Acetic acid (167 mM) and lactic acid (548 mM) completely inhibited growth of Saccharomyces cerevisiae both in minimal medium and in media which contained supplements, such as yeast extract, corn steep powder, or a mixture of amino acids. However, the yeast grew when the pH of the medium containing acetic acid or lactic acid was adjusted to 4.5, even though the medium still contained the undissociated form of either acid at a concentration of 102 mM. The results indicated that the buffer pair formed when the pH was adjusted to 4.5 stabilized the pH of the medium by sequestering protons and by lessening the negative impact of the pH drop on yeast growth, and it also decreased the difference between the extracellular and intracellular pH values (⌬pH), the driving force for the intracellular accumulation of acid. Increasing the undissociated acetic acid concentration at pH 4.5 to 163 mM by raising the concentration of the total acid to 267 mM did not increase inhibition. It is suggested that this may be the direct result of decreased acidification of the cytosol because of the intracellular buffering by the buffer pair formed from the acid already accumulated. At a concentration of 102 mM undissociated acetic acid, the yeast grew to higher cell density at pH 3.0 than at pH 4.5, suggesting that it is the total concentration of acetic acid (104 mM at pH 3.0 and 167 mM at pH 4.5) that determines the extent of growth inhibition, not the concentration of undissociated acid alone.The yeast Saccharomyces cerevisiae under aerobic conditions can use short-chain organic acids, such as acetic acid and lactic acid, as carbon sources. The process involves induction of certain anabolic pathways, enzymes, and specific transport mechanisms (2,4,5,6,17). If glucose is available in the growth medium, these pathways and permeases are repressed. Glucose-repressed yeast cells are unable to take up the anions of these acids (6), but the undissociated acids diffuse freely into the cells. Once inside, these acids dissociate because of the higher intracellular pH and cause acidification of the cytoplasm. Generally, eucaryotic cells maintain their intracellular pH within a narrow range despite wide variations that may occur in the extracellular pH (7). Under fermentation conditions, the intracellular pH of S. cerevisiae is usually maintained between 5.5 and 5.75 when the external pH is 3.0 (9) or between 5.9 and 6.75 when the external pH is varied between 6.0 and 10.0 (8). To maintain the intracellular pH within a physiological range optimum for metabolism, the cells pump out protons at the expense of metabolic energy (ATP). Increased diversion of energy (ATP) to pump out protons results in decreased molar growth yield with respect to glucose (Y glucose ) (19,24). It has also been reported that the Y ATP (grams of biomass produced per mole of ATP generated) decreased from 14 to 4 when the concentration of acetic acid was increased from 0 to 170 mM (19). As the gap between the extracellular pH and the intracellular pH widens, greater stress i...

We determined that paracrystailine protein surface arrays (S layers) protected gram-negative eubacteria from predation by Bdelovibrio bacteriovorus. Aquaspirillum serpens VHA and MW5 and AquaspiriUum sinuosum were resistant to predation by B. bacteriovorus 6-5-S when fully covered by their S layers. The

The life cycle, prey range and taxonomic status of a Bdellovibrio -like organism, strain JSST, were studied. Strain JSST was isolated from sewage in London, Ontario, Canada, in enrichment culture with Caulobacter crescentus prey cells. During predation, this strain remained attached to the outside of a stalked C. crescentus cell. No periplasmic growth stage was observed and no bdelloplast was formed. The stalked cells of C. crescentus retained their shape and, after predation, were devoid of cytoplasmic content, as shown by transmission electron microscopy. A periplasmic growth stage has been a definitive character in the description of members of the genera Bdellovibrio , Bacteriovorax , Bacteriolyticum and Peredibacter . This is the first description of an epibiotic predator in this group of prokaryotic predators. The G+C content of the genomic DNA of strain JSST was 46.1 mol%. 16S rRNA gene sequence analysis showed that this strain was related to Bdellovibrio bacteriovorus strains HD100T, 109J, 114 and 127 (90–93 % similarity). Phylogenetic analysis based on 16S rRNA gene sequences grouped strain JSST with the Bdellovibrio cluster, but at a distance from other Bdellovibrio isolates. On the basis of features of the life cycle and phylogenetic data, it was concluded that strain JSST merits classification as the type strain of a novel species, for which the name Bdellovibrio exovorus sp. nov. is proposed (type strain JSST  = ATCC BAA-2330T  = DSM 25223T).

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Normal-gravity (22 to 24°Plato) wheat mashes were inoculated with five industrially important strains of lactobacilli at ϳ10 5 , ϳ10 6 , ϳ10 7 , ϳ10 8 , and ϳ10 9 CFU/ml in order to study the effects of the lactobacilli on yeast growth and ethanol productivity. Lactobacillus plantarum, Lactobacillus paracasei, Lactobacillus #3, Lactobacillus rhamnosus, and Lactobacillus fermentum were used. Controls with yeast cells but no bacterial inoculation and additional treatments with bacteria alone inoculated at ϳ10 7 CFU/ml of mash were included. Decreased ethanol yields were due to the diversion of carbohydrates for bacterial growth and the production of lactic acid. As higher numbers of the bacteria were produced (depending on the strain), 1 to 1.5% (wt/vol) lactic acid resulted in the case of homofermentative organisms. L. fermentum, a heterofermentative organism, produced only 0.5% (wt/vol) lactic acid. When L. plantarum, L. rhamnosus, and L. fermentum were inoculated at ϳ10 6 CFU/ml, an approximately 2% decrease in the final ethanol concentration was observed. Smaller initial numbers (only 10 5 CFU/ml) of L. paracasei or Lactobacillus #3 were sufficient to cause more than 2% decreases in the final ethanol concentrations measured compared to the control. Such effects after an inoculation of only 10 5 CFU/ml may have been due to the higher tolerance to ethanol of the latter two bacteria, to the more rapid adaptation (shorter lag phase) of these two industrial organisms to fermentation conditions, and/or to their more rapid growth and metabolism. When up to 10 9 CFU of bacteria/ml was present in mash, approximately 3.8 to 7.6% reductions in ethanol concentration occurred depending on the strain. Production of lactic acid and a suspected competition with yeast cells for essential growth factors in the fermenting medium were the major reasons for reductions in yeast growth and final ethanol yield when lactic acid bacteria were present.

The antibiotic virginiamycin was investigated for its effects on growth and lactic acid production by seven strains of lactobacilli during the alcoholic fermentation of wheat mash by yeast. The lowest concentration of virginiamycin tested (0.5 mg Lactrol kg-1 mash), was effective against most of the lactic acid bacteria under study, but Lactobacillus plantarum was not significantly inhibited at this concentration. The use of virginiamycin prevented or reduced potential yield losses of up to 11% of the produced ethanol due to the growth and metabolism of lactobacilli. However, when the same concentration of virginiamycin was added to mash not inoculated with yeast, Lactobacillus rhamnosus and L. paracasei grew after an extensive lag of 48 h and L. plantarum grew after a similar lag even in the presence of 2 mg virginiamycin kg-1 mash. Results showed a variation in sensitivity to virginiamycin between the different strains tested and also a possible reduction in effectiveness of virginiamycin over prolonged incubation in wheat mash, especially in the absence of yeast.

Aims: The aim of this study was to evaluate interactions between Saccharomyces cerevisiae and selected strains of lactobacilli regarding cell viabilities, and production of organic acids and ethanol during fermentation. Methods and Results: Corn mashes were inoculated with yeasts and selected strains of lactobacilli, and fermented in batch or semi-continuous (cascade) mode. Ethanolic fermentation rates and viabilities of yeast were not affected by lactobacilli unless the mash was pre-cultured with lactobacilli. Then, yeast growth was inhibited and the production of ethanol was reduced by as much as 22%. Conclusions: Yeasts inhibited the multiplication of lactobacilli and this resulted in reduced production of acetic and lactic acids. The self-regulating nature of the cascade system allowed the yeast to recover, even when the lactobacilli had a head start, and reduced the size of the population of the contaminating Lactobacillus to a level which had an insigni®cant effect on fermentation rate or ethanol yield. Signi®cance and Impact of the Study: Contamination during fermentation is normally taken care of by the large yeast inoculum, although yeast growth and fermentation rates could be adversely affected by the presence of high numbers of lactobacilli in incoming mash or in transfer lines.

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