The UK Standards for Microbiology Investigations (SMIs) are a comprehensive referenced collection of over 200 documents, outlining clinical microbiology SOPs, algorithms (for virology and serology) and guidance notes. The development of the standards and algorithms is undertaken under the auspices of the HPA in conjunction with the NHS and the National Public Health Service for Wales and with professional societies including the Association of Medical Microbiologists, Association of Clinical Microbiologists, Institute of Biomedical Science, Clinical Virology Network and the Scottish Microbiology Association.

The National Standard Method (BSOP 30) describes general culture of faecal samples for bacterial pathogens and BSOPID 22 specifically describes identification of VTEC O157. The methods give some guidance for the identification of the sorbitol-fermenting VTEC O157 that have been found very rarely in the UK.  There are no specific methods for the isolation of other VTEC.

Presumptive VTEC O157 isolates  should be sent for confirmation and typing to the Laboratory of Gastrointestinal Pathogens (LGP), Gastrointestinal Infections Reference Unit (GIRU), Microbiology Services, Colindale which provides the national reference services for VTEC in England and Wales. Diagnostic laboratories are encouraged to report suspect cases by telephone to the local Health Protection Unit for rapid investigation.

The front-line typing method for the differentiation of VTEC O157 is a phage typing scheme.  This can designate over 85 types (PTs).  It is essential to confirm the presence of Vero cytotoxin genes VT1 and/or VT2 and this is performed by PCR.  Isolates are submitted to LGP that lack VT genes but have all the other properties of VTEC O157.  These may cause diarrhoea but are not associated with more serious illness.

If VTEC O157 is not isolated but the clinical symptoms are consistent with VTEC infection, the specimen may be sent LGP where it will be tested for the presence of VTEC other than VTEC O157.  It is also possible to obtain evidence of infection by testing patient’s serum for the presence of antibodies to the lipopolysaccharide of VTEC O157 and a limited number of other VTEC.

For consistency over the whole country, is it essential that diagnostic laboratories submit all potential VTEC isolates to the Reference Unit in LGP so that a true picture of the occurrence of isolates is recorded.

For information and advice on all aspects of microbiology relating to VTEC and the services of the GIRU, contact the unit head: Claire Jenkins.