Lactobacillus plantarum WCFS1 electron transport chains

Abstract

Lactobacillus plantarum WCFS1 requires both heme and menaquinone to induce respiration-like behavior under aerobic conditions. The addition of these compounds enhanced both biomass production, without progressive acidification, and the oxygen consumption rate. When both heme and menaquinone were present, L. plantarum WCFS1 was also able to reduce nitrate. The ability to reduce nitrate was severely inhibited by the glucose levels that are typically found in L. plantarum growth media (1 to 2% [vol/vol] glucose). In contrast, comparable mannitol levels did not inhibit the reduction of nitrate. L. plantarum reduced nitrate with concomitant formation of nitrite and ammonia. Genes that encode a bd-type cytochrome (cydABCD) and a nitrate reductase (narGHJI) were identified in the genome of L. plantarum. The narGHJI operon is part of a cluster of genes that includes the molybdopterin cofactor biosynthesis genes and narK. Besides a menaquinone source, isogenic mutants revealed that cydA and ndh1 are required for the aerobic-respiration-like response and narG for nitrate reduction. The ndh1 mutant was still able to reduce nitrate. The existence of a nonredundant branched electron transport chain in L. plantarum WCFS1 that is capable of using oxygen or nitrate as a terminal electron acceptor is proposed.

FIG. 1.

Genomic region of ndh1 and ack2.ndh1 (NADH dehydrogenase) forms part of an operon-like structure with a transcription factor (c) and an integral membrane protein (d) that neighbors the acetate kinase gene (ack2). ack2 itself is flanked by another integral membrane protein (a) and a putative acetyl transferase (b).

FIG. 2.

The genomic region upstream of the nar operon (narGHJI) in L. plantarum WCFS1. Genes coding for molybdopterin cofactor biosynthesis (moaABEDA, mobAB, and moeAB) (dark grey), for iron transport (fecBED and narGHJI) (light grey), and for nitrite extrusion (narK) and a kinase/response regulator system (rrp4 and hpk4) (black) are found in this region. Putative genes are indicated by white arrows.

FIG. 3.

Nitrate reduction and nitrite formation by L. plantarum WCFS1 in the presence of glucose. The bars indicate the average values ± the standard deviation. Wild-type cells were grown overnight in anaerobic batch cultures in modified MRS broth. Acetate and citrate were omitted from the standard MRS broth recipe. This medium was supplemented with heme, menaquinone, 16 mM nitrate, and various concentrations of glucose (glu): 0, 5, 10, 20, and 40 mM.

FIG. 4.

Nitrate reduction and nitrite formation by L. plantarum WCFS1 in the presence of mannitol. The bars indicate the average values ± standard deviations. Wild-type cells were grown overnight in anaerobic batch cultures in MRS broth from which acetate, citrate, and glucose were omitted (modified MRS broth). This medium was further supplemented with heme, menaquinone, nitrate, and various concentrations of mannitol (man) (in mM). Nitrate reduction is not repressed at high mannitol concentrations.

FIG. 5.

Proposed branched electron transport chain of L. plantarum WCFS1 that terminates in either a nitrate reductase A or a bd-type cytochrome complex. The dashed arrows represent the extracellular origin of menaquinone (vitamin K₂) and heme. Red. sub., reduced substrate; Oxid. sub., oxidized substrate; mQ, menaquinone; mQH₂, menaquinol.