Microbes and anti-microbial substances in pasteurized milk sold in Oman

Five brands of commercially sold milk samples were routinely evaluated for the presence of microorganisms and screened for anti-microbial substances using both the standard agar-well diffusion method and by a qualitative rapid antibiotic residue test kit for a period of 15 months. Bacillus spp. was recovered in higher frequencies in all the milk samples and milk was contaminated with coliforms, S. aureus, and S. agalactae. Anti-microbial activity were present in all brands of milk samples against the standard test strain of S. aureus using the standard agar-well diffusion method but were absent during the cooler months of the year. When milk samples with positive anti-microbial activity were subjected to heat treatment in order to denature milk proteins, the anti-microbial activity still persisted suggesting that these anti-microbial substances were not protein in nature. The rapid antibiotic residue detection method revealed the presence of antibiotics of the beta-lactam group. The presence of anti-microbial substances and widespread microbial contamination in the milk samples tested in Oman underlined the necessity to employ stricter enforcement of quality control procedures during the production of milk and milk products.