Oxygen-Inducible Conversion of Lactate to Acetate in Heterofermentative Lactobacillus brevis ATCC 367

Abstract

is an obligatory heterofermentative lactic acid bacterium that produces high levels of acetate, which improve the aerobic stability of silages against deterioration caused by yeasts and molds. However, the mechanism involved in acetate accumulation has yet to be elucidated. Here, experimental evidence indicated that aerobiosis resulted in the conversion of lactate to acetate after glucose exhaustion in ATCC 367 (GenBank accession number NC_008497). To elucidate the conversion pathway, analysis showed that lactate was first converted to pyruvate by the reverse catalytic reaction of lactate dehydrogenase (LDH); subsequently, pyruvate conversion to acetate might be mediated by pyruvate dehydrogenase (PDH) or pyruvate oxidase (POX). Transcriptional analysis indicated that the and genes of ATCC 367 were upregulated 37.92- and 18.32-fold, respectively, by oxygen and glucose exhaustion, corresponding to 5.32- and 2.35-fold increases in the respective enzyme activities. Compared with the wild-type strain, the transcription and enzymatic activity of PDH remained stable in the Δ mutant, while those of POX increased significantly in the Δ mutant. More lactate but less acetate was produced in the Δ mutant than in the wild-type and Δ mutant strains, and more HO (a product of the POX pathway) was produced in the Δ mutant. We speculated that the high levels of aerobic acetate accumulation in ATCC 367 originated mainly from the reuse of lactate to produce pyruvate, which was further converted to acetate by the predominant and secondary functions of PDH and POX, respectively. PDH and POX are two possible key enzymes involved in aerobic acetate accumulation in lactic acid bacteria (LAB). It is currently thought that POX plays the major role in aerobic growth in homofermentative LAB and some heterofermentative LAB, while the impact of PDH remains unclear. In this study, we reported that both PDH and POX worked in the aerobic conversion of lactate to acetate in ATCC 367, in dominant and secondary roles, respectively. Our findings will further develop the theory of aerobic metabolism by LAB.

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