Staphylococcus aureus Plasmids: Reservoirs of Antibiotic Resistance and Virulence Genes Original paper

What was studied?

This original research article investigated the prevalence and genetic composition of plasmids in Staphylococcus aureus strains of poultry origin, with a focus on determinants of antibiotic and heavy metal resistance and virulence-associated genetic elements. The study aimed to address the underrepresentation of complete plasmid sequences in public databases by sequencing and analyzing the complete plasmid content from 18 poultry-associated S. aureus strains. Researchers classified these plasmids into three groups: (I) poultry-associated plasmids (including pAvX and pAvY variants), (II) plasmids previously reported in S. aureus from diverse hosts, and (III) entirely novel plasmids characterized for the first time in this study. Functional genetic analyses were performed to identify resistance genes, virulence factor genes, and elements relevant for plasmid maintenance and mobility, including toxin-antitoxin (TA) and restriction-modification (RM) systems.

Who was studied?

The study analyzed 18 S. aureus strains isolated from poultry and game birds (Gallus gallus, Perdix perdix, Phasianus colchicus, and Meleagris gallopavo) collected from Belgium, the USA, the UK, and Poland between 1976 and 2016. The strains represent various sequence types (STs) according to multilocus sequence typing (MLST), with most originating from poultry farms. These strains were selected to provide a broad view of plasmid diversity in avian-associated S. aureus. The plasmid sequences were obtained through next-generation sequencing, followed by functional and comparative genomic analyses.

Most important findings

The study revealed a high prevalence and diversity of plasmids in poultry-associated S. aureus. Group I plasmids (pAvX and pAvY variants) were found in 72% of strains and are associated with virulence factors but do not carry resistance determinants. Group II plasmids, found in 39% of strains, included previously characterized elements known to carry antibiotic resistance against aminoglycosides, beta-lactams, macrolides, and tetracyclines, as well as heavy metal (cadmium and arsenic) resistance genes. Notably, these plasmids are also present in human clinical isolates, highlighting cross-species transmission. Group III comprised novel plasmids (28% prevalence), such as pPA3, which exhibited a mosaic structure derived from other plasmids and carried multiple resistance determinants (erythromycin, tetracycline, cadmium) and virulence genes. Many plasmids harbored toxin-antitoxin systems (e.g., pemIK-Sa1, yefM/yoeB-Sa2, sprF3/sprG3) and mobility elements (oriT or mimics, mob genes), enhancing their maintenance and potential for horizontal transfer. The mosaicism and mobilizability of these plasmids underscore the dynamic nature of resistance and virulence gene dissemination in S. aureus, especially across human and animal reservoirs.

Key implications

This research highlights livestock-associated S. aureus, especially poultry strains, as significant reservoirs for resistance and virulence determinants that can be mobilized and transmitted to human strains. The presence of identical or highly similar plasmids in both animal and human isolates demonstrates the ease of resistance gene flow between species, facilitated by mobile genetic elements and TA systems. The identification of previously unknown plasmids and novel mobility elements (such as new oriT mimics) suggests that the true diversity and mobilization potential of S. aureus plasmids are underestimated. For clinical practice and infection control, these findings stress the importance of comprehensive plasmid surveillance in both human and animal settings, as well as the need for complete plasmid sequencing in genomic studies to better understand and curb the spread of antimicrobial resistance.