A comparison of standard cultural methods for the detection of foodborne Salmonella

The sensitivity of the standard cultural method of the International Organization for Standardization (ISO 6579 and ISO 3565 combined) was compared to that of the Health Protection Branch (HPB) procedure for the detection of foodborne Salmonella. Of 195 foods tested, 84 (43.1%) were found to contain salmonellae by one or more cultural conditions. Of these, 75 (89.3%) and 68 (81.0%) were identified by the ISO and HPB methods, respectively. The apparent lack of agreement between both methods likely stemmed from the low indigenous numbers of salmonellae in several food homogenates, and unequal transfer of the target microorganism into homologous ISO and HPB pre-enrichment broths. The sensitivities of the commercially available Muller-Kauffmann tetrathionate broth (MKTBG43, Oxoid CM343), and a closely-related medium prepared with Oxoid CM29 tetrathionate base varied from 86.9 to 89.3%, and were deemed equivalent to that obtained with the ISO formulation of MKTBG43 (89.3%). Comparatively fewer contaminated samples were identified from selenite cystine (SC35) and selenite brilliant green (SBG35) enrichment cultures (82.1-83.3%). The high selectivity and saccharide-independent response of the bismuth sulfite agar medium warrants its consideration as a mandatory plating medium in ISO methodologies for the effective detection of typical and atypical biotypes of foodborne Salmonella spp.