Typing of mecD Islands in Genetically Diverse Methicillin-Resistant Macrococcus caseolyticus Strains from Cattle

摘要

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 (McRI(mecD)(-1) or McRI(mecD)(-2)) carrying the methicillin resistance gene mecD. To gain insight into the distribution of McRI(mecD) 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 McRI(mecD) 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. McRI(mecD)(-1) was found in different STs from Switzerland (n = 19) and England/Wales (n = 4). McRI(mecD)(-2) was only found in 7 strains from Switzerland, all of which belonged to ST6. A novel island, McRI(mecD)(-3), which contains a complete mecD operon (mecD-mecR1(m)-mecl(m), where the subscript m indicates Macrococcus) combined with the left part of McRI(mecD)(-2) and the right part of McRI(mecD)(-1), was found in heterogeneous STs from both collections (Switzerland, n = 7; England/Wales, n = 21). Two strains from England/Wales carried a truncated McRI(mecD)(-3). Phylogenetic analyses revealed no clustering of strains according to geographical origin or carriage of McRI(mecD)(-1) and McRI(mecD)(-3). Circular excisions were also detected for McRI(mecD)(-1) and McRI(mecD)(-3) by PCR. The analyses indicate that these islands are mobile and may spread by horizontal gene transfer between genetically diverse M. caseolyticus strains.