Occurrence, Diversity of Listeria spp. Isolates from Food and Food-Contact Surfaces and the Presence of Virulence Genes

This study evaluates the hazards posed by foodborne bacteria of the Listeria genus by analyzing the occurrence, diversity and virulence of Listeria spp.in food and food-manufacturing plants. Seventy-five isolates obtained from the routine analysis of 653 samples taken by three diagnostic laboratories in Northern Italy were genotypically differentiated by Repetitive Extragenic Palindrome (rep) PCR, with the GTG5 primer identified by sequencing the 16S rRNA gene and examined by specific PCR tests for the presence of L. monocytogenes virulence determinants occasionally found to occur in other species of the genus. Within this sample, 76% (n = 57) isolates were identified as L. innocua, 16% (n = 12) as L. monocytogenes, 6.6% (n = 5) as L. welshimeri and 1.3% (n = 1) as L. seeligeri. All L. monocytogenes isolates belonged to the serotype 1/2a and were predicted to be virulent for the presence of the inlJ internalin gene. Potentially virulent strains of L. innocua, L. seeligeri and L. welshimeri, carrying the L. monocytogenesinlA gene and/or hly gene, were identified, and most isolates were found to possess the toxin–antitoxin system mazEF for efficient adaptation to heat shock. Results indicated the need to reinforce food-contamination-prevention measures against all Listeria species by defining efficiently their environmental distribution.