A model of the specific growth rate inhibition by weak acids in yeasts based on energy requirements.

Zygosaccharomyces bailii, a spoilage yeast, capable of metabolic activity in food environments with low pH, low a(w) and in the presence of weak acid preservatives was chosen for a study on the effect of benzoic acid on growth parameters. In batch cultures, under controlled pH, this food preservative inhibited growth, decreasing the specific growth rate (mu) and the yield coefficient (Y(S)) on glucose. Data obtained at pH 3.5, 4.0 and 4.5 showed that this inhibition was exclusively promoted by the undissociated form of the acid since the effect was independent of pH when the concentration of the acid was expressed in this form. Moreover, the relationship between the values for mu and Y(S), provided evidence that the specific consumption rate of glucose (q(S)) was not affected by benzoic acid, indicating that the inhibition of growth should be completely explained by a decrease of Y(S). The outcome of parallel experiments performed in continuous culture was that the decrease of Y(S) was due to an increase of the maintenance coefficient (m), defined as the fraction of q(S) diverted from growth to cope with stress, represented in this case by the presence of the preservative. Based on these results a model was built, assuming that m increased hyperbolically with the concentration of benzoic acid, from zero in the absence of the acid up to q(S) when growth was completely inhibited. The concentration of the acid, for which m=q(S)/2, is a constant (K(W)), and represents a measure of the tolerance for a preservative, in this case benzoic acid. The simple equation mu/mu(0)=1+W/K(W) predicts the value of mu for a concentration (W) of the preservative, requiring the knowledge of two parameters: the specific growth rate in the absence of the preservative (mu(0)) and K(W). The equation fitted very well the data of the effect of benzoic acid on the specific growth rate of Z. bailii, having K(W)=0.96 mM benzoic acid. The model was also validated with other spoilage yeasts grown in the presence of benzoic acid in microtiter plates in an automated spectrophotometer. The values obtained for K(W) under these conditions confirm Z. bailii as the most tolerant (K(W)=2.1 mM) followed by Pichia sp. (K(W)=0.78 mM), Saccharomyces cerevisiae (K(W)=0.53 mM) and Debaryomyces hansenii (K(W)=0.11 mM).

ATP requirements for benzoic acid tolerance in Zygosaccharomyces bailii.

AIMS: To calculate the energetic requirements for benzoic acid tolerance in Zygosaccharomyces bailii in chemostat experiments. METHODS AND RESULTS: A 5.6-l stirred-tank chemostat was used. The yield of ATP (Y(ATP)) was calculated under nitrogen atmosphere, assuming equimolar ATP and ethanol production. Under these conditions Y(ATP), equal to 20 g mol(-1) of ATP, was not affected by the acid, whereas the maintenance coefficient (m(ATP)) increased from 1.0 mmol of ATP g(-1) h(-1) in the absence of the acid to 4.8 in the presence of 0.67 mmol l(-1) undissociated benzoic acid. These ATP requirements were similar to those found in Saccharomyces cerevisiae with other weak acids. CONCLUSIONS: No significant differences have been found in the energy expended to cope with the acid between sensitive and tolerant species. Therefore, the main difference between tolerant and sensitive species could rely on cellular features that would not need extra energy in terms of ATP. SIGNIFICANCE AND IMPACT OF THE STUDY: The potential mechanisms involved in the tolerance to weak acids in yeasts have been extensively studied but their actual relevance has not been assessed. Our results suggest that future efforts should concentrate on nonexpending energy features as membrane permeability and metabolic tolerance in the cytoplasm.

Terapia con rifampicina de portadores de Staphylococus aureus en tracto genital / Therapy with rifampin in patients with Staphylococcus aureus in the genital tract

Cuatrocientos diez mujeres de tres grupos socioeconómicos diferentes fueron investigadas en busca de la incidencia de colonización del tracto genital femenino por Staphylococcus aureus. El porcentaje global de portadoras fue de 27.5%. El grupo socioeconómico más alto fue al que se le encontró colonizado con más frecuencia. Cuarenta y siete portadores recibieron 600 mg diarias de rifampicina durante 10 días; en 87.5% de ellas quedó eliminado el estado de portadora