Salmonella persisters promote the spread of antibiotic resistance plasmids in the gut

The emergence of antibiotic-resistant bacteria through mutations or the acquisition of genetic material such as resistance plasmids represents a major public health issue 1 , 2 . Persisters are subpopulations of bacteria that survive antibiotics by reversibly adapting their physiology 3 – 10 , and can promote the emergence of antibiotic-resistant mutants 11 . We investigated whether persisters can also promote the spread of resistance plasmids. In contrast to mutations, the transfer of resistance plasmids requires the co-occurrence of both a donor and a recipient bacterial strain. For our experiments, we chose the facultative intracellular entero-pathogen Salmonella enterica serovar Typhimurium ( S . Typhimurium) and Escherichia coli , a common member of the microbiota 12 . S . Typhimurium forms persisters that survive antibiotic therapy in several host tissues. Here we show that tissue-associated S . Typhimurium persisters represent long-lived reservoirs of plasmid donors or recipients. The formation of reservoirs of S . Typhimurium persisters requires Salmonella pathogenicity island (SPI)-1 and/or SPI-2 in gut-associated tissues, or SPI-2 at systemic sites. The re-seeding of these persister bacteria into the gut lumen enables the co-occurrence of donors with gut-resident recipients, and thereby favours plasmid transfer between various strains of Enterobacteriaceae. We observe up to 99% transconjugants within two to three days of re-seeding. Mathematical modelling shows that rare re-seeding events may suffice for a high frequency of conjugation. Vaccination reduces the formation of reservoirs of persisters after oral infection with S . Typhimurium, as well as subsequent plasmid transfer. We conclude that—even without selection for plasmid-encoded resistance genes—small reservoirs of pathogen persisters can foster the spread of promiscuous resistance plasmids in the gut. The re-seeding of antibiotic-resistant persister subpopulations of Salmonella enterica into the gut lumen favours the transfer of resistance plasmids to gut-resident enterobacteria, showing that even small reservoirs of persister bacteria facilitate the spread of antibiotic resistance.