Indole is produced by tryptophanase during growth of enteric bacteria and accumulates in the culture medium. The physiological role of indole production is poorly understood. We discovered that enterohaemorrhagic Escherichia coli (EHEC) O157 : H7 with a tnaA deletion has decreased secretion of EspA and EspB via the type III secretion system and as a result there is reduced formation of attaching and effacing (A/E) lesions in HeLa cells. Addition of indole restored and enhanced secretion of EspA and EspB and formation of A/E lesions by the tnaA deletion mutant EHEC. Indole addition moderately increased the promoter activity of LEE4 genes, including espA and espB, in the locus of enterocyte effacement. Thus in EHEC indole can serve to signal EspA and EspB expression and secretion and stimulate the ability of EHEC to form A/E lesions on human cells.

The acrS regulatory gene is located upstream of the acrEF multidrug efflux system genes. However, the roles of AcrS in regulation of drug efflux pumps have not been clearly understood. Here we show that AcrS represses other multidrug efflux genes, acrAB, which encode a major efflux system in Escherichia coli.One of the important mechanisms underlying resistance to antibiotics involves extrusion of the compounds by drug efflux pumps. Drug efflux pumps are found in a variety of bacterial species, and their expression is often controlled by cognate regulatory proteins (14,19). In Escherichia coli, a major drug efflux system, AcrAB, has a broad substrate range and confers intrinsic drug resistance (12)(13)(14). The acrR gene is located upstream of acrA, and the AcrR protein represses expression of the acrAB operon. Deletion or inactivation of acrR results in enhanced expression of acrAB and increases fluoroquinolone resistance in clinical E. coli strains (6, 23).AcrEF also has a broad substrate range, similar to AcrAB. In contrast to acrAB, the expression level of acrEF is very low because of global repression by a histonelike protein, H-NS (15, 16). The acrS (formerly envR) gene is located upstream of acrE and encodes a putative repressor (9,18).In order to investigate the effects of AcrS and AcrR on the drug susceptibility of E. coli cells, the acrS or acrR gene was cloned into the pTrc99A expression vector. The resulting plasmids were transformed into the W3104 wild-type strain, and then the MICs of toxic compounds for these transformants were determined as described previously (15). When AcrS was

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