Purpose: This study determined the prevalence of Listeria spp. and L. monocytogenes in raw and processed cow milk sold in northern Ghana and assessed the resistance of isolated L. monocytogenes to selected antibiotics.
Methods: A total of 326 samples comprised of 114 raw cow milk, 56 boiled milk, 50 nunu (soured milk), 34 yoghurt, 54 woagashie (West-African soft cheese), and 18 hard/fried cheese were collected from dairy farms and market vendors for the isolation of Listeria spp. and L. monocytogenes. Isolates were characterized by biochemical tests and identified by PCR-techniques based on presence of virulence-associated genes (plcA, actA, hlyA, iap, and prfA). Listeria monocytogenes were further evaluated for resistance to ten antibiotics using the microdilution method.
Results: Listeria spp. was detected in 44 of 326 (13.5%) samples, of which 22 (6.7%) were positive for L. monocytogenes. Soft cheese had the highest prevalence (18.5%) of L. monocytogenes followed by raw milk (6.9%) and spontaneously fermented milk (4.0%). All L. monocytogenes were positive for plcA, prfA, actA, hlyA, and iap genes. In general, L. monocytogenes were susceptible to ampicillin (100%), gentamicin (100%), vancomycin (100%), streptomycin (98%), ciprofloxacin (96%) kanamycin (95), doxycycline (93%), and erythromycin (90%). However, 61% and 23% of L. monocytogenes isolates were resistant to neomycin and tetracycline, respectively.
Significance: This report on the presence of Listeria in raw and processed milk products in Ghana indicated the potential risk of listeriosis associated with the consumption of these products and, therefore, indicated the need to implement safety plans aimed at controlling Listeria in these products.