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Background: Little is known about the influence of the ageing process on the bacterial metabolism. The aim of this work was to study the inter-individual variation and the influence of age on metabolic characteristics of the faecal microflora from persons of three age groups., Methods: Nine enzyme activities and 11 metabolites were analysed in faecal samples of 21 elderly persons (69-89 years of age), 15 adults (30-46 years of age) and 14 children (3-15 years of age)., Results: They provide a pattern of the faecal microflora metabolism in a French population aged between 3 and 89 years. They showed large inter-individual differences in the three groups. Although the enzyme activities were not significantly different between groups, glycolytic activities appeared less variable in adults than in elderly persons and children. The major metabolites (acetate, propionate, butyrate and L-lactate) resulting from carbohydrate fermentation were similar in the three groups. Samples from elderly persons were characterized by higher D/L lactate ratio and higher concentrations of metabolites resulting from protein fermentation (ammonia, valerate, iso-butyrate and iso-valerate)., Conclusion: The data showed significant differences between elderly persons and younger adults and children, but the major metabolic characteristics of the faecal microflora were not greatly altered by the ageing process.

The effects of citric acid-modified pH (pH 2.5, 2.75, 3, 3.5, 4, 4.5, 5, and 5.5) and a 30% glucose-70% sucrose mixture (300, 400, 500, 600, 700, 800, 875, and 900 g/liter) on an osmophilic yeast, Zygosaccharomyces rouxii, were determined by using synthetic medium. One hundred experiments were carried out; 50-ml culture flasks were inoculated with 10(3) CFU ml(-1) by using a collection strain and a wild-type strain cocktail. The biomass was measured by counting cell colonies, and growth curves were fitted by using a Baranyi equation. The growth rate decreased linearly with sugar concentration, while the effect of pH was nonlinear. Indeed, the optimal pH range was found to be pH 3.5 to 5, and pH 2.5 resulted in a 30% reduction in the growth rate. Finally, we evaluated the performance of two nonlinear predictive models developed previously to describe bacterial contamination. Equations derived from the Rosso and Ratkowsky models gave similar results; however, the model that included dimensionless terms based on the Ratkowsky equation was preferred because it contained fewer estimated parameters and also because biological interpretation of the results was easier.

The growth, survival, and death of Listeria monocytogenes were studied in a synthetic medium as a function of temperature, NaCl content, and amount of liquid smoke, and the findings were validated in smoked fish products. The smoke preservative compound was simulated by adding liquid smoke, and the concentration was expressed as phenol concentration. The growth of L. monocytogenes was limited at a temperature as low as 4 degrees C or at a phenol concentration as high as 20 ppm. The predicted values were obtained using a mathematical model established in liquid medium in a previous study. They accurately fit values observed in L. monocytogenes challenge tests on smoked fish. After 21 days of storage the deviation between the predicted and experimental values was within 0.5 log for 60% of the data. This model may be useful in predicting Listeria contamination in smoked fish. Moreover, this study emphasizes the importance of phenol concentration to control the growth of Listeria spp. in smoked food products.

Predictive modelling consists in describing effects of environmental factors on microbial growth parameters. With food spoilage bacteria, this approach must be extended to both growth and food damage characterisation. In order to study the incidence of storage temperature on vegetable damage, using predictive microbiology tools, kinetics of pectic compound degradation were studied. Chryseomonas luteola has been chosen because of its ability to grow on post-harvested vegetables. Experiments were performed at refrigerated temperatures (0-10 degrees C) with low initial bacterial charges (10(1)-10(3.5) cfu/ml). Microbial specific growth rate (mu), stability phase before pectic degradation (Sp) and alteration percentage (Ap) were chosen as reference parameters. Then, sub-optimal temperature effects on these three parameters were estimated using modified Ratkowsky model. Results obtained in synthetic medium were compared with data observed in endive juice to appreciate the alteration of vegetable during post-harvest storage.

The genus Pectinatus was isolated recently and the deposited strains were classified as beer spoilage bacteria producing propionate as a major fermentation product. A recent investigation of this genus demonstrated the existence of two species: Pectinatus cerevisiiphilus, the type strain and Pectinatus frisingensis, a new species with a different pattern of growth substrates. Different culture media tested for both species demonstrated a higher specific growth rate for P. cerevisiiphilus. However, final biomass production was in every case around 20% higher in P. frisingensis. A 400% decrease of final biomass production was measured when the species were cultivated on poor culture medium; this decrease was found to be broadly proportional to amounts of acetate excreted in the medium. Both species produced CO2 from glucose; however, no significant modifications of biomass and volatile fatty acid production were demonstrated when varying head space composition with regard to CO2 levels. Growth experiments on glucose with increasing amounts of ethanol added in the culture, revealed a higher sensitivity of P. cerevisiiphilus to ethanol inhibition. Ethanol concentrations over 1.7 M resulted in a complete inhibition of growth for both Pectinatus species. Combined effects of culture medium pH, lactate, and glucose concentrations, demonstrated the prevailing role of glucose in the development of the bacteria. However, these three parameters had a different influence on growth characteristics of both Pectinatus species. P cerevisiiphilus grew very poorly with a glucose concentration of 5 mM for pH values below 4.1. This species had optimal pH for growth between 6 and 6.2 and it excreted increasing amounts of acetate with increase of pH, glucose or lactate in the culture medium. P. frisingensis showed a wide range of pH allowing a good growth. For glucose concentrations below 20 mM, highest final biomass productions were measured in the culture for pH values around 4.9; this also corresponded to a minimum in acetate excretion. The above results pointed at P. frisingensis as the prevailing species of Pectinatus in beer spoilage.

The interaction of temperature (10, 14, 25, 31, and 37 degrees C), pH (pH 5, 5.6, 6.5, 7.4, and 8), and NaCl (0, 2, 5, 8, and 10%) in a laboratory medium affects the specific growth of Staphylococcus aureus. From growth curves obtained by the turbidimetric technique, a nonlinear model in which the specific growth rate (mu) is fitted directly, without data transformation and with the residual error variations taken into account, is proposed. This model correctly fits experimental data and gives more biological information than the quadratic polynomial model. Moreover, the comparison of five strains of S. aureus was performed by a principal-component analysis in which the specific growth rate was the identifying characteristic for S. aureus strains. The results obtained from model coefficient comparison among the five strains and from multivariate data analysis allow the same classification of strains to be performed. Two of them have similar behaviors during food spoilage, two others could be distinguished by their capacity to grow at a low temperature, whereas the last one was markedly different from the others.