Hazard identification and characterization of Listeria monocytogenes in salad vegetables and milk products in Mymensingh district in Bangladesh

The objectives of the present study were to isolate and identify Listeria monocytogenes in salad vegetables and milk products sold in different markets of Mymensingh district in Bangladesh, and to characterize this organism (serogroups, virulence genes) along with antibiogram study. A total of 85 samples of salad vegetables (tomato, carrot and cucumber) were collected from six different markets of five upazilas of Mymensingh district, and 83 milk products such as cheese, yogurt and icecream of two available brands were collected from Mymensingh sadar upazila. Isolation and identification of Listeria monocytogenes was done by cultural, biochemical, PCR and MALDI-TOF assays. The presence of serogroups (lineage I, lineage II, lineage III) and virulence-associated genes (inlA, actA, prfA, and hlyA) were determined by multiplex PCR. The antimicrobial susceptibility test of the isolates was done with 22 antimicrobial agents using disk diffusion assay. The overall prevalence of L. monocytogenes was 7.1%. Among salad vegetables, contamination of cucumber was highest (19.2%), and among milk products, the highest contamination was in icecream (7.1%). Five isolates (41.7%) belonged to Lineage II and seven isolates (58.3%) to Lineage III. Only two isolates (16.67%) harbored one or more virulence-associated genes. The highest resistance was observed to oxacillin and nalidixic acid followed by erythromycin and meropenem. Findings of the present study would be useful to evaluate the risk posed by salad vegetables and milk products, and to the concerned authorities for eventual future food regulations.