LOD<sub>50</sub> is dependent on choice of Campylobacter strain and food matrix

Hazeleger, W.C.; Veljkovic Cvoric, L.; Jacobs-Reitsma, W.F.; Besten, H.M.W. den


Due to abundant growth of extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae during enrichment it is hard to isolate and recognize Campylobacter colonies on mCCDA. Therefore, in the current revision of the ISO protocol (ISO 10272-1), next to Bolton Broth (BB), Preston broth (PB) is suggested as enrichment broth to inhibit competitive fl ora in samples where high levels of background fl ora such as ESBLs are suspected.
To validate this revised ISO 10272-1, an Inter Laboratory Study (ILS) was performed where diff erent matrices were used in the enrichment procedures: frozen spinach, minced meat, raw milk and chicken skin. Each matrix was
inoculated with a diff erent strain of C. jejuni or C. coli and the results were expressed as LOD50 (Level of Detection) which is the lowest contamination level that can be detected with a probability of 50%. Since diff erent strains
were used for each matrix, results of the ILS are possibly infl uenced by the strains’ characteristics. Therefore, in this study we tested the enrichment procedures for spinach, minced meat, milk and chicken skin with each of
the strains used in the ILS. The LOD50 of all strains tested in spinach was in the range of 0.7 cfu/sample which complies with the ILS-results and which is also the theoretical value. Preliminary results for the other product types, however, show a large variation of LOD50 between strains which indicates that the choice of strain will infl uence the LOD50. In conclusion care should be taken to extrapolate the ILS results to other strains.