Occurrence and characteristics of methicillin resistant Staphylococcus aureus and methicillin resistant coagulase-negative staphylococci in raw milk manufacturing

Abstract

For monitoring the occurrence of MRSA (methicillin resistant Staphylococcus aureus) and MR-CNS (methicillin resistant coagulase-negative staphylococci), cow’s, goat’s, and sheep’s milks (bulk milks and individual samples) were investigated. Human nasal and throat swabs of the farm staff and nasal swabs of animals were also investigated as well. In total 1729 samples were examined and 634 strains were isolated by means of the cultivation method and used in this study. Generic identification of the staphylococci isolates was done performed by biochemical tests and all S. aureus and CNS isolates were checked by the PCR method for the presence of mecA gene which is responsible for methicillin resistance. The presence of the staphylococcal cassette chromosome mec (SCCmec), Panton-Valentine leukocidin (pvl) and genes encoding toxic shock syndrome toxin (tst) was detected in all strains confirmed as MRSA. The species were also examined for antimicrobial susceptibility by using disk diffusion method with antibiotic disks. S. aureus was the most frequently identified species from the samples tested (n = 557; 32.2%), followed by S. haemolyticus (n = 32; 1.9%), S. chromogenes (n = 24; 1.4%), S. epidermidis (n = 20; 1.2%), and S. caprae (n = 1; 0.16%). Among the resistant staphylococci (n = 49), S. aureus (n = 25; 51%) was found the most frequently, followed by S. epidermidis (n = 17; 34.7%), S. chromogenes (n = 6; 12.2%), and S. haemolyticus (n = 1; 2%). The resistant Staphyloccocus sp. occurred mainly in cow’s milk (MRSA, S. epidermidis, S. chromogenes, S. haemolyticus) and in animal’s swabs (S. epidermidis). One MRSA was also found in goat’s milk and one was isolated from human swab. No resistant strains were found in sheep’s milk. The negative results of the analysed genes presence (pvl, tst) were identical with all MRSA tested. The staphylococcal cassette chromosome mec (SCCmec) was classified as type IV or V.