Characterization of staphylococcal cassette chromosome mec in Staphylococcus haemolyticus ST3 and ST42 isolates from bovine mastitis milk: identification of novel variant in class C1 mec complex.

BACKGROUND

Methicillin-resistant Staphylococcus haemolyticus (MRSH) is an important pathogenic agent of bovine mastitis. Among the prominent clone lineages in dairy cows are MRSH sequence types ST3 and ST42. Little information is available on the complete characterization of SCCmec elements in MRSH.

OBJECTIVE

In this study, two clinical isolates of MRSH ST3 and ST42 from bovine mastitis milk were selected, and their nontypable SCCmec structures were compared.

METHODS

Two MRSH strains, MRSH-ST3 strain M62.3 and MRSH-ST42 strain M81.1, were identified from bovine mastitis milk in Thailand in 2022. Minimum inhibitory concentration was used to screen for antimicrobial resistance susceptibility. Oxford Nanopore Technologies and Illumina sequencing were performed in combination to complete the genome. Their gene organization and structure of SCCmec types were analysed and compared with the whole sequences of other strains in the same sequence types.

RESULTS

Both MRSH-ST3 strain M62.3 and MRSH-ST42 strain M81.1 possessed the class C1 mec complex but lacked the ccr gene complex. Notably, MRSH-ST42 strain M81.1 contained a novel variant of C1 mec complex, which consisted of IS431-mecA-ISSha1-paaZ-upgQ-IS431, with IS431 organized in the same orientation. Apart from class C1 mec and the heavy metal-resistant cluster, the gene composition and order of the SCCmec element varied. In ST3, variations in the SCCmec type, gene content and organization were observed.

CONCLUSIONS

The distinct evolution of the MRSH lineage was indicated by the various SCCmec elements. The insertion of ISSha1 resulted in a unique variant of class C1 mec complex that demonstrated the important role of the insertion sequence in SCCmec diversification.