7.2.1 Isolation of E. coli

Is the isolation of E. coli from a faecal specimen a diagnostic confirmation of an infection?

Answer

It depends. Unlike Salmonella, E. coli are usually harmless intestinal commensals. Their isolation from faeces is trivial, but it has a diagnostic value if followed by the identification of specific virulence factors, such as the K99 factor in an E. coli isolated from a calf with diarrhoea.

Is the isolation of E. coli from a sterile site specimen indicative of an infection?

Answer

Recovery of these bacteria in pure culture from sterile site specimens is suggestive of an infection, although agonic and post-mortem invasion should also be considered.

A common isolation and identification protocol for E. coli from sterile sites is shown in Figure 11.

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Figure 11 Isolation and identification scheme for E. coli from sterile site specimens
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A flowchart depicting the isolation and identification scheme for E. coli from sterile site specimens.

Figure 11 Isolation and identification scheme for E. coli from sterile site specimens

These organisms are normally cultured using solid media. Note that to cover Gram-negatives and Gram-positives, in this scheme specimens are plated in parallel on blood agar and selective-differential MacConkey agar. E. coli is a lactose fermenter, producing red colonies on MacConkey (Figure 12). The isolate is then tested by a panel of biochemical tests for identification.

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Figure 12 Red/pink colonies of E. coli on MacConkey agar due to lactose fermentation
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Red/pink colonies of E. coli on MacConkey agar due to lactose fermentation.

Figure 12 Red/pink colonies of E. coli on MacConkey agar due to lactose fermentation

7.2 Escherichia coli

7.3 Salmonella species