|Abstract||This NSM describes the identification of presumptive Vero cytotoxin-producing Escherichia coli O157 (VTEC O157) isolated from faeces. These strains are associated with a wide spectrum of disease including haemolytic uraemic syndrome (HUS).Issue 3|
Vero cytotoxin-producing E. coli O157 (VTEC O157) is a member of the genus Escherichia and the family Enterobacteriaceae.
VTEC O157 is a Gram-negative rod. On sorbitol MacConkey agar (SMAC) or SMAC containing
cefixime and tellurite (CTSMAC), the colonies are colourless and 2-3mm in diameter. VTEC O157
differs from other members of the genus Escherichia in that it does not usually ferment sorbitol (a
characteristic that is exploited by the selective media3) and is â-glucuronidase-negative. Most VTEC O157 are motile and possess the flagellar antigen H7, but at least 20% in England and Wales are phenotypically non-motile. They are facultatively anaerobic. Strains are oxidase-negative and usually produce gas from glucose. Some strains show atypical biochemistry eg they are anaerogenic, nonlactose fermenting, indole-negative or urea-positive. Some VTEC O157 strains have been found to ferment sorbitol and to be â-glucuronidase-positive.
VTEC O157 is highly infective, and the infective dose is less than 50 organisms. Laboratory acquired infections have been documented.
Principles of Identification:
Presumptive VTEC O157 isolates from primary culture are identified by colonial appearance on
CTSMAC, serology (agglutination with O157-specific antisera) and biochemical tests. Some
commercial biochemical tests may give a doubtful or a low percentage profile for E. coli O157 as the fermentation of sorbitol is heavily weighted for other E. coli strains8, and care must be taken with the interpretation of the profile. All isolates of presumptive E. coli O157 should be referred to the Reference Laboratory for confirmation of identification, serotyping, phagetyping and Vero cytotoxin testing. All identification tests should ideally be performed from non-selective agar to take into account the variations that may occur with biochemical tests such as sorbitol fermentation.
Where the clinical evidence is suggestive of VTEC infection (particularly in children under 15 years and adults over 65 years) and no presumptive sorbitol non-fermenting E. coli O157 colonies are observed on SMAC or CTSMAC/SMAC agar, we recommend that clinical laboratories should:
• Test sorbitol fermenting colonies for agglutination with E. coli O157 antiserum
• Confirm the identification of agglutination-positive O157 colonies as E. coli
• Send presumptive isolate(s) to the Reference Laboratory (LEP, CfI, HPA Colindale) for
confirmation, phage typing and detection of VT genes
Faecal samples from appropriate cases from whom VTEC O157 have not been isolated should be submitted to LEP for detection of VTEC strains belonging to serogroups other than O157.