Isolation and characterization of a slowly milk-coagulating variant of Lactobacillus helveticus deficient in purine biosynthesis

A slowly milk-coagulating variant (Fmc(-)) of Lactobacilius helveticus CRL 1062, designated S1, was isolated and characterized, Strain S1 possessed al l the known essential components required to utilize casein as a nitrogen s ource, which include functional proteinase and peptidase activities as well as functional amino acid, di- and tripeptide, and oligopeptide transport s ystems. The amino acid requirements of strain S1 were similar to those of t he parental strain. However, on a purine-free, chemically defined medium, t he growth rate of the Fmc- strain was threefold lower than that of the wild -type strain. L. helveticus S1 was found to be defective in IMP dehydrogena se activity and therefore was deficient in the ability to synthesize XMP an d GMP. This conclusion was further supported by the observation that the ad dition of guanine or xanthine to milk, a substrate poor in purine compounds , restored the Fmc(+) phenotype of L. helveticus S1.