The suitability of the ISO 11290-1 method for the detection of Listeria monocytogenes

Published data suggest that detection of Listeria monocytogenes is affected by presence of other Listeria species in matrix. In this study efficacy for traditional standard detection method (ISO 11290-1) of L. monocytogenes was investigated, focussing on the growth in Fraser broth (FB). The multiplication ability of different mixtures of L. monocytogenes strains and Listeria innocua strain was investigated in half FB (hFB) and FB. Growth parameters were determined by DMFit software using the Baranyi-model. When initial cell concentration of L. innocua was higher than that of L. monocytogenes inhibition of the latter was observed. Based on the results it was concluded that the strain of L. monocytogenes was overgrown by L. innocua in Fraser broth. When their growth was monitored during co-culturing in hFB the lag phase of L. monocytogenes was prolonged by 3.4 h. When L. monocytogenes grew in the presence of L. innocua in 1:1 ratio a three log cycle difference was observed at the end of Fraser culturing step. Testing the suitability of ALOA plates for detection of L. monocytogenes it was determined that halo formation has to be checked after 24, 34 and 48 h incubation. All these difficulties might lead to false negative results.