Isolation and identification of Salmonella typhimurium in milk by conventional and PCR methods

Salmonella is one of the important food poisoning agents. The common method for its identification in foodstuffs is by using media cultures. This method is timeconsuming and does not have good precision. In the present study we compared PCR as an alternative, rapid and more precise method in identification of Salmonella Typhimurium serovar in milk purchased from the local supermarkets. Six pasteurized milk sample bags were purchased and divided in 2 parts; 3 were contaminated manually with different numbers of Salmonella typhimurium (10-1 –103), the other, non-contaminated, 3 being used as control. All were enriched in BPW (buffer peptone water) media and analyzed by both the PCR and common bacterial culture methods. The PCR method could identify 10, 5, and 1 Salmonella typhimurium in the contaminated samples after 8, 12, and 16 hours of enrichment with BPW, respectively. Identification by the common culture media required a longer time (4-6 days) and was less precise. All the purchased milk samples were found to be free of any contamination with Salmonella typhimurium, using either both methods. These findings show that by using the PCR method it is possible to isolate and identify Salmonella typhimurium in milk in less than 24 hours with at least one bacterium in the sample, while with the culture media methods the time required is 4-6 days. Furthermore, in spite of good agreement between results obtained by the 2 methods, the PCR method has a higher precision and sensitivity.