Detection and diagnosis of Escherichia coli O157 and other verocytotoxigenic E. coli in animal faeces

摘要

Escherichia coli O157:H7 is increasingly often a cause of serious disease in humans with asymptomatic animal carriers cited as a reservoir of infection. Carrier animals excrete the organism intermittently and in low numbers in faeces. Current diagnostic procedures depertd upon enrichment and isolation and have been developed primarily for recovery from human clinical samples and food rather than from animal faeces. Non-selective enrichment is preferred followed by culture on a variety of media which contain supplements and indicators for the differentiation of E. coli O157 from other E. coli serotypes. Immunomagnetic separation (IMS) using antibodies specific for the O157 lipopolysaccharide enhances recovery from enrichment broths and IMS followed by plating on selective media is accepted as the gold standard for isolation of the serotype. However, this approach is laborious and not appropriate for large scale screening, particularly of animal faeces. Additionally, this approach does not detect other, non-0157 verocytotoxic E. coli. Isolation of these organisms relies upon direct analysis of colonies on agar plates by immunoblotting or DNA probing for verocytotoxin production, which is both laborious and technically demanding. There is a requirement, therefore, for simple, rapid tests for screening large numbers of samples. Those techniques that lend themselves to automation (enzyme-linked immunosorbent assays, PCR, immunomagnetic electrochemiluminescence) hold most promise, whether screening for the O157 antigen or for verocytotoxin production. Confirmation of the identity of potential human pathogens and characterisation for epidemio-logical studies ultimately requires the isolation of pure cultures but preliminary screening of samples would reduce the number needing to be cultured.