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.