Integrative analysis of fitness and metabolic effects of a large multidrug-resistant plasmid in Salmonella across diverse serovars

Plasmids, particularly those of the IncHI2 type, are key drivers of multidrug resistance in Salmonella . However, the fitness costs imposed by multidrug-resistant (MDR) IncHI2 plasmids across different serovars and the underlying mechanisms remain poorly understood. Here, we report that carriage of the MDR IncHI2 plasmid pMDRHI2 imposes a fitness cost that varies in magnitude across Salmonella hosts. This phenotypic variation is accompanied by serovar-specific differences in both transcriptional and metabolic reprogramming. Notably, despite this heterogeneity, plasmid carriage consistently perturbed the expression of chromosomal loci involved in propanediol utilization ( pdu ), ethanolamine utilization ( eut ) and sulphur and glycerophospholipid metabolism, though in a strain-dependent manner. Integrative transcriptomic and metabolomic analyses revealed that the fitness costs are linked to distinct metabolic lesions in a serovar-specific manner. Exogenous metabolite supplementation further confirmed that these lesions – such as dysregulated propanoate metabolism or impaired redox homeostasis – underlie the observed fitness costs. Collectively, these findings demonstrate that the fitness burden imposed by MDR IncHI2 plasmid carriage is dictated by serovar-specific host factors and arises from plasmid-mediated remodelling of central metabolic pathways.