Prevalence of low-virulence Listeria monocytogenes strains from different foods and environments.

ABSTRACT

Various studies have demonstrated variations in the levels of virulence of different L. monocytogenes strains. In our laboratory, a plaque-forming assay followed by subcutaneous footpad inoculation of mice enabled us to estimate the prevalence of the low-virulence strains. This value fell from 16.3% to 1.7% with bacteria collected before 1994 and after 1997 respectively. This could be related to the modification in 1997 of the reference method EN ISO 11 290-1 of Listeria detection which recommended the use of polymyxin-acriflavine-LiCl-ceftazidime-aesculin-mannitol (PALCAM) medium. The aim of this study was to determine whether the percentage of low-virulence strains detected has changed due to the modification of the detection method recommending the use of the ALOA medium. After analyzing 380 L. monocytogenes strains, no increase in the percentage of low-virulence strains could be detected. The prevalence reached only 2.6% (ten of the 380 strains tested). The low virulence of L. monocytogenes strains was not related to rare serotypes and was also observed in serotypes usually involved in human disease. Low-virulence strains were found in dairy, meat, ready-to-eat products and also in the environment, highlighting the absence of one specific source. These results are discussed in terms of detection methods and the definition of low virulence.