Summary 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 transcri...