Antibiotic-residues in milk

Abstract

The presence of antibiotic residues in foods represents a potential health hazard to man which at the present time is difficult to assess. Nevertheless, some problems have already been defined and legislature has been enacted to prevent or reduce the occurrence of antibiotic substances in food intended for human consumption. In addition, antibiotics in milk intended for the production of cheese or for the production of milk products requiring the use of bacterial (or yeast) cultures may result in the killing of these cultures with subsequent substantial losses to the dairy industry. Only limited information on the incidence of antibiotic residues in milk in Kenya is available. A survr.; was therefore carried out on pooled milk samples obtained from various sources associated with the Kenya Cooperative Creameries. The agar diffusion method using Micrococcus luteus as the test organism was used for screening milk samples for inhibitory action on growth. Whenever inhibition of growth was observed, the milk sample was heated at 820C for 5 minutes to inactivate heat-labile inhibitory substances of a non-antibiotic nature occasionally found in milk. Furthermore, attempts were made to identify the antibiotic present by using penicillinase. A total of 1,725 samples of raw milk were examined for the presence of heat-stable inhibitory substances to M. luteus. 89 samples (5.1%) were inhibitory, and 29 of these were shown to contain penicillin, i.e. 1.7% of the total number of samples, or 33% of all inhibitory samples. The inhibitory substances in 67% of positive tests could not be identified. Quantitation of the penicillin concentration revealed a range from 0.02 to 0.03 iu per rnl., milk. Minimum inhibitory concentrations of penicillin and oxytetracycline on streptococcus lactis and Lactobacillus bulgaricus were deteDTIined. The results were as follows: strept.lactis 0.26 unit/ml (Penicillin) 0.60 ~g/ml (Oxytetracyline) 0.39 unit/ml (Penicillin) 0.70 ~g/ml (Oxytetracycline) The above results ~how that low concentrations of anti- Lact. bulgaricus biotics in milk can inhibit dairy "starter" cultures and cause economic losses to cheese and fermented milk industries. Taking into account that milk from treated cows when added to the central milk supply is diluted, the amounts of antibiotic residues detected in the milk samples of the present investigation, however, were not likely to result in inhibition of starter cultures since they were far below the values demonstrated to have such effects. Excretion of penicillin in milk of treated cows was also measured. Two routes of administration were used: the intramuscular and intramammary .. A total of 12 milking cows were used (i.e. 6 cows per group) and the withholding periods for penicillin turned out as follows: Route of administration Withholding period 1. Intramuscular 2 days 2. Intrami3rrmary 4 days (infused quarters) 1 day (non-infused quarters) The results of this study emphasize the importance of preventing antibiotics from entering milk supplies by strictly adhering to the appropriate withholding periods specified for the antibiotics used.