Assessment of enrichment broths for their ability to recover Listeria monocytogenes from spiked milk

Abstract

Commercially available selective enrichment media like Fraser broth (FB), Listeria Enrichment Broth (LEB) and University of Vermont medium (UVM) were evaluated for their ability to recover cells of L. monocytogenes from spiked milk, simultaneously their ability to inhibit the background microflora like E. faecalis , L. casei , S. aureus and B. cereus which are invariably present in the milk and milk products was also studied. Milk spiked with five different strains of L. monocytogenes (ATCC 15313, 19111, 19118, 15313, BAA-751) and other contaminants such as E. faecalis, L. casei, S. aureus and B. cereus was inoculated in three selective enrichment media i.e. LEB, UVM and FB. LEB showed maximum recovery followed by UVM for all five strains of L. monocytogenes used in this study and average counts were achieved up to 9.283 and 9.179 log 10 CFU/mL after 24 h of incubation , while least recovery was obtained in FB where 8.225 log 10 CFU/mL counts were observed after 24 h of incubation at 35±2.0 o C. However, efficacy of FB was established a better inhibitory medium in context to the growth of contaminants other than Listeria spp. The study revealed that selective agents already present in FB possess some negative impact on the recovery of L. monocytogenes and but more inhibition towards background microflora was observed.