English abstract
Anti-microbial agents are used for the prevention and treatment of bacterialinfections in animals as well as in humans. They are also used in livestock feed as growthpromoters to increase the rate of weight gain and decrease the amount of feed needed.Emergence of microbial resistance is a global issue today. Part of this concern is due tothe inappropriate use of antibiotics in human and veterinary medicine. In addition, highmilk production is generally associated with a higher incidence of clinical and subclinical mastitis. The affected milk quarters are routinely treated with intra-mammaryinfusions using several combinations of antibiotics.
In Oman, several imported and locally produced pasteurised milk are availablein the market. Due to the potential hazardous effects of antibiotic residues in milk forhuman health, this study was conducted to detect the presence and the frequency ofoccurrence of antimicrobial substances in milk over a period of one year. In addition,mik quality was also assesed in these milk samples. Pasteurized fresh milk samples werepurchased randomly to represent both locally produced and imported milk which areconsumed in Oman. The milk samples were from Al-Marai, Al-Rawabi, Al-Safwa, Soharand SQU. Milk samples were analyzed for total heterotrophic microbial populationsincluding the standard plate count (SPC), total coliforms (TC), total fecal coliforms (FC),total Staphylococcus aureus and total Streptococcus agalactae counts. Milk sampleswere screened for antimicrobial activity using both the standard agar-well diffusionmethod against three frank and opportunistic standard strains; Escherichia coli NCTC
10418, Psedomonasaeroginosa NCTC 10662 and Staphylococcus aureus NCTC 6571and by qualitative rapid antibiotic residue test kit.
SPC was highest (P<0.05) in Al-Safwa milk samples. TC were higher (P<0.05) in both Al-Safwa and Sohar milk samples. FC was higher (P<0.05) in Al-Safwa and SQU milk samples but FC in SQU milk samples were similar (P>0.05) to other milk samples.The presence of S. aureus was highest (P<0.05) in Al-Safwa milk samples. S. agalactaewas present only in Al-Safwa and Sohar milk samples. There was no effect of season onthe frequency of the occurrence of microbial population in all milk samples. Theantimicrobial substances were present in all milk samples. Al-Rawabi milk samples hadthe least mean inhibition zones along with the lowest frequency of antimicrobial activity.The occurrence of antimicrobial substances in milk samples was absent (P<0.05) duringthe cooler months of the year. The rapid detection method revealed the presence of Betalactam group which is active against Gram-positive bacteria. These qualitative resultswere similar to the results of the presence of antimicrobial substances against thestandard test strain of S. aureus using the standard agar-well diffusion method. Whensamples with positive antimicrobial activity were subjected to temprature treatment inorder to denature milk proteins, the antimicrobial activity still persisted suggesting thatthere was no natural antimicrobial substances that are protein in nature in these milksamples. The rapid antibiotic residue test along with a positive test using the standardagar-well diffusion test confirmed the presence of antibiotic residues in milk samplesthat were sold in Oman. Based on this and the amount of microbial contamination in themilk samples tested warrant the enforcement of strict quality control procedures beforemarketing the product.
