Abstract
SUMMARY: Costilow & Humphreys's (1955) observation that certain strains of Lactobacillus plantarum reduced nitrates under certain conditions was confirmed. Two strains of L. fermenti also reduced nitrates. In static culture, agar and anaerobiosis were not essential for nitrate reduction, contrary to speculations in the literature. Nitrate reduction was possible only in media with restricted carbohydrate and with the pH value maintained at a relatively high value within the activity range of nitrate reductases. For good growth, lactobacilli for the nitrate test have been customarily grown in media with high carbohydrate content, with consequent low final pH values. This seems to be the essential reason why the genus Lactobacillus had previously been defined as unexceptionally nitratase-negative.