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A presumptive identification of E. coli is made from typical colony morphology, appearing as a flat lactose-fermenting colony on MacConkey agar, in conjunction with a positive spot indole test. However, up to 10% of E coli strains may be lactose-negative and/or may produce an atypical colony, often mucoid in appearence. Most (if not all) clinical isolates of E. coli giving a negative spot indole test result will be indole-positive in an over-night tube indole test. E. coli is also frequently beta-hemolytic on sheep blood agar, while most other enteric Gram-negative rods are not. E. coli is the most frequent (70 - 90%) causative agent of UTI. | |
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Typically non-lactose-fermenting colonies isolated from stool cultures can be distinguished as possible Salmonella or Shigella colonies according to the following phenotypic characteristics:  (The combination of gas production and a positive indole test excludes an identification of Shigella.) Shigella spp. are usually recovered from a rectal swab or a stool specimen. Unlike Salmonella spp., Shigella tend to remain within the GI tract, although occasionally they can be found in the bloodstream. They produce a somewhat fringy non-lactose-fermenting colony on MacConkey agar and have a characteristic odor. Although closely related to Escherichia coli, Shigella are generally biochemically inert, including being non-motile and lysine decarboxylase negative. Unlike Salmonella spp., Shigella does not produce H2S from sodium thiosulfate. The most commonly isolated species in the U.S. is S. sonnei, and the least commonly isolated species is S. dysenteria. S. boydii and S. flexneri are isolated infrequently. | |
