Gastrointestinal Tract - Bacterial Identification

Laboratory testing algorithm for Salmonella, Shigella, Vibrio, E. coli, Campylobacter jejuni, Yersinia enterocolitica, Enterococcus, and Bacillus cereus



Stool culture must be performed to identify specific pathogens, because stool contains an abundance of commensal bacteria. A standard stool culture identifies Salmonella, Shigella, Campylobacter, and Yersinia enterocolitica. Since E. coli constitutes a substantial amount of gut flora, then specific serotypes known to cause illness must be identified, such as serotype O157:H7 producing a shiga-like toxin that can lead to hemolytic-uremic syndrome (HUS).

  • Salmonella typhi starts as a gastrointestinal infection, but can become a systemic disease. The more common Salmonella enterica produces cramping abdominal pain and diarrhea; it is often found in contaminated poultry products.

  • Shigella organisms are virulent and can produce a necrotizing colitis and cause dysentery (a bloody diarrhea).

  • Campylobacter jejuni can cause watery, and sometimes bloody, diarrhea and abdominal pain. Aninmals are often implicated as the source.

  • Yersinia enterocolitica is a less common organisms in the family Enterobacteriaceae that can cause diarrhea (bloody if severe), abdominal pain, and vomiting. It often involves the terminal ileum, resulting in signs and symptoms resembling appendicitis. Mesenteric lymph node involvement may occur, from where sepsis may originate.

  • Vibrio species involving the GI tract include the highly virulent V. cholera that elaborates a toxin causing a profuse watery diarrhea. The organism is not invasive, but the toxin is powerful, and treatment requires prompt fluid and electrolyte replacement.

    V. parahemolyticus produces a less severe diarrhea, and contaminated shellfish are often implicated as the source.

  • Enterococcal species within a genus of Streptococcus are gut commensals that are not ordinarily a problem until there is perforation of a viscus. Many of them have become antibiotic resistant.

  • Bacillus cereus is a cause for food poisoning, with prominent vomiting. These spore forming organisms can survive in harsh environments. Therefore, they may survive refrigerator temperatures or heating. Reheated fried rice is the classic source.