Evidence of transfer of antimicrobial resistance genes from the porcine pathogen Streptococcus suis to human clinical isolates of Streptococcus agalactiae in a major pig-producing region of Spain

The emergence of antimicrobial resistance (AMR) is a global threat, with livestock antibiotic use proposed as a significant contributor. We investigated Streptococcus suis, a multidrug-resistant porcine pathogen, as a potential source of AMR gene dissemination to human-associated streptococci in Aragón, Spain. Among 91 porcine isolates recovered across Spain, erythromycin and tetracycline resistance was linked to erm (B) and tet (O) genes, frequently co-located on Tn 5252 -family integrative and conjugative elements (ICEs) and integrative and mobilizable elements (IMEs). Tn 5252 -family ICEs shared 32–100% of their sequence and carried different AMR gene combinations. Screening of 2388 human clinical streptococcal isolates of different species obtained in Aragón revealed ~20% erythromycin resistance, particularly S. pneumoniae and S. agalactiae . Analysis of erm (B)- tet (O) linkage in a set of erythromycin-resistant isolates and subsequent genome sequencing, revealed twelve isolates carrying Tn5252-family ICEs with both AMR genes. Eight S. agalactiae isolates harbored Tn 5252 -family ICEs with >95% nucleotide identity across >91% of their sequence compared with two different ICEs identified in S. suis isolates recovered in the same region and period. Conjugation assays confirmed ICE transfer from S. suis to S. agalactiae, S. pneumoniae , and S. pyogenes , while in vitro experiments demonstrated that recombination between ICEs promotes diversification of AMR gene cargo within ICEs. Our results identify S. suis as a reservoir and generator of ICEs carrying distinct AMR gene combinations that can be horizontally transferred to other human pathogenic streptococci, leading to multidrug resistance in a single step. These findings provide strong evidence supporting interspecies AMR dissemination in regions with intensive pig production and underscore the need for integrated One Health strategies combining veterinary and clinical genomic surveillance, and targeted interventions to limit the spread of mobile genetic elements across species.