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

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 degrees 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 degrees C to fully eliminate MRSA from it. However, quantitative analysis showed that a heat treatment for 10 min at 60 degrees C already significantly reduced the number of viable MRSA in highly inoculated raw milk. Heating colostrum and raw milk above 60 degrees 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 degrees C for at least 10 min to reduce the likelihood of transmitting MRSA. In addition, the 60 degrees C heat-treated colostrum/raw milk should be fed to the calves as soon as possible to avoid re-growth of viable MRSA.