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 milk
products, given its role in the foodborne illness listeriosis. Twelve milk samples were collected and processed using standard
microbiological techniques. Listeria isolates were identified through biochemical characterization and reference to Bergey's
Manual of Systematic Microbiology. Antibiotic susceptibility was assessed adopting the Kirby-Bauer method, and Multiple
Antibiotic Resistance (MAR) indices were calculated. Plasmid profiling was conducted via PCR, followed by a curing process and
post-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 was
previously resistant, suggesting plasmid-mediated resistance. Conversely, an isolate without a detectable plasmid remained
resistant after curing, implying a non-plasmid-mediated resistance mechanism. Notably, both isolates (YS and CA) remained
resistant to cephalosporin after curing. All isolates demonstrated full susceptibility to gentamycin. This study confirmed the
presence of Listeria species in some of the milk products tested.