Update on media for isolation of Enterobacteriaceae from foods

Testing for 'total' Enterobacteriaceae, coliforms and Escherichia coli as m arker organisms in foods and detection of specific pathogens of the family Enterobacteriaceae, including pathogenic E. coli, Salmonella, Shigella and Yersinia spp. is widely applied in many food control laboratories. This rev iew describes some recent developments in culture media for these organisms . Methods for enumeration of E. coli include the standard MPN technique, a membrane-filter method and the use of media containing chromogenic and fluo rogenic indicators for beta-D-glucuronidase (GUD) activity. Shiga toxin-pro ducing E. coli O157 strains usually do not ferment sorbitol and an GUD-nega tive. These characteristics are used in selective media for these organisms , such as cefixime tellurite sorbitol MacConkey agar. For the detection of salmonellae, motility enrichment in Modified Semisolid Rappaport-Vassiliadi s (MSRV) medium shows equal or better results than the use of standard Rapp aport-Vassiliadis broth. Addition of nitrofurantoin to diagnostic semisolid salmonella agar and to xylose lysine desoxycholate agar favours the isolat ion of S. enteritidis. Recently developed salmonella media use different se lective and diagnostic properties, such as acid formation from propylene gl ycol, glucuronate fermentation, fermentation of glycerol and addition of Te rgitol 4 as selective agent. The isolation of Shigella spp. from foods is r ather difficult and further evaluation of suggested isolation systems and t he development of more effective methods for the isolation of this pathogen are needed. Yersinia enterocolitica includes both pathogenic and nonpathog enic biotypes and serogroups. As no single procedure will recover all patho genic strains of Y. enterocolitica, the use of two isolation procedures in parallel is recommended. (C) 1998 Elsevier Science B.V. All rights reserved .