FIG. 1.

Impact of amino acids on toxin production in C. difficile. Toxin yield (U/ml) in 48-h cultures of (A) strain VPI 10463 grown in PY or PY supplemented with cysteine, proline, or the seven amino acids glycine, isoleucine, leucine, methionine, threonine, tryptophan, and valine and (B) a clinical isolate of C. difficile grown in PY or BHI with or without cysteine or proline. Amino acid concentrations are given in Materials and Methods. The mean and standard error of three experiments are shown. The asterisk indicates that the toxin yield was below the detection limit of 0.2 U/ml.