The Acr AB-TolC efflux system is involved in multidrug and bile salt resistances. In addition, this pump has recently been suggested to increase the invasion of Salmonella enterica serovar Typhimurium (S. Typhimurium) into host cells in vitro and could therefore have an important clinical relevance for multidrug-resistant strains. The arm of this study was to investigate the role of the TolC outer membrane channel and the AcrB transporter in the interaction of multidrug-resistant S. Typhimurium strains with eukaryotic cells, especially in relation to the expression of the type III secretion system-1 (TTSS-1) required for Salmonella invasion. Deletion of toIC led to a reduced transcription of the Salmonella pathogenicity island-1 genes sipA. invF and hilA, demonstrating that all genes required for TTSS-1 biosynthesis are down-regulated in this mutant. Consequently, toIC Mutants secreted smaller amounts of the TTSS-1 effector proteins SipA and SipC, and invasion tests performed with one mutant showed that it was significantly less able to invade HT-29 epithelial cells than its parental strain. This control seems specific to the TTSS-1 among the three TTSS of Salmonella as no down-regulation of expression of TTSS-2 or flagella was observed in this mutant. By contrast, acrB mutants behaved as their parents except that they secrete a slightly greater amount or SipA and SipC proteins. These data indicate that ToIC but not Acr-B mediates the uptake of multidrug-resistant S. Typhimurium into target host cells. Therefore, this role or ToIC in the invasion of the intestine in addition to its role in bile salt resistance reinforces the interest of targeting ToIC for fighting multidrug-resistant Salmonella. (C) 2008 Elsevier GmbH. All rights reserved.