Abstract
Several strains of acetic acid bacteria belonging to the genus Acetobacter, showing strong acetate oxidation, were screened and their microbiological aspects in acetate oxidation were investigated. When all available carbon and energy sources were exhausted and only acetic acid remained in the late stationary phase, the bacteria started to consume the acetic acid that had been accumulated in the culture medium for vinegar fermentation. They grew rapidly, showing the second stationary phase and a typical biphasic growth curve was observed. The cells from the first growth phase were acid tolerant, while the cells from the second growth phase turned over to become acid sensitive. However, no distinct acetate oxidation took place when oxidizable ethanol and other available carbon sources still remained in the culture medium. Moreover, no apparent acetate oxidation was observed in vinegar mash in which more than 4.5% of acetic acid was allowed to accumulate. There was a threshold in acetate concentration since the most selected strains oxidized acetate when the final concentration of acetic acid accumulated was less than 3.7%. When only acetic acid was administrated as the sole carbon and energy sources, the organisms finally used acetic acid after a long lag time. The lag time was shortened by the addition of a small amount of readily usable energy source, such as ethanol. From enzymatic analysis, only acetyl-CoA synthetase increased much among the enzymes concerning acetyl-CoA formation from acetate, while the enzyme activities of acetate kinase and phosphotransacetylase were not changed significantly. The enzyme activities or isocitrate lyase and malate synthase also increased significantly in the cells when acetate was consumed. These results indicate that acetic acid is converted to acetyl-CoA by acetyl-CoA synthetase to put acetate into the TCA cycle as well as to the glyoxylate cycle allowing the bacteria to grow rapidly on acetic acid after ethanol exhaustion. Taking together with growth experiments and enzymatic data accumulated, it was strongly suggested that cells different in physiological characteristics from the first growth phase emerged in the second growth phase.
