FURTHER-STUDIES OF CLINICAL ISOLATES OF CYSTEINE-REQUIRING ESCHERICHIA-COLI AND KLEBSIELLA AND POSSIBLE MECHANISMS FOR THEIR SELECTION IN-VIVO

Cysteine-dependent (cys(-)) Escherichia coli and Klebsiella spp., defe ctive in sulphate assimilation, were isolated from urine and stool sam ples of infected patients. These isolates reverted to prototrophy unde r conditions of cysteine deprivation but the revertant strains and a p rototrophic wild-type E. coli strain became auxotrophic for cysteine i n a cysteine-enriched medium. This suggested that excess cysteine acts as a repressor of the cys HIJ operon known to control aspects of cyst eine biosynthesis. A group of mostly elderly patients infected with cy s(-) 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 t hese raised levels of cysteine and related compounds may enhance the s election of cys(-) strains in vivo.