Phenotypic and genotypic characterization of antimicrobial resistance in coagulase-negative staphylococci from bone...

Lameness in broiler chickens, frequently caused by bacterial chondronecrosis with osteomyelitis (BCO), poses a major challenge to intensive poultry production due to its adverse impact on animal welfare and farm economics. While Staphylococcus aureus has traditionally been considered the principal staphylococcal pathogen in poultry, coagulase-negative staphylococci (CoNS) are increasingly implicated in skeletal infections. This study aimed to characterize the phenotypic and genotypic antimicrobial resistance profiles of 93 CoNS isolates representing 12 Staphylococcus species. Isolates were recovered from bone and joint lesions of lame broiler chickens from 25 commercial flocks in Poland and were characterized using disk diffusion testing, PCR-based detection of selected resistance genes, pulsed-field gel electrophoresis (PFGE), and exploratory whole-genome sequencing (WGS).

Disk diffusion testing and PCR screening revealed a high prevalence of resistance to penicillin, doxycycline, and erythromycin, with bla_Z (37.6%; 35/93), lnu(A) (37.6%; 35/93), tet(L) (25.8%; 24/93), tet(M) (22.6%; 21/93), and erm(C) (12.9%; 12/93) being the most commonly detected determinants. The mecA gene, conferring resistance to β-lactams, was detected in a subset of isolates (16.1%; 15/93) although phenotypic resistance did not always correlate with mecA presence. Femoral-head lesions (FHN/FHT/FHS; n = 50) yielded isolates with resistance profiles broadly similar to those observed in the overall collection. PFGE of S. cohnii demonstrated genetic heterogeneity within the analyzed subset. Whole-genome analysis of S. cohnii strain SC27 identified multiple AMR-associated determinants and mobile genetic elements.

These findings indicate that CoNS associated with broiler skeletal lesions can harbor diverse AMR determinants linked to several widely used antimicrobial classes. Our findings are primarily relevant from an epidemiological and surveillance perspective. The study also adds new genomic and epidemiological context to current knowledge of AMR in broiler-associated CoNS and supports their inclusion in ongoing veterinary and One Health-oriented surveillance efforts.