Background
The interconnectedness of human, animal, and environmental health drives emerging threats, such as antimicrobial-resistant pathogens. The widespread use of the same antimicrobials in both human and livestock may play a role in interspecies bacterial transmission by disrupting natural microbial communities and creating an environment favouring resistant bacteria. Pigs and poultry receive high levels of antimicrobials and are reservoirs of multidrug-resistant bacteria, including Streptococcus suis, a zoonotic pig pathogen. S. suis detection in non-porcine hosts, particularly poultry, raises a critical question: is this due to transient spillover or does it represent sustained host jumps and adaptation?
Results
Analysing over 3000 S. suis genomes from a diverse range of hosts—including pigs, wild boar, humans, cats, dogs, cattle, fish, otter, and birds—we identify a multidrug-resistant lineage, distinct from the lineage responsible for most zoonoses, that has undergone multiple host jump events into birds. Unlike transmission to humans, which is exclusively derived through contacts with pigs, we find evidence of S. suis adaptation to birds. This includes phylogenetic persistence, independent acquisition of bird-specific mobile genomic islands, enhanced survival in chicken versus pig blood, and subsequent transmission from poultry to wild birds.
Conclusions
While chickens may not be a source of zoonotic S. suis infections, shared antibiotic usage in pigs and poultry may have promoted host jumps of multidrug-resistant S. suis, leading to onward transmission to wild bird populations. Our results suggest that antibiotic use in livestock production may promote transmission of antimicrobial-resistant bacteria to other hosts, thereby expanding the ecological range of bacterial pathogens.
