Biomedical subjects
Publications and source records attributed to R Ratomahenina.
Bacteria and mould were isolated from microflora of Cantal cheese rind. They were identified and the influence of pH, temperature and water activity on growth rate were studied. An equation for growth rate mu(h-1) relative to temperature is described according to a mathematical model.
A mould, Scopulariopsis brevicaulis was isolated from a Cantal cheese. This mould was responsible for the "violine" phenomenon. It's optimum pH growth was about 7 and the mould was able to grow suitably at 10 degrees C, the temperature of cellar for cheese ripening. It was inhibited by high sodium chloride contents or low water activities.
The effects of various physicochemical parameters on the growth of three yeast strains were investigated, including: pH, sodium chloride content, water activity in the medium and temperature. All strains were unaffected by pH changes. Growth was optimal at pH 5.5, while these yeasts were able to develop within the pH 2.5-8 range. Suitable growth was obtained at temperatures of 4 degrees C-44 degrees C and the optimal temperature for growth was 32 degrees C for all three strains. Modelling of this latter parameter is described. Growth of microorganisms was considerably modified by increased NaCl or decreased water activity in the medium.
The change in yeast flora on the surface of two batches of Roquefort cheese was monitored over a period of 6 months. 401 isolates were determined and their technological properties were investigated. The main species isolated were: Debaryomyces hansenii and its non sporulating form Candida famata, Kluyveromyces lactis and its non sporulating form Candida sphaerica and Candida species. The species Debaryomyces hansenii inoculated on the surface of the cheese in one of the batches just before the salting phase was abundant throughout the ripening phases but never exceeded 50% of the yeast count. About 80% of the isolates of each species were resistant to 15% (w/v) of sodium chloride. Most of the species were able to assimilate lactose and lactic acid. 50-90% of the isolates of each species were able to hydrolyze rapeseed oil and glycerol tributyrate. Ten isolates among 401 hydrolyzed gelatin. Most of them were able to assimilate cadaverine, histamine, putrescine and tyramine.
A virus-like particle (VLP) of 35 nm diameter has been isolated from the lipolytic yeast Candida curvata. The VLP contains a linear, double stranded RNA molecule of 1.55 microns in length.
Saccharomyces cerevisiae TYKF2 was obtained by cloning in S. cerevisiae the gene coding for beta-glucosidase in Kluyveromyces fragilis Y610 (ATCC 12424). The beta-glucosidases of both organisms were purified and their biochemical characteristics were determined. The two beta-glucosidases had the same enzymatic properties as those previously described in the literature. The strain S. cerevisiae TYKF2 is able to produce enhanced amounts of enzyme.
The biosynthesis of the beta-glucosidase enzyme was studied in a transformed yeast obtained by cloning in Saccharomyces cerevisiae the structural gene coding for beta-glucosidase in Kluyveromyces fragilis. The enzyme biosynthesis was found to be non-adaptative, and repressed by glucose. These features are similar to those observed in K. fragilis. beta-Glucosidase activity in the transformed yeast was much higher than in K. fragilis. We attempted to ferment cellobiose with the transformed yeast: practically no cellobiose was consumed, growth and ethanol production were negligible. Warburg experiments showed that cellobiose fermentation did not occur when the respiratory chain was not functioning.
Dekkera intermedia and Brettanomyces custersii were shown to have a respiratory pathway resistant to cyanide, antimycin A, and azide. This respiration remained sensitive to salicylhydroxamic acid (SHAM). The "cyanide-resistant" respiration was induced mainly at the end of the growth phase and could reach 50% of total respiratory capacity. The mitochondrial "petite colony" mutation had no effect on this oxidation pathway. The presence of this respiration pathway in these strains constitutes a compensation mechanism for the reducing activity of acetaldehyde dehydrogenase. This alternate pathway would thus be a fundamental element of the Custer effect, a characteristic feature of these strains.
The fermentation of cellodextrins by eight yeast species capable of fermenting cellobiose was monitored. Only two of these species, Torulopsis molischiana and T. wickerhamii, were able to ferment beta-glucosides with a degree of polymerization between one and six. These two species showed exocellular beta-glucosidase activity. Four other species were able to ferment cellotriose, and the last two species only fermented cellobiose. These latter six species produced a beta-glucosidase capable of attacking cellodextrins, but this enzyme was endocellular.
Methylotrophic bacteria, Gram-positive, with the serine pathway, were shown to have their growth inhibited by 0.5% glycine. The effects of this amino acid on individual enzyme activities were studied in wild and mutant strains of Micrococcus varians and Bacillus licheniformis. The enzymes studied were glycerate dehydrogenase (EC 1.1.1.29), isocitrate lyase (EC 4.1.3.1), serine hydroxymethyltransferase (EC 2.1.2.1) and glycine--oxaloacetate aminotransferase (EC 2.6.1.35). The last-named enzyme was found to be inhibited, the kinetic constants having been determined for two strain types.
A glycine-resistant mutant was isolated from a methylotrophic strain of Pseudomonas species possessing serine pathway. This mutant presents some improvements in regard to growth parameters, and is able to excrete a fluorescent pigment under certain culture conditions. This pigment is capable of accelerating the reduction rate of formaldehyde to formate coupled with NAD. The same cannot be said for the wild type.
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