Novel antibiotic resistance gene in milk

Researchers at the University of Bern have identified a new antibiotic resistance gene in Macrococcus caseolyticus strains from dairy cows. This gene confers resistance to all beta-lactam antibiotics including the last generation of cephalosporins used against methicillin-resistant Staphylococcus aureus (MRSA). A transfer to S. aureus (which is possible according to the researchers) would jeopardize the use of reserve antibiotics to treat human infections caused by multidrug-resistant bacteria in hospitals.

Macrococcus caseolyticus is a harmless bacterium naturally found on the skin of dairy cows which can spread to milk during the milking process. It can also be present in dairy products made from raw milk.

The researchers investigated M. caseolyticus present in milk of dairy cows suffering from mastitis. These strains showed an unusual resistance pattern to beta-lactam antibiotics with a resistance profile resembling that of MRSA, but the known genes responsible for resistance (mecA, mecB and mecC) were missing. Using Next Generation Sequencing (NGS), the researchers rapidly found that the M. caseolyticus isolates acquired a novel antibiotic resistance island which contains a new methicillin resistance gene designated mecD. This discovery is published in Scientific Reports.

The researchers demonstrated that the novel methicillin resistance gene mecD confers resistance to all classes of β-lactams including anti-MRSA cephalosporins. It was located on a resistance island which has been acquired by M. caseolyticus. Further experimental investigations of the resistance island showed that it also has the potential for integration into the chromosome of S. aureus.

M. caseolyticus containing the novel mecD gene has been so far mainly found in cattle, but in one case it has been isolated from skin infection in a dog indicating that this bacterium has the potential to colonize different animal species.

Read article: Novel methicillin resistance gene mecD in clinical Macrococcus caseolyticus strains from bovine and canine sources by Sybille Schwendener, Kerstin Cotting and Vincent Perreten, published in Scientific Reports (2017) 7, article number: 43797, doi:10.1038/srep43797

[SOURCE: University of Bern]