Un-ravelling the network of regulation of multidrug resistance in Salmonella enterica

Every year people are exposed to non-typhoidal salmonella bacteria, typically from contaminated foodstuffs. Some people succumb to infection and get ill. This is particularly dangerous for the very old and young, and these people often need treatment with antibiotics. Unfortunately, antibiotic resistance in salmonella is not uncommon. In this project we will perform experiments that will allow us to understand how multiple antibiotic (drug) resistance (MDR) is caused. We will identify genes and/or the proteins that control the production of the AcrAB-TolC system, which exports antibiotics from the bacterial cell and so allows it to continue to grow and cause infection. If this system is inactivated the bacterium becomes very susceptible to antibiotics, finds it difficult to colonise the host (e.g. poultry) and antibiotic resistant salmonella are hard to select. We anticipate that we will find proteins that confer MDR and that can also be used as targets for drugs that will inhibit production of AcrAB-TolC. Such new drugs would make it difficult for the salmonella to cause infection and be antibiotic resistant.

The AcrAB-TolC efflux pump in Salmonella enterica serovar Typhimurium confers innate multiple drug resistance (MDR), and is important in colonisation and persistence in poultry ? the primary reservoir. In isolates from humans and animals it confers clinically relevant MDR. Genetic inactivation of AcrAB-TolC confers antibiotic hyper-susceptibility, attenuates the organism and makes it difficult to select antibiotic resistant bacteria. In salmonella RamA, a member of the AraC-XylS family of transcriptional activators, can regulate expression of acrAB and tolC. Over-production of RamA confers MDR whereas in Escherichia coli MarA plays this role. However, in salmonella there are data to indicate that expression of acrAB and tolC is regulated, and MDR also conferred, by other factors. The purpose of this project is to advance our understanding of the mechanisms by which MDR via over-production of AcrAB-TolC is conferred. Four hypotheses will be explored (1) in salmonella RamA and MarA have distinct, but over-lapping, regulons; (2) RamA is the primary regulator of expression of the AcrAB-TolC efflux pump in those Enterobacteriaeceae that possess this gene, and MarA only plays a role in those bacteria that do not possess ramA (e.g. E. coli and Shigella spp); (3) both ramA/RamA and marA/MarA are regulated by other factors, possibly another regulator, which when mutated confers MDR; (4) expression of acrAB and tolC is regulated by a factors and/or gene/s other than local repressors, ramA or marA. A systems biology approach will be taken using microbiology, molecular genetics, cell biology and in vivo models and include new and novel techniques including ChIP and DNA sampling. This research project will make new and important scientific advances in how efflux pumps are regulated and this information will be relevant to those serovars that cause enteric fever, Typhi and Paratyphi, plus other Enterobacteriaceae.