Addictive plasmids drive hospital transmission of mupirocin-resistant Staphylococcus aureus

Background Mupirocin, a widely used topical agent for decolonization of Staphylococcus aureus, is increasingly compromised by resistance. Although plasmid-mediated mupirocin resistance is a recognized cause of decolonization failure, its role in facilitating hospital-wide transmission is unknown. Methods We conducted genomic surveillance of S. aureus at two interconnected urban hospitals where mupirocin decolonization is routine. Genome sequencing of >10,000 isolates was integrated with patient data to identify transmission and resistance determinants. Bacterial phenotypes and fitness were evaluated in vitro and in murine colonization models. Findings Genome sequencing identified 475 hospital transmission events; none were detected by conventional surveillance. The mupA (ileS2) resistance determinant, carried on conjugative plasmids, was enriched eightfold in methicillin-resistant S. aureus (MRSA) relative to methicillin-susceptible strains. mupA was associated with nearly a threefold greater chance of hospital transmission, especially within endemic healthcare-associated MRSA lineages, and was enriched twofold in hospital-onset infections compared with admission colonizing isolates. Multiple independently evolved inactivating mutations in the essential chromosomal gene ileS1 co-occurred with mupA, creating plasmid addiction in which mupA became indispensable for bacterial survival. Addiction arose most frequently within the dominant community-acquired MRSA lineage, where plasmid carriage reduced colonization fitness in mice. Plasmid-containing strains exhibited stringent-response activation, explaining the fitness costs and collateral tolerance to disinfectants, such as ethanol and peroxide. Although addiction reduced S. aureus fitness, it increased plasmid transfer, and addicted variants spread across hosts, demonstrating adaptation that mitigates these costs. Unexpectedly, we identified a mupirocin-dependent vulnerability to isoleucine limitation, revealing a potential strategy to target mupA-mediated resistance. Interpretation Plasmids promote hospital transmission of mupirocin-resistant S. aureus and create an evolutionary trap in which antibiotic use selects for bacterial dependence on otherwise costly resistance elements. This dependence revealed a collateral bacterial vulnerability that could be exploited to target resistant strains and preserve the effectiveness of mupirocin.