Clinical and bacteriological findings in seven cases of urinary tract infection with cysteine-requiring strains of are described. The organisms were isolated from patients with long-standing urinary tract abnormalities and grew as small (. 1 mm) colonies on MacConkey agar. The organisms failed to grow in a minimal medium supplemented with sodium sulphate but grew when the medium was supplemented with cysteine sulphinic acid, sodium sulphide or L-cysteine. The smallest amount of cysteine required for optimal growth in a chemically denned medium was 20 mg/L. Cysteine-requiring strains of had previously been shown to require a similar amount of cysteine and to be unable to reduce sulphate to sulphite; this suggests a common influence in the selection of cysteine auxotrophs However, the amino acid inhibited the growth of at concentrations which only slightly altered growth of the strains. Problems with the isolation, identification and sensitivity testing of cysteine-requiring were also observed and methods by which these may be minimised are suggested.


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