50. Studies in the Metabolism of the Lactic Acid Bacteria. I. Nitrogen Metabolism. J. G. Davis and A. T. R. Mattick

Abstract

1. The true lactic acid bacteria cannot grow in a medium in which the sole source of nitrogen is an amino acid mixture or an ammonium salt. 2. The presence of growth factors (yeast extract, marmite and peptones) does not permit the utilisation of this “simple” nitrogen. 3. The streptococci grow fastest with those peptones containing the greatest amount of the higher fractions, Bactoproteose being superior to the other peptones tested; the lactobacilli, however, show a considerably greater fermentation rate with those peptones containing most amino nitrogen, Fairchild's being the best. Similarly, the streptococci thrive best upon a peptic casein digest which is about 15 days old, further digestion impoverishing the growth; the lactobacilli, however, grow better the longer the digestion. The significance of these observations is discussed from the aspects of bacterial metabolism, the sequence of flora in ripening cheese, and the preparation of casein digest media.