Antibiotics Resistance and Plasmid Curing of Listeria Monocytogens Isolates from Milk and Milk Products

This study investigated the prevalence and antibiotic resistance profiles of Listeria monocytogenes in milk and milkproducts, given its role in the foodborne illness listeriosis. Twelve milk samples were collected and processed using standardmicrobiological techniques. Listeria isolates were identified through biochemical characterization and reference to Bergey'sManual of Systematic Microbiology. Antibiotic susceptibility was assessed adopting the Kirby-Bauer method, and MultipleAntibiotic Resistance (MAR) indices were calculated. Plasmid profiling was conducted via PCR, followed by a curing process andpost-curing sensitivity testing on isolates carrying resistance plasmids. Listeria was isolated from four of the twelve samples,specifically from nunu, yogurt, cheese, and cow milk. The isolates exhibited varying levels of resistance to the tested antibiotics.Two isolates displayed MAR indices of 0.41, indicating multidrug resistance. Plasmid profiling detected a plasmid in one isolate.Following the curing process, the isolate harboring the plasmid became susceptible to erythromycin, an antibiotic to which it waspreviously resistant, suggesting plasmid-mediated resistance. Conversely, an isolate without a detectable plasmid remainedresistant after curing, implying a non-plasmid-mediated resistance mechanism. Notably, both isolates (YS and CA) remainedresistant to cephalosporin after curing. All isolates demonstrated full susceptibility to gentamycin. This study confirmed thepresence of Listeria species in some of the milk products tested.