Prevalence of Listeria monocytogenes in food samples from retail shops and street vendor stalls in Pretoria and the evaluation of bacterial probiotics as potential control measure

Abstract

Listeria monocytogenes is a foodborne pathogen that causes listeriosis. This study investigates the prevalence of L. monocytogenes in food samples acquired from retail stores and street vendors around Pretoria and evaluates various probiotics as a control measure. The pathogen was isolated by selective enrichment and plating on selective media. All presumptive colonies were confirmed by morphological analysis, 16SrRNA gene BLAST analysis and inlA amplification. Probiotic antagonistic tests against two L. monocytogenes isolates acquired from food samples and three controls was performed. The probiotics used were Bifidobacterium animalis, Lactobacillus acidophilus, Lactobacillus plantarum and Pediococcus acidilacti during spot inoculation tests and well diffusion assays using probiotic cell free supernatants (CFS). The effect probiotics and their cocktails had on avocado and cucumber samples spiked with L. monocytogenes strains was assayed. Of 167 food samples, only two, avocado and cucumber, tested positive for L. monocytogenes. Samples positive for L. monocytogenes were obtained from street vendors exhibiting poor hygiene. B. animalis inhibited growth of four out five L. monocytogenes strains, L. acidophilus three, L. plantarum one while P. acidilacti inhibited none. Inhibition by all probiotic strains, their CFSs and cocktails in both food matrixes was statistically insignificant, except for L. monocytogenes T62 inhibition in avocado by B. animalis. Properties of L. monocytogenes, food medium and storage conditions used in the study could have had an impact on their inhibitory effects.