Heat Inactivation of Methicillin-Resistant Staphylococcus aureus Strains from German Dairy farms in Colostrum and Raw Milk

SIMPLE SUMMARY: Methicillin-resistant Staphylococcus aureus (MRSA) is a mastitis pathogen in dairy cows. It is common on dairy farms to feed calves milk that cannot be marketed for human consumption for various reasons. Thus, one possible route of MRSA transmission into young stock is via the feeding of contaminated colostrum or raw milk. The aim of our study was to evaluate whether heat treatment of colostrum or raw milk prior to feeding will eliminate MRSA from the colostrum/raw milk and therefore reduce the risk of introducing MRSA into the calf population during the feeding process. We demonstrate that heating colostrum or raw milk at 60 °C causes a substantial reduction in MRSA in these two matrices. However, depending on the MRSA concentration, it may not be sufficient to eliminate all viable MRSA and heat-resistant MRSA can multiply again. Thus, heated colostrum and raw milk should be fed to the calves shortly after the treatment to avoid re-growth of viable MRSA. ABSTRACT: Methicillin-resistant Staphylococcus aureus (MRSA) may cause difficult-to-treat infections in dairy cattle. One possible route of MRSA transmission into calves is via the feeding of contaminated waste milk. We tested the heat resistance of 17 MRSA strains isolated from German dairy farms in colostrum and raw milk in a laboratory approach. Heating colostrum or raw milk at 60 °C for 30 min eliminated all viable MRSA in the milk, provided the MRSA inoculation rate is low (10(3) cfu mL(−1)). In contrast, raw milk highly inoculated with MRSA (10(6) cfu mL(−1)) required a holding time of at least 30 min at 70 °C to fully eliminate MRSA from it. However, quantitative analysis showed that a heat treatment for 10 min at 60 °C already significantly reduced the number of viable MRSA in highly inoculated raw milk. Heating colostrum and raw milk above 60 °C may destroy immunoglobulins which are crucial for the calf’s health. Therefore, we suggest that colostrum and raw milk that is to be fed to calves on MRSA-positive dairy farms is heated at 60 °C for at least 10 min to reduce the likelihood of transmitting MRSA. In addition, the 60 °C heat-treated colostrum/raw milk should be fed to the calves as soon as possible to avoid re-growth of viable MRSA.