The patient's evaluation should be based on the clinical assessment and the need to do the following:
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Further evaluate the seriousness of the condition (degree of dehydration and electrolyte derangement).
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Determine a specific etiologic agent.
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Evaluate the patient for noninfectious etiologies.
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Patients who require further workup include those who appear seriously ill or dehydrated; those who have high fevers, bloody stools, severe abdominal pain, or persistent diarrhea; and those who are immunocompromised or whose condition is suspected of having an epidemic diarrheal etiology.
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History, epidemiologic considerations, and the physical examination should be the primary guides in determining whether any further diagnostic evaluation is necessary, followed by microscopic examination of the stool.
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Hektoen enteric agar with Escherichia coli colonies. Different growth media are necessary for identifying different enteric pathogens, suppressing the growth of nonpathogens, and allowing for chemical reactions to assist in identification. The appearance results from the organism's ability to ferment lactose placed in the medium. This results in the production of acid, which lowers the pH and causes a change in the pH indicator placed in the medium. Salmonella and Shigella organisms do not ferment lactose.
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Example of Salmonella on Hektoen enteric agar. The medium also contains ferric ammonium citrate, which indicates the production of hydrogen sulfide by the appearance of a black precipitate.
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The MacConkey medium is commonly used and differentiates lactose fermenters, which produce acid, decrease the pH, and cause the neutral red indicator to give the colonies a pink-to-red color.
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The Christensen method is used to determine if an organism produces the enzyme urease (Yersinia) or not (Salmonella, Shigella, Vibrio). Hydrolysis of urea produces ammonia and carbon dioxide, alkalinizing the medium and turning the phenol red from light orange to magenta (pink).
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Often, a combination of methods may be used for identification. The tube on the left is triple sugar iron (TSI) agar. The alkaline slant and acid butt (K/A) indicates an organism that ferments glucose only (not lactose or sucrose). The middle tube is indole positive, as indicated by the pink ring, and indicates the organism's ability to split tryptophan to form indole. The tube on the right is urease negative. Taken together, these tests indicate the organism is likely Shigella.
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Gram stain may be helpful in identifying an etiologic agent. This stain shows gram-negative bacilli, which could be Salmonella or Shigella with 2 polymorphonucleocyte cells (PMNs).
