Effect of the presence of colostrum on microbial screening methods for antibiotic detection in goats' milk

The aim of this work was to study the response of microbial tests used for the detection of antibiotics in colostrum and in colostrum-containing goats' milk. For this purpose, colostrum and milk samples were collected from 43 Murciano-Granadina goats every 12 h during the first 7 days of lactation. All samples were analysed by the microbial inhibitor tests BRT MRL, Delvotest SP-NT MCS and Eclipse 100. The results show that the tests were not suitable for the analysis of goat colostrum because they presented a high percentage of doubtful and positive results for samples of the first 24 h post-partum. Only the Delvotest SP-NT MCS could be used successfully for samples from 36 h post-partum onwards, as it shows a very high specificity, while this was not obtained for BRT MRL and Eclipse 100 until 96 h post-partum. According to the logistic regression model, the occurrence of non-compliant results for these screening tests is mainly related to the high protein content of colostral secretions. Furthermore, to study the influence of the presence of colostrum on goats' milk, antibiotic and colostrum-free farm tank milk was used, to which different concentrations (n = 12) of colostrum obtained at different time points (12, 24, 36 and 48 h) were added. Positive results appeared in BRT MRL for milk mixed with relatively low concentrations of colostrum collected at 12-24 h post-partum (7.5-10%, respectively), while in the Delvotest SP-NT MCS and Eclipse 100 non-compliant results only occurred for milk with high concentrations (≥35%) of colostrum for both time points. Moreover, high concentrations ≥20% of colostrum from 36 to 48 h are needed to affect the BRT MRL. Low concentrations of colostrum in milk that may cause non-compliant results can be avoided if good farming practices of identification and separation of goats at the beginning of lactation are respected.