Submitted to: Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/3/2006
Publication Date: 9/29/2006
Citation: Flythe, M.D., Russell, J.B. Fermentation acids inhibit amino acid deamination by clostridium sporogenes md1 via a mechanism involving a decline in intracellular glutamate rather than protonmotive force. Microbiology. 152(9):2619-2624. Interpretive Summary: American cattle consume over 150 million tons of fermented plant materials (silage) each year. However, silages are frequently contaminated with clostridia. Clostridia ferment amino acids, produce ammonia and increase silage pH. If the silage pH increases, molds and other toxic microorganisms proliferate. Amino acid fermenting clostridia were isolated from fresh alfalfa, fresh corn, and silages, and all of them could ferment amino acids at acidic pH values. Previous experiments indicated that the clostridia took up amino acids, produced ammonia and grew even if they did not have a membrane potential. Fermentation acids inhibited amino acid deamination by Clostridium sporogenes MD1 via a mechanism involving a decline in intracellular glutamate rather than protonmotive force. Research on silage microbiology has the potential to decrease the cost of American cattle production and protect cattle and consumers from potentially toxic bacteria.
Technical Abstract: Fermentation acids inhibited the growth and ammonia production of the amino acid fermenting bacterium, Clostridium sporogenes MD1, but only when the pH was acidic. Such inhibition was traditionally explained by the ability of fermentation acids to act as uncouplers and decrease protonmotive force (_p), but C. sporogenes MD1 grows even if the _p is very low. Cell suspensions incubated with additional sodium chloride produced ammonia as rapidly at pH 5.0 as 7.0, but cells incubated with additional sodium lactate were sensitive to even small decreases in extracellular pH. Similar results were obtained if the sodium lactate was replaced by sodium acetate or propionate. When extracellular pH declined, _pH increased even if sodium lactate was present. The cells accumulated intracellular lactate anion when the pH was acidic, and intracellular glutamate declined. Because amino acid deamination is linked to a transamination reaction involving glutamate dehydrogenase, the decrease in ammonia production could be explained by the decrease in intracellular glutamate. This latter hypothesis was consistent with the observation that extracellular glutamate addition restored amino acid deamination even though glutamate alone did not allow for the generation of ammonia.