Defense systems and mobile elements in Staphylococcus haemolyticus: a genomic view of resistance dissemination.

Staphylococcus haemolyticus is a multidrug-resistant opportunistic pathogen and a major reservoir of antimicrobial resistance (AMR) genes within the Staphylococcaceae family. Its high genomic plasticity, frequent association with mobile genetic elements (MGEs), and prevalence in clinical settings underscore its relevance as both a threat and a conduit for resistance dissemination. In this study, we performed a comprehensive pan-genomic analysis of the S. haemolyticus defensome - including restriction-modification (RM), abortive infection (Abi), and CRISPR-Cas systems - across 692 high-quality genomes. Our results reveal a highly diverse and modular repertoire of immune systems, often organized in physical clusters and frequently associated with MGEs. We identified evidence of antagonistic interactions, with both defense and anti-defense elements encoded on plasmids and prophages. CRISPR spacer analysis showed a predominant targeting of phages, and genomes encoding CRISPR-Cas systems exhibited a lower abundance of MGEs and AMR genes, suggesting a trade-off between defense and gene acquisition. RNA-seq data from one reference strain indicate that only a fraction of the defensome is actively transcribed under standard conditions, hinting at environment-responsive regulation. Together, these findings provide new insights into the genomic strategies sustaining the persistence and adaptability of S. haemolyticus in clinical environments. The interplay between its immune systems and mobilome likely contributes not only to its evolutionary trajectory, but also to its role in the horizontal transfer of resistance determinants among pathogenic staphylococci. A deeper understanding of this immune-mobilome interface may help inform future strategies to limit the spread of resistance.