Listeria monocytogenes incidence and distribution on a selective/differential plating medium in Listeria-positive environmental samples from ready-to-eat food facilities

摘要

One thousand sixty-three environmental samples (sponges) from 9 meat, dairy, fish and bakery ready-to-eat food processing facilities were analyzed for Listeria. The ratios and distribution of Listeria monocytogenes to non-L. monocytogenes were determined by use of the chromogenic L. monocytogenes plating medium (LMPM). LMPM differentiates among the Listeria spp. by detecting phosphatidylinositol-spectfic phospholipase C activity specific for Listeria monocytogenes/L ivanovii. L. monocytogenes and L. ivanovii form turquoise colonies, whereas all other Listeria spp. produce white colonies with a blue tinge. L. monocytogenes confirmatory medium and acid production from rhamnose were used to rapidly differentiate L. monocytogenes from L. ivanovii, andall 136 turquoise colonies tested were identified as L. monocytogenes. Two hundred six samples were identified as Listeria-positive. On LMPM-positive plates, the ratios of L. monocytogenes to non-L. monocytogenes spp. were arranged into 5 categories onthe basis of colony color. In the two distribution extremes, 34 percent of the Listena-positive LMPM plates contained 100 percent non-L. monocytogenes Listeria and 23.3 percent contained 100 percent L. monocytogenes colonies, accounting for 57.3 percentof the total positive samples. Eighteen percent of the Listerio-positive LMPM plates contained a majority (> 75 percent) of white (non-L. monocytogenes Listeria spp.) and < 25 percent turquoise (L. monocytogenes) colonies, and on half these plates therewere 10 or fewer turquoise colonies. Of the remaining distributions, Usterio-positive LMPM plates containing a majority of turquoise colonies (> 75 percent, compared with < 25 percent white Listeria spp. colonies) and plates with approximate equal proportions of Listeria turquoise to white constituted 15.5 and 9.2 percent of the total positive samples, respectively. Thus, a selective/differential plating medium capable of differentiating L. monocytogenes/L. ivanovii from varying concentrations of otherListeria spp. is a useful tool when L. monocytogenes is a minority component of the total Listeria present.