CHEESE-RIPENING STUDIES: Nitrogen Requirements of Lactic Acid Bacteria: II. THE INFLUENCE OF DEFINED NITROGEN SOURCES ON THE SUGAR-FERMENTING ABILITIES OF LACTIC ACID BACTERIA

The influence of 36 nitrogen sources on the sugar-fermenting abilities of five cultures isolated from cheese has been studied. The five cultures are Gram positive coccus forms that fail to liquefy gelatin: some appear as chains in young milk culture and some are seen as pairs. Three sugars, glucose, mannose and lactose have been used for the work. The fermentation studies have been done after the manner of Orla-Jensen: the suitability of a nitrogen source being interpreted in terms of the total titratable acidity produced by the organisms from defined sugars, after 14 days incubation at the appropriate temperature.For each organism, peptic casein digest broth is a very suitable source of nitrogen when the standard method of preparation is followed, and the total nitrogen content of the broth is approximately 1%. If the broth be diluted to contain 0.5% total nitrogen, the total titratable acidity obtainable is commonly less by one-third. Containing approximately 1% total nitrogen or 0.5% total nitrogen, tryptic casein digest broth is unsatisfactory as a nitrogen source for cultures EMB1 173 and 195; but is very suitable for cultures EMB2 166, 168 and 173,—providing the total nitrogen content of the broth is 0.5% rather than 1% total nitrogen..In the broth prepared from two commercial peptones, the nitrogen distribution is of the same order, but in each case dissimilar from the nitrogen distribution in peptic casein digest or in tryptic casein digest: one peptone broth—1% total nitrogen content—is a very suitable source of nitrogen for all the organisms; but, apart from the fermentation of mannose by two strains, the other peptone broth is no more satisfactory than is a peptic casein digest containing 0.5% nitrogen. In a commercial hydrolyzed casein broth, the nitrogen distribution is something of a composite picture of the distribution in peptic casein digest broth and tryptic casein digest broth. This source is less suitable for culture EMB1 173 than is peptic casein digest broth, equally suitable with peptic casein digest for culture EMB1 195 and, for cultures EMB2 166, 168 and 173, is the best nitrogen source investigated. The hydrolyzed casein broth containing 1% total nitrogen is much more suitable for each culture than is the same broth diluted to contain 0.5% total nitrogen.Differentiation as between cultures EMB1 173 and 195, on the one hand, and cultures EMB2 166, 168 and 173, on the other hand, may be obtained by employing certain of the nitrogen sources investigated.It has been shown that, when the nitrogen sources fractionated are employed as the substrate for fermentation studies, the suitability of a source is not necessarily indicated by the nitrogen distribution picture: even so, it is to be seen that, on the whole, the biological significance of the nitrogen distribution in a source is reflected in the influence on the sugar-fermenting abilities of the lactic acid bacteria reported upon..The results of the fermentation study show clearly that if the "kind" of nitrogen made available is suitable, the "amount" of nitrogen supplied is then equally important.