A novel role for antibiotic resistance plasmids in facilitating S almonella adaptation to non‐host environments

It is believed that the main role of plasmids that encode multiple antibiotic resistance is to confer their hosts the ability to survive in the presence of antimicrobial compounds. In the pathogenic bacterium S almonella , plasmids of the incompatibility group HI 1 account for a significant proportion of antibiotic resistance phenotypes. In this work, we show that plasmid R 27 has a strong impact on the global transcriptome of S almonella T yphimurium strain SL 1344 when cells grow at low temperature and enter the stationary phase. Down‐regulated genes include pathogenicity islands, anaerobic respiration and metabolism determinants. Up‐regulated genes include factors involved in the response to nutrient starvation, antimicrobial resistance, iron metabolism and the heat shock response. Accordingly, cells harbouring R 27 are more resistant to heat shock than plasmid‐free cells. The use of a different IncHI1 plasmid, pHCM1 , provided evidence that these plasmids facilitate adaptation of S almonella to environmental conditions outside their host(s). This is consistent with the fact that conjugative transfer of IncHI1 plasmids only occurs at low temperature. A significant number of the R 27‐dependent alterations in gene expression could be correlated with expression of a plasmid‐encoded orthologue of the global modulator H ‐ NS , which is up‐regulated when cells grow at low temperature.