Septicaemia caused by cysteine-requiring isolates of Escherichia coli

The clinical and bacteriological findings in five cases of septicaemia with cysteine-requiring isolates of Escherichia coli are reported. Infections with these nutritionally-dependent organisms have been found previously in the urinary tract only, associated usually with chronic rather than acute conditions. The urinary tract was considered to be the source of the septicaemia in our patients and that site should be investigated when such strains are isolated from blood cultures. When first isolated the organisms characteristically form small translucent colonies on media deficient in appropriate growth factors. Their nutritional requirement for cysteine can be determined by a simple auxanographic technique, thereby enabling the appropriate supplementation of media necessary for reliable identification and antibiotic-sensitivity testing.