Summary: The cultural characters of a strain of and used in the microbiological assay of riboflavin and nicotinic acid proved to be typical, excepting that neither strain fermented xylose and the strain of fermented rhamnose but not raffinose. Neither produced catalase, so that a strongly positive catalase test in these cultures indicates probable contamination by common air-borne micro-organisms.

Modifications of the medium to achieve maximum acid production were investigated by altering, omitting or adding various constituents. A modified medium was adopted allowing a slightly higher acid production. In this medium the maximum acid production was obtained in the presence of natural substances like peptone. The maximal acid production obtainable with nicotinic acid alone was slightly lower, suggesting an additional specific factor in peptone. No confirmatory evidence, however, could be obtained of its existence. Linoleic acid in a concentration of 640 μg./10 ml. medium depressed the acid production, and its action was antagonized by cholesterol. Other fatty acids and lipids had no effect.

A good reproducibility and a small coefficient of variation was found between tubes at various levels of nicotinic acid within any one assay, but between separate assays the variation was high (coefficient of variation = 12--19%).


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