PATHOGENIC ENTEROBACTERIACEAE

Be aware that we will not include all pathogenic members, and that details can be found elsewhere.

Common features of enterobacteriaceae:

• They are ubiquitous in the environment, and some are commensals.
• They are a significant cause of disease, particularly urinary tract and gastrointestinal infections.
  The following characteristics are helpful in identification:
• The bacteria in this family share a common surface antigen.
• They are facultative anaerobes.
• The ferment glucose and reduce nitrate
• They are catalase positive and oxidase negative
• They have common virulence factors, including endotoxin, antibiotic resistance, and others.

Ferments lactose: Escherichia coli

• When grown in MacConkey’s agar, lactose-fermenters, including most strains of E. coli, produce bright pinkish-purple colonies; in Eosin methylene blue agar, lactose-fermenters produce colonies with a bright green sheen.
  Escherichia coli produce extra-intestinal and gastrointestinal infections, depending on the bacterial strain.

• Extra-intestinal infections are typically caused by opportunistic pathogens, such as commensals that migrate to new anatomical niches.
  – Primary cause of urinary tract infections.
  – Significant cause of neonatal meningitis.

• Gastrointestinal infections are caused by pathogenic strains in the GI tract; they are usually introduced via contaminated food or water.
• Pathogenesis and symptoms vary by pathotype
• Infants and children are most likely to suffer long-term consequences of dehydration due to severe diarrhea and/or vomiting.
• Pathotypes that tend to produce their effects in the small intestine and cause watery diarrhea and vomiting (illness is often referred to as “traveler’s diarrhea):
  – Enteropathogenic, Enterotoxigenic, and Enteroaggregative; Enteroaggregative E. coli also affects the large intestine.
• Pathotypes that primarily affect the large intestine and produce watery, then bloody diarrhea:
  – Shiga toxin-producing Escherichia coli, as its name suggests, produces Shiga-like toxin.
  Enterohemorrhagic Escherichia coli (EHEC) is usually considered a subset of Shiga toxin-producing Escherichia coli.
  In severe infection, a condition called hemolytic uremic syndrome (HUS) can develop when red blood cells and platelets are destroyed; the damaged cells can obstruct the renal filtration system and cause renal failure.
  Antibiotics increase the risk of HUS in patients with Shiga toxin-producing Escherichia coli infection.
  – Enteroinvasive Escherichia coli is similar to Shigella, which produces inflammatory lesions and ulcers in the colon.

Enterobacteriaceae that do not ferment lactose (or do so very slowly)

• They produce colorless colonies on MacConkey’s agar, as opposed to the bright pink colonies of E. coli.

Salmonella

• Transmitted via the fecal-oral route, usually via contaminated foods.
• Causes typhoid and paratyphoid fevers, which are characterized by flu-like, nonspecific and gastrointestinal symptoms.
  – More specific signs of typhoid and paratyphoid fever include *gallbladder infection due to bacterial replication, liver and spleen enlargement, and the development of so-called rose spots over the skin of the torso.
• Also causes gastroenteritis without typhoid fever; this is the most common form of infection in the United States.
• Can cause extra-intestinal problems, such as osteomyelitis and septicemia; recurrent septicemia is an AIDS-defining condition.

Shigella

• Transmitted via the fecal-oral route, and show that it is an intracellular pathogen.
• Shigellosis is most common in children, and is easily transmitted due to its low infectious dose.
• Associated with watery, then bloody diarrhea.
• Shigella strains that produce the Shiga toxin can cause Hemolytic uremia syndrome (HUS), often in children.

Yersinia

• Zoonotic infections.
• Fleas transmit the strain of Yersinia that causes the plague, which is characterized by fever, chills, and weakness.
  Three key forms of the plague: bubonic, system, and pneumonic.
• Bubonic plague is characterized by infection of the lymphatic system, and that bacterial replication in the lymph nodesproduces buboes.
  – Localized lymph node destruction produces these inflamed, raised areas that are tender or even painful.
• Septicemic plague occurs when the bacteria enter the bloodstream. Patients experience extreme weakness, as well as pain in the abdominal area.
  – Disseminated intravascular coagulation occurs, producing necrosis and even organ failure without treatment.
• Pneumonic plague can occur if bubonic plague spreads to the lungs, or, in other cases, as the result of human-human transmission via respiratory droplets.
  – Inflammation and necrosis in the lungs can rapidly lead to respiratory failure.

• Entercolitis is caused by other strains of Yersinia
• Transmitted via consumption of infected animals, especially pork products.
  – Whereas younger children are likely to have bloody diarrhea, older children tend to experience pain the right abdomen that may be mistaken for appendicitis.