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. 2001 Apr;67(4):1846-50.
doi: 10.1128/AEM.67.4.1846-1850.2001.

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

E M Hebert  1 , G S De Giori, R R Raya
Affiliations

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

E M Hebert et al. Appl Environ Microbiol. 2001 Apr.

Abstract

A slowly milk-coagulating variant (Fmc(-)) of Lactobacillus helveticus CRL 1062, designated S1, was isolated and characterized. Strain S1 possessed all the known essential components required to utilize casein as a nitrogen source, which include functional proteinase and peptidase activities as well as functional amino acid, di- and tripeptide, and oligopeptide transport systems. The amino acid requirements of strain S1 were similar to those of the parental strain. However, on a purine-free, chemically defined medium, the 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 dehydrogenase activity and therefore was deficient in the ability to synthesize XMP and GMP. This conclusion was further supported by the observation that the addition of guanine or xanthine to milk, a substrate poor in purine compounds, restored the Fmc(+) phenotype of L. helveticus S1.

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Figures

FIG. 1
FIG. 1
SDS-PAGE analysis of α-casein (A) and β-casein (B) hydrolysis by L. helveticus CRL 1062 (lane 2) and S1 (lane 3) after growth in CDM. Lane 1, starting substrates.
FIG. 2
FIG. 2
Growth of L. helveticus CRL 1062 (●) and S1 (○) in SCDM (A), SCDM supplemented with adenine, guanine, and inosine (B), and SCDM plus adenine (C), guanine (D), or inosine (E). Cell growth was expressed as ln x/x0, where x0 is biomass produced at initiation of the experiment and x is biomass at the indicated time. Values are averages from three independent experiments.
FIG. 3
FIG. 3
Metabolic pathways potentially involved in the biosynthesis and interconversion of purines in L. helveticus CRL 1062. Enzymes assayed in this study are identified by their gene symbols: guaB, IMP dehydrogenase; guaA, GMP synthetase. AICAR, aminoimidazolecarboxalamide ribonucleotide; APRT, adenine phosphoribosyltransferase; GPRT, guanine phosphoribosyltransferase; HPRT, hypoxanthine phosphoribosyltransferase. Data were adapted from reference .
FIG. 4
FIG. 4
Rates of acidification during cultivation of L. helveticus S1 in RSM without nucleotides (○), L. helveticus S1 in RSM supplemented with inosine (▾), adenine (□), xanthine (⋄), or guanine (▪), and L. helveticus CRL 1062 in RSM (●).
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