Abstract
Cysteine-dependent (cys-) Escherichia coli and Klebsiella spp., defective in sulphate assimilation, were isolated from urine and stool samples of infected patients. These isolates reverted to prototrophy under conditions of cysteine deprivation but the revertant strains and a prototrophic wild-type E. coli strain became auxotrophic for cysteine in a cysteine-enriched medium. This suggested that excess cysteine acts as a repressor of the cys HIJ operon known to control aspects of cysteine biosynthesis. A group of mostly elderly patients infected with cys- strains suffered a disproportionate amount of renal impairment as compared with a control group. In renal impairment, sulphur compounds, including cysteine, are retained. This raises the possibility that these raised levels of cysteine and related compounds may enhance the selection of cys- strains in vivo.