Antibiotic Resistant Profiles of Food (Fresh Raw Milk) and Environmental (Abattoir Effluents) Isolates of Listeria monocytogenes from the Six Zones of Nigeria

The prevalence of Listeria monocytogenes in fresh raw milk and abattoir effluents in the six zones of
Nigeria was determined. Antibiotic resistant profile of the isolates was examined using the Bauer-
Kirby disc diffusion assay. A total of 626 food and environmental samples were cultured on selective
media out of which 54 (8.6%) were positive for L. monocytogenes. Chloramphenicol was the most
effective antibiotic against the isolates with the least resistance (3.70%) while nalidixic acid proved to
be least effective with resistance of 90.74%. The multiple-antibiotic resistant pattern of the isolates
showed nalidixic acid/cloxacillin (35.2%), nalidixic acid/colistin (31.5%) and cloxacillin/colistin/nalidixic
acid (29.6%) to be most prominent. The least value was observed in
chloramphenicol/nitrofurantin/cotrimoxazole with 5.6%. The modal values of the minimum inhibitory
concentrations (MICs) of the antibiotics to the isolates range between 4.0 and >16.0 μg/ml.
Chloramphenicol, nitrofurantin and gentamycin recorded the highest MIC compared with other
antibiotics. This study has demonstrated that a wide and rapidly expanding range of undesirable and,
in some cases, multi-resistant determinants is currently present in L. monocytogenes.