Detection of Plasmid in Listeria monocytogenes Isolated from Fresh Raw Milk in Nigeria

Abstract

Aims: To determine the presence of plasmids in Listeria monocytogenes strains isolated from fresh raw milk from healthy cattle in Nigeria and also determine its role in antibiotics resistance since this organism is a food borne pathogen of public health importance. Methods and results: Isolation of Listeria monocytogenes was done using selective enrichment and isolation protocol, as recommended by United States Department of Agriculture (USDA) and Association of Analytical Chemists/ International Dairy Federation (AOAC/ IDF). Identification was by phenotypic characterization and confirmed using β-haemolytic activity, phosphatidyl inositol phospholipase-C (PIPLC) assay and polymerase chain reaction (PCR) . Strains identified as Listeria monocytogenes from the six zones were determined and results expressed as rate of prevalence in graph. Twenty-three strains of Listeria monocytogenes isolated from 305 samples of fresh raw milk were screened for the presence of plasmid DNA using the method of Ehrenfeld and Clewel and agarose gel electrophoresis. One isolate was found to harbour one plasmid with molecular weight of 14.62kb. The plasmid was cured using different concentrations of acridine orange. Antibiotic sensitivity test was carried out on both the wild strain (with plasmid) and cured strain using disc diffusion method of Bauer-Kirby and it was found that the wild strain was resistant to seven of the twelve antibiotics used while the cured strain was resistant to only one. Conclusions: There is a possibility of the presence of strains of Listeria monocytogenes that can harbour plasmids. This could confer antibiotic resistance to the strains and thus make antibiotic therapy unsuccessful. The increase in the number of antibiotics which the cured strain of Listeria monocytogenes was susceptible to in this study showed that plasmid may play a significant role in antibiotics resistance. The Fulani herdsmen who take fresh raw milk without boiling or pasteurization may serve as carriers of resistant strains of Listeria monocytogenes and help in its spread in the environment through faecal pollution.