Listeria monocytogenes Strains Isolated From Dry Milk Samples in Mexico: Occurrence and Antibiotic Sensitivity

Dry milk is a particular concern in Mexico, as approximately 150,000 metric tons of dry milk are imported every year at a cost of around $250 million. Dry milk is used to make many products, most of which are dairy products widely distributed among the population covered by welfare programs. The purpose of the study described in this article was to determine the presence of Listeria spp. in imported dry milk samples in Mexico, and to determine the sensitivity of the Listeria monocytogenes isolates to different antimicrobial agents. Listeria isolates (7.8% of 550 bacterial isolates) were identified as L. monocytogenes (53.49%), L. innocua (30.23%), L. seeligeri (13.95%), and L. ivanovii (2.33%). L. monocytogenes strains isolated showed multiresistance to ampicillin, erythromycin, tetracycline, dicloxacillin, and trimethoprim-sulfamethoxazole (9%-14%). The results provide additional evidence of the emergence of multiresistant Listeria strains both in nature and in widely consumed dairy products, representing a potential threat to human health.