Glucose toxicity in Prevotella ruminicola: Methylglyoxal accumulation and its effect on membrane physiology

When the ruminal bacterium Prevotella ruminicola B14-M was grown in a defined medium with an excess of glucose (3.6 mM ammonia and 50 mM glucose), the cells accumulated large amounts of cellular polysaccharide and the viable cell number decreased at least 1,000-fold. This decrease in viability was correlated with an accumulation of methylglyoxal in the supernatant (3 to 4 mM). Other genetically distinct strains of P. ruminicola produced methylglyoxal, but methylglyoxal production was not ubiquitous among the strains. When P. ruminicola B14-M was grown in continuous culture (dilution rate, 0.1 h-1) with an excess of glucose, there was an oscillating pattern of growth and cell death which was correlated with the accumulation and washout of methylglyoxal from the culture vessel. Mutants which resisted an excess of glucose took up glucose at a slower rate and produced less methylglyoxal than the wild type. These mutants were, however, not stable. There was always a long lag time, and the mutants could only be maintained with a daily transfer schedule. When the mutants were transferred less frequently, methylglyoxal eventually accumulated and the cultures died. The mutants transported glucose at a threefold-slower rate than the wild type, and in each case the carrier had more than one binding site for glucose. Because glucose transport could not be driven by phosphoenolpyruvate or ATP, the glucose carrier of P. ruminicola is probably a proton symport system. When P. ruminicola B14-M cultures were treated with 4 mM methylglyoxal, the delta-psi decreased even though intracellular ATP concentrations were high. The decrease in delta-psi was associated with a decline in the intracellular potassium level and an inhibition of high-affinity glucose transport. Dicyclohexylcarbodiimide, an inhibitor of the F1F0 ATPase, had a similar effect on delta-psi, intracellular potassium level, and high-affinity glucose transport.