Novel Lsa(F) gene confers pleuromutilin, lincosamide, streptogramin A resistance in *Streptococcus parasuis*

The escalating crisis of antimicrobial resistance (AMR) poses a severe global public health threat, driven by the emergence of multidrug-resistant pathogens. A significant mechanism of resistance involves Lsa homologs, which confer resistance to a critical class of antibiotics including pleuromutilins, lincosamides, and streptogramin A (PLSA). These antibiotics are vital for treating various bacterial infections, and the spread of resistance mechanisms, particularly those mediated by mobile genetic elements, severely limits treatment options. Understanding novel resistance genes and their dissemination pathways is crucial for developing effective countermeasures against AMR.

This study aimed to characterize a novel PLSA resistance gene, designated Lsa(F), initially identified in Streptococcus parasuis. Researchers performed sequence analysis to determine the identity of the Lsa(F) protein, comparing it to known Lsa variants. Functional expression studies were conducted by introducing Lsa(F) into Streptococcus suis and Staphylococcus aureus to confirm its ability to confer resistance to PLSA. Furthermore, genetic context analysis was performed to identify associated mobile genetic elements (MGEs), such as plasmids and integrative and conjugative elements (ICE), and to investigate potential mechanisms facilitating its movement. Transferability was assessed by attempting to transfer the Lsa(F) gene from S. parasuis to S. suis.

The novel Lsa(F) protein was found to exhibit 41.9-58.7% sequence identity with previously characterized Lsa variants, confirming its unique genetic signature. Structurally, Lsa(F) shares features typical of the F lineage within the ATP-binding cassette (ABC) protein superfamily, known for conferring ribosome protection. Functional expression experiments demonstrated that Lsa(F) successfully conferred resistance to PLSA in both Streptococcus suis and Staphylococcus aureus strains, validating its role in antibiotic resistance. Although the prevalence of Lsa(F) among S. parasuis isolates was relatively low, database analyses revealed its primary presence in Streptococcus and Lactococcus species. In Streptococcus, Lsa(F) was predominantly associated with various mobile genetic elements (MGEs), including plasmids, integrative and conjugative elements (ICE), and ICE-derived elements. In Lactococcus, it was primarily located on plasmids and at a specific chromosomal locus. Genetic context analysis identified the insertion sequence ISLll1 downstream of Lsa(F) on almost all chromosomal MGEs and plasmids, suggesting a critical mechanism for its mobility.

Notably, the Lsa(F) gene was successfully transferred from S. parasuis to Streptococcus suis, highlighting a significant risk of its spread to other pathogenic bacteria.