DETECTION OF LISTERIA SPECIES AND LISTERIA MONOCYTOGENES IN RAW MILK AND MILK PRODUCTS IN THE NORTHERN REGION, GHANA

Abstract

There is little information about the quality of the milk and its derived products either at farm level or at market level in Ghana. As a result it is important to investigate the quality and safety of milk and its derived products in order to improve the nutritional base of the increasing Ghanaian population and the marketing of the milk and its derived products. The purpose of this study was therefore to detect and determine the prevalence of Listeria species and L. monocytonenes in milk and milk products in the Northern Region of Ghana using different selective culture media and PCR-based techniques. Prior to the collection of samples of milk and milk products for bacteriological analysis, a questionnaire based survey was administered to 85 dairy farm owners and processors about factors that help in reducing the level of contamination of milk and its products by Listeria species. All the respondents indicated that cleaning/disinfection of cattle udder before milking is not commonly practice. The milk products commonly made by producers include Nunu, soft cheese, yoghurt and butter. Milk intended to be sold directly to consumers is boiled by the producers. All (100%) the cattle farmers feed their cattle by means of free range system. About 39.2% of dairy farmers regularly vaccinate their cattle against some of the common cattle diseases. A total of 163 samples of milk and milk products were purchased from producers, traders and vendors to examine the presence of Listeria species and L. monocytogenes by conventional isolation, colony PCR and enrichment PCR methods. The conventional method detected 34/163 (20.9%) and 38/163 (23.3%) Listeria species of the experimental samples based on growth on oxford and palcam selective media respectively. While colony PCR detected 21 Listeriacontaminated samples, Enrichment PCR detected 22 Listeria contaminated samples. The prevalence of L. monocytogenes was 6.7% (11/163) in both colony and enrichment PCR methods. Among the studied samples of milk and milk products, soft cheese had the highest prevalence (33.3%) of Listeria, followed by raw milk (17.5%) and then spontaneously fermented yoghurt (11.5%). Listeria species and L. monocytogenes were not detected in pasteurized milk on market, fried cheese and yoghurt produced with commercial starter culture. The efficiencies of oxford agar and palcam agar in this study were 91.1% and 87.7% respectively, however, there was no significant difference among the two media in the detection of Listeria species. Again, there was no significant difference between the enrichment PCR technique and the colony PCR (P >0.05). In conclusion, this study reveals that consumption of raw milk, soft cheese and spontaneously fermented milk could serve as potential risks of listeriosis in this region. The scenario warrants that milk is pasteurized and safety practices ensured during processing before consumption.