There are an estimated 2 billion cases of diarrhea that occur yearly, and it is the leading cause of death in many underdeveloped countries. It is the second leading cause of death in children younger than 5 years, taking the lives of approximately 1.9 million children each year. [14, 15] Approximately 30-50% of visitors to developing countries return with diarrhea.
In May 2011, a shiga-toxin–producing E coli (O104:H4), eventually classified as enteroaggregative pathotype, started in Germany and affected 3000 or more individuals, with 900 (30%) or more developing hemolytic–uremic syndrome (a very high percentage) and with an unusual number of adults affected and a high mortality rate compared with prior shiga-producing E coli strains. The German outbreak is unique as horizontal genetic exchange appears to have resulted in this unique O104:H4 strain, which has a prophage encoding shiga toxin 2 and additional virulence and antibiotic-resistance factors. Fieldwork suggested the source was fresh vegetables. [16]
In a systematic review and meta-analysis of data on the prevalence or incidence of norovirus and acute gastroenteritis in Latin America, the overall prevalence of norovirus in acute gastroenteritis cases was 15%, and 37%-100% of cases were associated with GII.4 strains (but only 7% of asymptomatic norovirus patients were affected with this strain). [17]
In a 2017 report that estimated the healthcare costs of acute gastroenteritis and human Campylobacter infection in Switzerland, investigators reported an annual cost of approximatedly 29-45 million euros, of which about 9 to 24.2 million euros related to physician visits without a stool diagnostic test being obtained; about 12.3 million euros for patients with negative Campylobacter species stool tests and 1.8 million euros for those with positive positive Campylobacter species stool tests; and 6.5 million euros for inpatients with Campylobacter infection. [18]
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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).
