End products and fermentation balances for lactic streptococci grown aerobically on low concentrations of glucose

Effect of oxygen on lactose metabolism in lactic streptococci

Three strains of Streptococcus lactis, two of Streptococcus cremoris, and one of Streptococcus thermophilus metabolized oxygen in the presence of added carbohydrate primarily via a closely coupled NADH oxidase/NADH peroxidase system. No buildup of the toxic intermediate H(2)O(2) was detected with the three S. lactis strains. All six strains contained significant superoxide dismutase activity and are clearly aerotolerant. Lactose- or glucose-driven oxygen consumption was biphasic, with a rapid initial rate followed by a slower secondary rate which correlated with factors affecting the in vivo activation of lactate dehydrogenase. The rate of oxygen consumption was rapid under conditions that led to a reduction in lactate dehydrogenase activity (low intracellular fructose 1,6-bisphosphate concentration). These conditions could be achieved with nongrowing cells by adding lactose at a constant but limiting rate. When the rate of lactose fermentation was limited to 5% of its maximum, nongrowing cells of S. lactis strains ML3 and ML8 carried out an essentially homoacetic fermentation under aerobic conditions. These same cells carried out the expected homolactic fermentation when presented with excess lactose under anaerobic conditions. Homoacetic fermentation leads to the generation of more energy, by substrate-level phosphorylation via acetate kinase, than the homolactic fermentation. However, it was not observed in growing cells and was restricted to slow fermentation rates with nongrowing cells.

Growth of enterococci in unaltered, unseeded beach sands subjected to tidal wetting

Enterococci are indicator bacteria used to assess the risk of acquiring enteric disease from swimming in marine waters. Previous work identified beach sands as reservoirs of enterococci which can be transported from the sand to the sea, where they may instigate beach advisories. The present study establishes that naturally occurring enterococci can replicate in beach sands under environmentally relevant conditions. In unseeded, nonsterile microcosm experiments, it was shown that intermittent wetting of sands by seawater, like that which would occur at the high tide line, stimulates the transient replication of enterococci at rates of 0.20 to 0.63 per day (equivalent to doubling times of 1.1 to 3.5 days). Replication was not observed in control microcosms that were not subjected to wetting. Enterococci were enumerated using both culture-dependent (membrane filtration and mEI media) and culture-independent (quantitative PCR [QPCR], 23S rRNA gene based) techniques, which allowed tracking of both culturable and total enterococcus populations. Inhibition of QPCR and DNA extraction efficiencies were accounted for in the interpretation of the QPCR results. The results provide evidence that enterococci may not be an appropriate indicator of enteric disease risk at recreational beaches subject to nonpoint sources of pollution.

Metabolism of glucose and cellobiose by cellulolytic mesophilic Clostridium sp. strain H10

The metabolism of strain H10, a cellulolytic mesophilic Clostridium sp., was studied on glucose and cellobiose as energy and carbon sources. The main products of fermentation of both sugars were acetate, lactate, and ethanol. At low sugar levels, molar growth yields were better for cellobiose than for glucose. In both cases, an inhibition of growth was observed between 1 and 2 g/liter and a total inhibition after the latter concentration. Inhibition was not the result of low pH due to acid formation; growth under static pH conditions was limited in the same way. On the other hand, acetate and lactate had no inhibitory effect when added at concentrations equal to the final titers. Concomitant with the inhibition of growth was a change in metabolic pathways for sugar concentrations between 1 and 2 g/liter, i.e., the production of lactate was higher. After complete inhibition of growth, an accumulation of carbohydrates which were neither glucose nor cellobiose was observed.

Energetics of growth of Microbacterium thermosphactum at low temperatures

Microbacterium thermosphactum was grown at 5 degrees C in glucose-limited continuous cultures. The end products of glucose metabolism were L-lactate and ethanol, and these compounds accounted for 86--92% of the glucose utilized. With input glucose concentrations less than 3 mV, YgluMax was found to be 40--43, YATPMax 20--21 and ms 0.1--0.2. These values are almost identical to those found previously for cultures at 25 degrees C and show that this psychrotroph grows with a very high energetic efficiency over a wide range of temperatures. With a higher (but still limiting) input glucose concentration of 5.6 mM at 9 degrees C, cellular efficiency declined as there was a marked reduction in Yglu. This decrease was accounted for in mathematical terms by an increase in ms to 0.7, whilst YgluMax and gh energetic efficiency over a wide range of temperatures. With a higher (but still limiting) input glucose concentration of 5.6 mM at 9 degrees C, cellular efficiency declined as there was a marked reduction in Yglu. This decrease was accounted for in mathematical terms by an increase in ms to 0.7, whilst YgluMax and gh energetic efficiency over a wide range of temperatures. With a higher (but still limiting) input glucose concentration of 5.6 mM at 9 degrees C, cellular efficiency declined as there was a marked reduction in Yglu. This decrease was accounted for in mathematical terms by an increase in ms to 0.7, whilst YgluMax and YATPMax remained high at 38 and 19 respectively.

Change from homo- to heterolactic fermentation by Streptococcus lactis resulting from glucose limitation in anaerobic chemostat cultures

Lactic streptococci, classically regarded as homolactic fermenters of glucose and lactose, became heterolactic when grown with limiting carbohydrate concentrations in a chemostat. At high dilution rates (D) with excess glucose present, about 95% of the fermented sugar was converted to l-lactate. However, as D was lowered and glucose became limiting, five of the six strains tested changed to a heterolactic fermentation such that at D = 0.1 h(-1) as little as 1% of the glucose was converted to l-lactate. The products formed after this phenotypic change in fermentation pattern were formate, acetate, and ethanol. The level of lactate dehydrogenase, which is dependent upon ketohexose diphosphate for activity, decreased as fermentation became heterolactic with Streptococcus lactis ML(3). Transfer of heterolactic cells from the chemostat to buffer containing glucose resulted in the nongrowing cells converting nearly 80% of the glucose to l-lactate, indicating that fine control of enzyme activity is an important factor in the fermentation change. These nongrowing cells metabolizing glucose had elevated (ca. twofold) intracellular fructose 1,6-diphosphate concentrations ([FDP](in)) compared with those in the glucose-limited heterolactic cells in the chemostat. [FDP](in) was monitored during the change in fermentation pattern observed in the chemostat when glucose became limiting. Cells converting 95 and 1% of the glucose to l-lactate contained 25 and 10 mM [FDP](in), respectively. It is suggested that factors involved in the change to heterolactic fermentation include both [FDP](in) and the level of lactate dehydrogenase.

Aerobic growth thermograms of Streptococcus lactis obtained with a complex medium containing glucose

With different culturing methods both simple and complex thermograms were obtained with Streptococcus lactis grown aerobically in a complex medium containing growth-limiting concentrations of glucose. The thermogram profiles have been interpreted in relation to growth rate, glucose degradation, and molar growth yields calculated for different time intervals during growth.