, 2008). In Salmonella, the T3SS-1 genes invH and ABT199 sopA were highly expressed under iron-rich conditions (Bjarnason et al., 2003), and 2,2′ dipyridyl represses expression of the SPI-1 transcriptional activator hilA and subsequent protein secretion via T3SS-1 (Ellermeier & Slauch, 2008; this study). Furthermore, Fur was recently reported to activate hilA expression (Ellermeier & Slauch, 2008). To investigate whether inhibition

of Salmonella T3SS-1 is dependent on Fur-regulation of SPI-1, proteins secreted via T3SS-1 were prepared from culture supernatants of S. Typhimurium SL1344 wild-type and SL1344 Δfur strains grown in the presence of INP0403 or DMSO and analysed by SDS-PAGE. Levels of the T3SS-1-secreted protein SipC were quantified by scanning of gels stained with a fluorescent total protein stain (Fig. 5). The location of SipC is known from peptide sequencing of S. Typhimurium secreted proteins and Western blotting (data not shown). Densitometric analysis of secreted SipC in cultures of the wild-type strain indicated a mean fold reduction of 7.97±2.71 in the presence of INP0403 relative to the DMSO-treated control. The Δfur mutant exhibited a reduction in secreted SipC of 3.61±0.67-fold compared with the wild-type

in the presence of DMSO, consistent with the role of Fur in the activation of SPI-1 (Ellermeier & Slauch, 2008). In the presence of INP0403, there was a further reduction in SipC secreted by the Δfur mutant of 3.50±0.53-fold relative to DMSO-treated SL1344 Δfur. This indicates that the effect of INP0403 on secretion of SipC occurs, at least in AZD4547 clinical trial part, independently of Fur. No effect Oxymatrine of INP0403 on fur transcription was observed by transcriptome analysis. In conclusion, inhibition of T3S by a candidate salicylidene acylhydrazide anti-infective agent is associated with modulation of gene expression in a manner that may be linked to iron sequestration. We show that INP0403 is capable of restricting iron supply

to Salmonella, and that inhibition of T3SS-1 by INP0403 is reversible by exogenous iron and, at least in part, independent of the iron-response regulator Fur. These data contrast with recent observations that such molecules may impair assembly of the Shigella flexneri T3S needle complex (Veenendaal et al., 2009), and raise the possibility of inhibitor- and species-specific modes of action. Taken together with data on the iron-sensitive activity of salicylidene acylhydrazides against Chlamydia (Slepenkin et al., 2007), our data reinforce the need for future studies on the mode of action of such molecules to address the potential for pleiotropic effects related to iron supply. The authors gratefully acknowledge the financial support from the Biotechnology and Biological Sciences Research Council (BBSRC), including grant D010632/1 to E.E.G. and M.P.S., and a BBSRC core strategic grant to J.C.D.H. We thank Innate Pharmaceuticals AB for providing inhibitors, and Dr Simon Andrews, University of Reading, for providing S.