Assessment of stress resistance of industrial strains of Saccharomyces cerevisiae for designing selection media

The two main methods for obtaining microbial strains with specific characteristics for application in the industry are isolation from natural sources and using random mutagenesis. Characterization of all isolated strains is very time-consuming and expensive. In this study the tolerance of some strains of Saccharomyces cerevisiae to different stresses was measured and the association between these stresses and tolerance to osmotic pressure and production of intracellular trehalose determined, aiming at applying the results to designing selection media. The viability [percent cell survival] of different strains of Saccharomyces cerevisiae was assessed by exposure to a 3M Nacl solution, a 40% sorbitol solution, a freezing shock at -20°C, and a heat shock at 52°C. In addition, the intracellular accumulation of trehalose was determined by the antrone reagent. The associations between these factors and resistance of Saccharomyces cerevisiae strains were then determined using statistical tests. Strong correlations were observed between resistance to NaCl- and sorbitol-introduced stress and strains of Saccharomyces cerevisiae [p<0.01]. There was also a strong association between intracellular trehalose accumulation and resistance to heat shock [p<0.01]. While sugars can not select osmotolerant cells, Nacl is a very strong selector for more specific isolation of more resistant cells in a suspension. Similarly, heat shock stress is very efficient in selecting cells with a higher intracellular trehalose accumulation in a suspension

effect of organic acids and nisin at sub-minimum inhibitory concentrations on growth of a strain of bacillus cereus isolated from cheese

One of the methods for preserving foods is decontamination by using food preservatives. Food preservatives exert antagonistic effects on microorganisms through interference with the structure and function of their cell membrane, enzyme action, or genetic structure. The effects of different preservatives on the growth of Bacillus cereus isolated from cheese were determined. First the growth curve of the bacteria was studied without addition of the preservatives. Then the curve was re-drawn after adding acetic acid and nisin as a chemical and a natural preservative, respectively, at sub-minimum inhibitory concentrations [sub-MIC]. Finally growth curves were drawn in the presence of both citric acid and potassium sorbate at different concentrations, the pH changes being determined. Bacillus cereus did not grow well in acetic acid at a concentration of half-MIC, and the pH was 5 and did not change. However, it caused changes in the pH at 1/4-MIC and 1/8-MIC of acetic acid and grew. Furthermore, it could increase the pH in the presence of nisin and grow after a lag phase of 12 hours. The simultaneous use of citric acid and potassium sorbate showed a synergistic preservative effect. Bacillus cereus isolated from cheese is resistant to sub-MIC concentrations of different preservatives. Therefore, it is recommended not to use preservatives at such concentrations

[ study of effect bacteriocin producing Lactoco ccus lactis on Listeria monocytogenes and Bacillus cereus]

Dairy products often associated with problems such as short shelf life and poor hygiene control. A novel approach is to utilize bacteriocin or bacteriocin producer strains, to control undesirable micro flora as Listeria monocytogenes and Bacillus cereus in foods. Hence, we studied the effect of nisin like producing Lactococcus lactis against Listeria monocytogenes and Bacillus cereus, in order to compare the isolated strain within different countries. In this research we studied the effect of nisin like producing Lactococcus lactis, with producer spot test method. We also used supernatant from 24 h culture of Lactoccus lactis. Moreover, we studied the effect of bacteriocin on Listeria monocytogenes and Bacillus cereus growth curves. The growth of both strains was inhibited by the bacteriocin. According to our results, the bacteriocin could be used in liquid food with bacteriocin added directly or as a starter culture in fermentation. This would inhibit the growth of Listeria monocytogenes; furthermore, Bacillus cereus is used to reduce food poisoning for fermented food products