine protects children from infection.

Stages of Pertussis

• Incubation period lasts approximately one week.
• Catarrhal stage lasts one to two weeks.
  – This stage is characterized by non-specific symptoms of fever, runny nose, sneezing, and cough.
• Paroxysmal stage can last for several weeks.
  – Characterized by episodes of forceful expirations followed by a loud inspiration – the “whoop!” of “whooping cough”.
  – Because the fits are prolonged, oxygen levels can drop, leading to cyanosis, and post-pertussive vomiting can occur.
• Convalescent stage is characterized by reduced coughing; complications are possible, including pneumonia and encephalopathy.

Legionella pneumophila

• Transmitted via aerosolized water.
  – For example, in the mist from air humidifiers, shower heads, etc.
• Legionnaire’s disease is characterized by severe pneumonia and fever; older and less healthy individuals are more susceptible to disease.
• Pontiac fever is characterized by fever, myalgias, and other flu-like symptoms.

Francisella tularensis

• Transmitted via infected rabbits or bites from ticks and deerflies; thus, tularemia is a zoonotic disease.
• Tularemia can manifest in different forms:
  – Ulceroglandular tularemia produces skin lesions, often with black areas; the lymph nodes also become infected and produce tender, painful bulges in the skin – it is sometimes said to resemble the bubonic plague.
  – Pneumonia*
  – Tularemia can also affect other organs, including the eye, mouth, and throat.

SHARED TRAITS

• Salmonella, Shigella, and Yersinia are members of the family Enterobacteriacea.
  – This family also includes Esherichia coli, Citrobacter, Serratia, Proteus, Klebisiella, and Morganella.
  – To learn more about their general properties and common virulence factors, please see our overview tutorial on E. coli.
• Salmonella, Shigella, and Yersinia gain access to the host by using Peyers’ patch M-cells to cross the intestinal epithelium.

SALMONELLA

• Be aware that Salmonella nomenclature is somewhat contested and highly confusing. Currently, the full name of an isolate includes italicized genus and species with non-italicized serovar type.
  – So, for example, the full name of the isolate that causes typhoid fever is: Salmonella enterica serovar Typhi; most authors shorten this to Salmonella Typhi.

• Salmonella causes foodborne infections.
• Does not ferment lactose, unlike the other members of Enterobacteriacea;
• Produces hydrogen sulfide.
• Infects both humans and non-human animals
  – Exception: Typhoid-causing strains are strict human pathogens.
• Intracellular pathogens that can invade all tissue types, including the brain.
• Virulence genes are encoded on Pathogenicity islands I and II:
  – Type three secretion systems inject effector proteins into host cells.
  – Salmonella-secreted invasion proteins, which facilitate entry into host cells.
  – Immune evasion proteins.
• Infection is acquired after consumption of contaminated foods, especially eggs, poultry, and dairy.

Salmonellosis

• Gastroenteritis, characterized by inflammation of the intestinal lining, is the most common form of Salmonellosis in the U.S.
  – Symptoms include nausea, vomiting, diarrhea, fever, and cramps.
  – Gastroenteritis is usually self-limiting, and requires only supportive care (i.e., water and ion replenishment).
  – Salmonella Typhimurium, the isolate often associated with gastroenteritis, resides in Salmonella-containing vacuoles (SCVs) within phagocytes; this protective endosome allows the bacteria to survive and replicated safely inside host cells.
• Septicemia causes typical bacteremia symptoms, including fever, chills, and low blood pressure.
  – In a small subset of patients, localized infections can develop, particularly in the joints and cardiovascular endothelium.
  – Salmonella septicemia is most common in young children and older adults, and in individuals with HIV or other immune deficiencies.
  – Be aware that recurrent septicemia is an AIDS-defining condition.
  – Treatment includes Fluoroquinolone or Cephalosporin antibiotics.

• Enteric fevers
• Typhoid fever is associated with Salmonella Typhi
• Paratyphoid fever, a milder form is associated with Salmonellaserotypes Paratyphi A, Paratyphi B, and Paratyphi C
  Be aware that S. Paratyphi B is also referred to as Salmonella Schottmeulleri, and S. Paratyphi C is also referred to as Salmonella Hirshfeldii
• Symptoms & Pathogenesis:
  – Approximately 10 days after ingestion of the bacteria, individuals experience fever, fatigue, and other non-specific symptoms; indicate that some patients develop “rose spots,” which are small red macules on the trunk.
  – Approximately 20 days after ingestion, patients experience gastrointestinal symptoms, which can include constipation or diarrhea, nausea, and vomiting.
  • We show that Typhoid-associated strains, such as SalmonellaTyphi, move through the lining of the intestine and are engulfed by macrophages.
    Macrophages carry the microbes to the bone marrow, liver, and spleen, where bacterial replication occurs.
    This sets the stage for bacteremia and the non-specific symptoms patients experience.
    Infection increases splenic cellularity and leads splenomegaly in some patients; liver enlargement has also been reported.
    Ultimately, the bacteria colonize the gallbladder and re-infect the intestines, producing gastrointestinal symptoms.
• Treatment includes administration of fluoroquinolones or ciprofloxacin.
• Write that asymptomatic carriers maintain typhoid-associated strains within populations; for example, enteric fever is endemic in Southeast Asia and parts of Africa.

SHIGELLA

• So closely related to Escherichia coli, particularly Enteroinvasive E. coli, that some authors call Shigella an E. coli biogroup.
• Shigella is an intercellular pathogen that only infects humans.
• Asymptomatic carriers are a key reservoir.
• Virulence factors are encoded on a plasmid, but regulated by chromosomal genes; this means that the plasmid, alone, is not enough to promote virulence.
  – Type three secretion systems inject effectors
  – Ipa’s A-D, facilitate invasion of epithelial cells and macrophages.
• Within host cells, Shigella lyses phagosomes and replicates within the cytoplasm; notice that this is unlike Salmonella, which replicates within special vacuoles.
• Actin tails facilitate Shigella migration to adjacent cells, which allows the bacteria to spread without exposure to agents of innate immunity.

Shigellosis

• Primarily affects small children; outbreaks are associated with daycares and preschools, and other places where fecal-oral transmission via contaminated hands is likely.
• Shigella has a low infectious dose.
• Upon infection, Shigella enterotoxin initially causes watery diarrhea.
• Then, as bacteria invade the colon mucosa, bloody diarrhea with pus, cramping, and fever occur.
  – The presence of neutrophils, red blood cells, and mucus in the stool is a helpful diagnostic criterion.
• Most patients with Shigellosis recover on their own; supportive care to prevent dehydration is all that is needed.
  – In severe cases, antibiotics can be administered.
• There are multiple pathogenic strains of Shigella:
  – Shigella sonnei is responsible for most cases of Shigellosis in the U.S.
  – Shigella flexneri is responsible for most cases of Shigellosis in developing countries.
  – Shigella dysenteriae causes dysentery, the most severe form of Shigellosis.
  These strains produce Shiga exotoxin, which impairs protein synthesis.
  As a result, damaged intestinal epithelium produces bloody diarrhea.
  Damaged renal endothelial cells can lead to Hemolytic uremia syndrome (HUS); this occurs most often in infected children.

YERSINIA

• Zoonotic infections.
• Short rods, aka, coccobacilli
• Bipolar Gram-staining: their tips stain darker than their middles.
• Type three secretion systems secrete immune-suppressingeffectors into host cells, which allows Yersinia to survive and replicate.

Plague

• Yersinia Pestis causes plague.
• Infected fleas and lice transmit the bacteria to humans.
• Virulence factors and their genes:
  – Antiphagocytic protein capsule (Factor 1 gene).
  – Outer membrane proteins degrade complement proteins C3b and C5a; this prevents bacterial opsonization and phagocyte migration (Plasminogen activator gene).
  – **Plasminogen activator proteases **also degrade fibrin clots, which may facilitate bacterial spread in the host (Plasminogen activator gene).
• Bubonic plague is characterized by fever, headache, chills, and weakness; replication in the lymph nodes produces swelling and tenderness – “buboes” refers to the visible lymph node swellings that typically appear in the axilla or groin.
• Septicemic plague occurs when infection spreads throughout the body via the bloodstream or lymph; it produces fever, chills, and extreme weakness, as well as abdominal pain, and, possibly, shock or bleeding into the skin or other organs.
• Pneumonic plague can quickly lead to respiratory failure and death; and, because is spread in respiratory droplets, transmission is human–to-human.
• Treatment includes streptomycin or gentamycin; because disease progression is rapid, and infection can spread to the lungs, early administration of these antibiotics is crucial.

Yersiniosis

• Yersinia enterocolitica and Yersinia pseudotuberculosiscause enterocolitis, especially in children.
• They are associated with *undercooked pork.
• Young children tend to experience bloody diarrhea, cramps, and fever.
• Older children tend to experience fever, and a pain in the right side of the abdomen that may be confused with appendicitis.
• In most cases, supportive care is sufficient.

ENTEROPATHIC E. COLI (EPEC)

• Watery diarrhea and vomiting.
• Occurs most often in toddlers and infants in developing countries.

EPEC gastroenteritis

• Adhesion and effacing lesions formed on the epithelial cells of the small intestine.
  – The lesions destroy the microvillus brush border, which is where the final stages of carbohydrate and protein digestion and absorption take place.
• Virulence factors and gene locations:
  – Bundle-forming pili (BFP)* mediate initial attachment to host and bacterial cells and form microcolonies.
  BFP genes are found on the EPEC adherence factor plasmid in typical strains.
  – Interactions between intimin and the translocated intimin receptor (Tir) mediate more intimate attachment; their genes are found on the Locus of Enterocyte Effacement (LEE) pathogenicity island.
  – The Type 3 Secretion System (T3SS) translocates Tir and various cytotoxic effectors into the host cell cytoplasm; EPEC cytotoxic effectors are encoded by pathogenicity islands and phages.

ENTEROTOXIGENIC E. COLI (ETEC)

• Watery diarrhea with vomiting, fever, and abdominal cramps.
• Travelers and infants in developing countries.

ETEC gastrotenteritis

• Attaches to epithelial cells of the small intestine via colonization factor antigens I-III; the genes for CFAs are carried on plasmids.
• Flagellar adhesions and outer membrane proteins Tia and TibA also promote adherence; these traits are carried on pathogenicity islands.
• Heat-stable and heat-labile enterotoxins cause water and ion loss by altering activity in the cells of the small intestine; as a result, water and ions are flushed out of the body in diarrhea. The genes for both enterotoxins are on plasmids.
  – Heat-stable enterotoxin (STa) increases cGMP in host cells; As a result, sodium absorption decreases and water secretion increases.
  – Heat-labile enterotoxin increases cAMP; as a result, chloride secretion increases and sodium and chloride absorption decreases.

ENTEROAGGREGATIVE E. COLI (EAEC)

• Watery diarrhea with inflammatory cell infiltration, vomiting, and dehydration.
• Causes gastroenteritis in travelers and infants; in some children, it can lead to chronic diarrhea and growth retardation.

EAEC gastroenteritis

• Characterized by aggregative adherence to each other and the epithelial cells of the small and large intestines; this results in a “stacked brick” aggregation pattern
  – Attachment is mediated via aggregative adherence fimbriae; the genes for AAF are carried on a plasmid.
• AAF also trigger mononuclear cell infiltration and inflammation.
• Cytotoxins damage the intestinal mucosa.
  – The specific cytotoxins vary by strain.
  – Plasma-encoded toxin (Pet), which is coded for by plasmid genes, and induces exfoliation.
  – Be aware that some isolates produce Shigella enterotoxin 1 and EAST1.

SHIGA TOXIN-PRODUCING E. COLI (STEC)

• Watery diarrhea that can progress to hemorrhagic colitis with severe abdominal cramping; some patients develop hemolytic uremic syndrome.
• Children younger than 5 years old are most commonly infected by STEC.
• Enterohemorragic E. coli (EHEC) is generally considered to be a subset of STEC, and there is phenotypic variation even within EHEC isolates.
• Animals, particularly cattle, are key reservoirs for STEC.
• Humans are infected upon ingestion of foods, especially undercooked meat and raw fruits and vegetables; water is also a source of infection.
• Undercooked hamburger was once the main cause of STEC infection in the U.S.; increased awareness of proper handling has reduced the rate of hamburger-associated infections.
• Because STEC has a low infectious dose, person-to-person transmission is also possible.
• Antibiotics increase the risk of HUS complications, and should NOT be given to patients with STEC infections; it is thought that antibiotics cause phage lysis and increase toxin release.

STEC gastroenteritis

• Some strains produce adhesion and effacing lesions that damage microvilli brush border; as in EPEC strains, this is mediated by intimin and Tir;
• Hemorrhagic coli pilus is a type IV pilus.
• Cytotoxins cause necrosis and cell death.
  – More specifically Shiga toxins damage the microvasculature; the genes for these toxins are on phages.
• In some cases of severe infection, Shiga toxins destroy red blood cells resulting in hemolytic uremia syndrome, aka, HUS.
  – HUS is characterized by microvascular thrombi, thrombocytopenia, hemolytic anemia and renal damage that leads to renal failure.

ENTEROINVASIVE E. COLI (EIEC)

• Watery, then bloody, diarrhea and abdominal cramps.
• Fever helps to distinguish EIEC from STEC.
• EIEC infection is rare, and the bacteria are closely related to Shigella. In fact, many authors consider them to be members of the same species, and much of what we know about EIEC comes from research on Shigella.
• Unlike most other strains of E. coli, EIEC are non-motile, and do not ferment lactose.

EIEC gastroenteritis

• EIEC bacteria are intracellular pathogens that invade cells of the colon.
• Genes for the following invasion virulence factors are on the Invasion plasmid (InvP):
  – The type 3 secretion system and IpaC effector protein promote bacterial entry into cells
  – IpgD, IpaA, and VirA effectors facilitate bacterial invasion of phagosomes
  – IpaB, IpaC, and IpaD effectors facilitate phagosome escape
  – OpsB, OpsF, and OpsG effectors inhibit host immune responses
• Intra- and intercellular movement is facilitated by VirG, which promotes actin polymerization; these actin tails push the bacteria within and between host cells.
• Severe infection can lead to Shigellosis, aka, bacillary dysentery, which is characterized by colon epithelium destruction and inflammation. However, the EIEC-associated form is rare.

GENERAL PRINCIPLES

• Gram-negative Rods
• Colon microbiome
  – E. coli are common members of the colon microbiome.
  – Opportunistic pathogens: upon transfer to new anatomical niches, these strains can cause endogenous infections.
• Found in soil and water
  – As a result of animal or human fecal contamination.
  – Upon ingestion, pathogenic strains cause exogenous infections in humans.
• Because they reside in the intestines of animals and humans, E. coli are “enteric” bacteria.
• E. coli common pilus
  – Aids in adhesion to host surface and biofilm formation.
• Flagellar motility facilitates movement.
• Rapid growth
  – Simple nutritional requirements
• Facultative anaerobe
• Catalase-positive
• Oxidase-negative
• Reduce nitrate
• Ferments glucose
• Ferments lactose
  – Coliforms: Gram-negative enteric lactose fermenters.
  – Also includes strains from Klebisella, Serratia, Klebsiella, and Citrobacter.
  – Eosin methylene blue agar and MacConkey’s agar are two types of selective media used to identify coliforms; both inhibit Gram-positive bacterial growth and signify acid production from lactose fermentation.
  EMB agar: lactose fermentation and strong acid production yield a characteristic greenish hue.
  MacConkey’s agar: E. coli lactose fermentation produces a pinkish-purple color.

COMMON VIRULENCE FACTORS

Many virulence factors are acquired via horizontal gene transfer of plasmids, bacteriophages, and pathogenicity islands.

• E. coli endotoxin comprises heat-stable lipopolysaccharide:
  – Its A-antigen has classic endotoxin activity: it triggers the host inflammatory responses, disseminated intravascular coagulation, and hypovolemic shock.
  – The core polysaccharide is common to all Enterobacteriaceae.
  – The O-antigen is used to classify E. coli serotypes.
• Exotoxins promote cell death or fluid loss.
• Adhesins are present on pili, fimbriae, or strain-specific surface antigens.
• Type III secretion systems inject virulence effector proteins into host cells.
  – Virulence effectors promote attachment, invasion, and cell destruction.
• Phase variation alters the expression of O, K, and flagellar H antigens and pili.
  – Alteration of surface antigens allows E. coli to avoid host immune system and, possibly, to adjust energy expenditure according to environmental factors.
• Antibiotic resistance
  – Resistant phenotypes are rapidly spread via HGT, which makes antibiotic treatment of E. coli infections particularly difficult.
• Siderophores, special receptors, and transporters
  – Acquire iron from the host; recall that iron is necessary for bacterial growth.
• Polysaccharide capsule lends repels phagocytes and avoids serum killing.

E. COLI INFECTIONS

Extraintestinal Infections

• Causative strains are called Extraintestinal pathogenic E. coli (ExPEC).
• ExPEC are often commensals that act as opportunistic pathogens when exposed to new anatomical sites.

• Urinary Tract Infections
  – Uropathic E. coli (UPEC) causes most urinary tract infections.
  – Adhesins: Type 1 and P pili and Dr fimbriae attach to urothelium.
  – Hemolysin A causes cell damage and facilitates bacterial movement through superficial cell layers.
  – Strains lacking these virulence factors are easily removed from the body in the urine.

• Neonatal Meningitis
  – E. coli is the second most common cause of bacterial neonatal meningitis.
  – Neonatal Meningitis-causing E. coli (NMEC) have special virulence factors that enable translocation across the blood-brain barrier:
  Type I pili and Outer Membrane Protein A (OmpA) attach to brain microvascular endothelial cells.
  Ibe (invasion of brain endothelial cells) proteins, CNF1 (cytotoxic necrotizing factor 1) and other mediators facilitate invasion of the blood brain barrier.
  K1 capsular antigens are thought to prevent lysosome fusion to allow successful movement of live bacteria across the barrier.

• Septicemia
  – Septicemia can occur when urinary or gastrointestinal tract infections spread to the blood; this can happen, for example, upon trauma to the abdomen.

Gastroenteritis

• E. coli pathotypes responsible for gastroenteritis are collectively referred to as “Intestinal Pathogenic E. coli” (IPEC).
  Infection typically follows ingestion of contaminated food or water; human or animal feces is often the source of the bacteria.
• IPEC can affect the small and/or large intestine:
  – Small intestine: Enteropathogenic E. coli and Enterotoxigenic E. coli
  – Small and Large intestines: Enteroaggregative E. coli
  – Large intestine: Enteroinvasive E. coli and Shiga toxin-producing E. coli
  Virulence factors specific to each pathotype determine the pathogenesis of infection.