Evidence of <i>in vitro mecB</i> -mediated β-lactam antibiotic resistance transfer to <i>Staphylococcus aureus</i> from <i>Macrococcus psychrotolerans</i> sp. nov., a psychrophilic bacterium from food-producing animals and human clinical specimens

Macrococci are usually found as commensals on the skin and mucosa of animals and have been isolated from mammal-derived fermented foods; however, they can also act as opportunistic pathogens. Here, we used whole-genome sequencing, comparative genomics, extensive biotyping, MALDI-TOF mass spectrometry, and chemotaxonomy to characterize <i>Macrococcus</i> sp. strains isolated from livestock and human-related specimens. Based on the results of polyphasic taxonomy, we propose the species <i>Macrococcus psychrotolerans</i> sp. nov. (type strain NRL/St 95/376^T = CCM 8659^T = DSM 111350^T) belonging to the <i>Macrococcus caseolyticus</i> phylogenetic clade. It grows at 4°C, and the core genome of the isolates contains suspected genes contributing to low-temperature tolerance. Variable genetic elements include prophages, chromosomal islands, a composite staphylococcal cassette chromosome island, and many plasmids that affect the overall genome expansion and adaptation to specific ecological settings of the studied isolates. Large plasmids carrying the methicillin resistance gene <i>mecB</i> were identified in <i>M. psychrotolerans</i> sp. nov. strains and confirmed as self-transmissible to <i>Staphylococcus aureus in vitro</i>. In addition to plasmids with circular topology, a 150-kb-long linear plasmid with 14.1-kb-long inverted terminal repeats, harboring many IS elements and putative genes for a type IV secretion system was revealed. The described strains were isolated from human clinical material, food-producing animals, meat, and a wooden cheese board and have the potential to proliferate at refrigerator temperatures. Their presence in the food chain and human infections indicates that attention needs to be paid to this potential novel opportunistic pathogen.IMPORTANCEThe study offers insights into the phenotypic and genomic features of a novel species of the genus <i>Macrococcus</i> that occurs in livestock, food, and humans. The large number of diverse mobile genetic elements contributes to the adaptation of macrococci to various environments. The ability of the described microorganisms to grow at refrigerator temperatures, enabled by genes that are predicted to contribute to low-temperature tolerance, raises food safety concerns. Confirmed <i>in vitro</i> conjugative transfer of plasmid-borne <i>mecB</i> gene to <i>S. aureus</i> poses a significant risk of spread of broad β-lactam resistance. In addition, the intergeneric plasmid transfer to <i>S. aureus</i> is indicative of horizontal gene transfer events that may be more frequent than generally accepted. Determining a complete sequence and gene content of linear megaplasmid with exceptional topology for the <i>Staphylococcaceae</i> family suggests its possible role in shuttling adaptive traits through an exchange of genetic information.