Typing of mecD islands in genetically diverse methicillin-resistant Macrococcus caseolyticus strains from cattle

Abstract

Macrococcus caseolyticus belongs to the normal bacterial flora of dairy cows and does not usually cause disease. However, methicillin-resistant M. caseolyticus strains were isolated from bovine mastitis milk. These bacteria had acquired a chromosomal island (McRImecD-1 or McRImecD-2) carrying the methicillin resistance gene mecD. To gain insight into the distribution of McRImecD types in M. caseolyticus from cattle, 33 mecD-containing strains from Switzerland were characterized using molecular techniques, including multilocus sequence typing, antibiotic resistance gene identification, and PCR-based McRImecD typing. In addition, the same genetic features were analyzed in 27 mecD-containing M. caseolyticus strains isolated from bovine bulk milk in England/Wales using publicly available whole-genome sequences. The 60 strains belonged to 24 different sequence types (STs), with strains belonging to ST5, ST6, ST21, and ST26 observed in both Switzerland and England/Wales. McRImecD-1 was found in different STs from Switzerland (n=19) and England/Wales (n=4). McRImecD-2 was only found in 7 strains from Switzerland, all of which belonged to ST6. A novel island, McRImecD-3, which contains a complete mecD operon (mecD-mecR1m-mecIm [where the subscript m indicates Macrococcus]) combined with the left part of McRImecD-2 and the right part of McRImecD-1, was found in heterogeneous STs from both collections (Switzerland, n=7; England/Wales, n=21). Two strains from England/Wales carried a truncated McRImecD-3. Phylogenetic analyses revealed no clustering of strains according to geographical origin or carriage of McRImecD-1 and McRImecD-3. Circular excisions were also detected for McRImecD-1 and McRImecD-3 by PCR. The analyses indicate that these islands are mobile and may spread by horizontal gene transfer between genetically diverse M. caseolyticus strains.