SUMMARY: P60 ( P60) requires serine for growth under normal conditions. On a medium containing relatively high concentrations of glycine it can dispense with serine provided that () pyridoxal is present, and () either Leucovorin (folinic acid) is added or the atmosphere is enriched with CO Increasing concentrations of CO or Leucovorin decrease the concentration of glycine required to support growth without added serine. Pyridoxal is not necessary for optimal growth with added serine, but the required concentration of the latter is twice as great.

Higher concentrations of -aminobenzoic acid or Leucovorin are required for growth on glycine than on serine. Leucovorin replaces -aminobenzoic acid for growth on serine, and both this factor and CO for growth on glycine. With the basal medium used (containing purines and all other amino-acids) thymidine supports growth in the absence of -aminobenzoic acid only when serine is added.

Growth is not inhibited by sulphonamide when Leucovorin is present; with -aminobenzoic acid there is the usual competition. Pteroylglutamic acid is inactive with this organism.

A valid assay for serine is possible, even with high concentrations of glycine present, when precautions are taken to destroy either Leucovorin or pyridoxal in the samples, and to prevent the atmosphere becoming enriched with CO


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