Rates of Drying and Survival of Rhizobium meliloti Strains During Storage at Different Relative Humidities

An investigation was made of the survival of six strains of Rhizobium meliloti filtered on membrane filters and held in atmospheres of controlled relative humidities (RH) of from 0 to 100% at 30 degrees C in the presence of air. The rate of water loss in the desiccator was determined by the humidity-controlling solution used. Drying was accelerated by a mild evacuation of the desiccator during the drying step. Survival rates of R. meliloti strains were much higher after slow drying to 0% RH than immediately after rapid drying. Fast drying (drying period less than 3.4 h) was shown to adversely affect the tolerance to storage at all RH values tested (no survival after 2 to 5 days of storage). When survival during storage was measurable (after slow drying), the optimum RH values for storage were 43% for strains A145 and Wu498, 22 to 43% for strains RCR2011, Wu499, and Ar16, and 83% for strain RCR2004. The most favorable drying periods were 8, 9.2, 14.2, and 50.1 h for the subsequent storage of strain RCR2011 at RH values of 0, 22, 43, and 83%, respectively. The damaging effects of rapid drying on the tolerance of strain RCR2011 to storage at different RH values could be prevented either by rehydration and subsequent slow redrying or incomplete rapid drying followed by slow drying. It is suggested that R. meliloti strains are susceptible to desiccation stresses. However, the quantitative differences among strains appear to be large enough to permit selection with regard to tolerance to desiccation and storage in dried states.

Effects of water activity, NaCl and smoke concentrations on the growth of Lactobacillus plantarum ATCC 12315

The effect of water activity (a(w)), NaCl and liquid smoke concentrations on the growth of Lactobacillus plantarum, was studied in MRS broth under anaerobic conditions. A complete 2(3) factorial design was used to determine influential factors and interactions among these factors. NaCl concentration and a(w) had a major effect on the maximum biomass obtained but no interaction influenced this response. Smoke did not affect either biomass nor acidifying capacities.

Behavior of Listeria spp. in smoked fish products affected by liquid smoke, NaCl concentration, and temperature

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.

[Bacteriological quality of spring and well water in Yaounde (Cameroon)]

The drinking water supply in Yaounde, Cameroon, often fails to meet demand and this leads many people to seek water from springs and wells the microbiological quality of which is unknown. We carried out a microbiological survey of five spring water points and ten wells. The fifteen study sites were located in various residential districts and were selected according to geographical location and number of users. Microorganisms were isolated on specific culture media using the membrane filter technique. We found that these water supplies contained many types of bacteria including Pseudomonas aeruginosa, Aeromonas hydrophila and indicators of fecal contamination. Maximum annual concentrations differed at the various sites and over time, from 750 to 98,000 CFU/l for P. aeruginosa, 40 to 78,000 CFU/l for A. hydrophila and 300 to 58,000 CFU/l for fecal indicator bacteria. The density of fecal bacteria was strongly correlated (p < 0.01) with those of P. aeruginosa in spring water and A. hydrophila in well water. The degree of correlation between microbial concentrations differed between sites. The source of fecal contamination differed between sites and was animal, human or both. The densities of bacteria recorded were high. Therefore, consumers of water drawn from these sources are at short-term risk of contracting waterborne diseases.

Modeling growth for predicting the contamination level of guava nectar by Candida pelliculosa under different conditions of pH and storage temperature

The combined effects of temperature (2-46 degrees C) and pH (1.55-6.25) on the growth of Candida pelliculosa isolated from guava nectar produced in Cameroon were studied using a turbidity method, ie measurement of optical density at 630 nm. A quadratic polynomial model was constructed to predict the effects and interactions of these two environmental conditions on the maximal optical density obtained (i2 = 0.97). The relation between optical density and population density of C. pelliculosa (CFU ml-1) was also established using an exponential regression (2 = 0.99). According to the model, maximal growth conditions were 37 degrees C and pH 6.25 for obtaining the maximal optical density of 1.25 corresponding to about 60 x 10(6) CFU ml-1. A good agreement of the model was found between the predicted values and the observed values of maximal optical density. The model was validated by the experimental values of maximal optical density obtained in the growth of C. pelliculosa in commercial guava nectar (pH 3.15).

[Hygiene and microbiologic quality of ice creams produced in Cameroon. A public health problem]

Three hundred samples of ice cream produced in two main towns in Cameroon (Douala and Yaoundé) are evaluated for their hygienic quality. The microbiologic examinations show that many of them are contaminated with bacteria of faecal origin, pathogenic Staphylococcus and Salmonella respectively in 71.3, 49.6 and 5% of the products examinations in the study. The use of non potable water and the disrespect of the hygienic rules during the production are the main causes of contamination. Recommendations are made to preserve the public health in the developing country.

Heat Resistance of Bacteria Isolated From Preparations Based on Seafood Products

The market for vacuum-packed pasteurized seafood products is developing rapidly. The stability and safety of pasteurized products rely on pasteurization processes and subsequent storage at refrigeration temperatures. The aim of this work was to study the heat resistance of characteristic bacterial flora responsible for spoilage in pasteurized seafood products and to determine the minimal pasteurizing value required for the destruction of nonsporeforming bacteria. The D_T and z-values were determined for various bacteria selected from among the most heat-resistant bacteria isolated from seafood products ( Pseudomonas paucimobilis , Pseudomonas putida , Micrococcus varians , Enterococcus faecium and Staphylococcus aureus ). Thermal resistance was measured in different media (fish fillet, fish terrine and M/15 phosphate buffer). The most heat-resistant microorganism found was Pseudomonas paucimobilis : the D₇₀ varied between 1.16 and 3.36 min, and the z-value, between 5.8 and 9.1°C, depending upon the medium. The minimal pasteurizing values required to destroy a sufficient proportion of nonsporulated bacteria were determined. In France, the pasteurizing value is designated P_T^Z. The reference temperature was 70°C (the official reference temperature in France). In fish, the minimal pasteurizing value, p₇₀⁶ = 15 min, should be applied to ensure the destruction of almost any vegetative forms of bacteria. In a fish terrine, the minimal pasteurizing value should be P₇₀⁹ = 30 min.

Compétition entre souches de Rhizobiaceae: mise en évidence de différents modes d'adhésion sur les racines de soja

Adhesion of 11 Rhizobiaceae strains on soybean roots was investigated. Whether these strains were able or not to nodulate soybean, they were found to adhere to these roots. Bacterial concentration above 1 × 1010 cells∙mL−1 was necessary to saturate root surfaces. Homologues and heterologous strains were used in mixed inoculation with Bradyrhizobium japonicum G49 StrR, used as reference strain in low number. Competition for adhesion was obtained at saturating concentrations of competitive strains. Only partial inhibition (< 55%) of the G49 StrR strain adhesion in the presence of heterologous strains was obtained, suggesting the occurrence of two modes of G49 StrR strain adhesion on the host roots: (i) a nonspecific adhesion of the B. japonicum and (ii) a specific adhesion of the B. japonicum or of strain G49 StrR. With homologous strains, inhibition of G49 StrR strain adhesion indicated a different competitive behavior between the four strains used belonging to the two DNA homology groups. Thus, the existence of species-specific recognition for all strains of B. japonicum, on soybean roots, remains to be confirmed.Key words: Rhizobiaceae, soybean, competition, adhesion, root.

[In vitro study of glucuronoconjugation of the toxin from Penicillium roqueforti (P.R.T.) and its metabolites]

Rat liver microsome UDP-glucuronyltransferase and labelled UDP-glucuronic acid were incubated either with P.R.T. or the compounds obtained by the in vitro metabolism of the toxin. Under the same conditions, labelled P.R.T. or its labelled metabolites were incubated with UDP glucuronyltransferase. Radioactive metabolites were produced with Eremofortin C and Eremofortin C alcohol and in each case, were identified as the corresponding beta-glucuronide conjugate. No measurable glucuronidation of P.R.T. or P.R.T. alcohol was observed. The results outlined in this paper show a good correlation between the biological effects and the ability of forming a glucuronide conjugate.