Marie T Thomas, Mark Shepherd, Robert K Poole, Arnoud H M van Vliet, David J Kelly, Bruce M Pearson

Campylobacter jejuni, a major food-borne intestinal pathogen, preferentially utilizes a few specific amino acids and some organic acids such as pyruvate and L- and D-lactate as carbon sources, which may be important for growth in the avian and mammalian gut. Here, we identify the enzymatic basis for C. jejuni growth on L-lactate. Despite the presence of an annotated gene for a fermentative lactate dehydrogenase (cj1167), no evidence for lactate excretion could be obtained in C. jejuni NCTC 11168, and inactivation of the cj1167 gene did not affect growth on lactate as carbon source. Instead, L-lactate utilization in C. jejuni NCTC 11168 was found to proceed via two novel NAD-independent L-LDHs; a non-flavin iron-sulfur containing three subunit membrane-associated enzyme (Cj0075c-73c), and a flavin and iron-sulfur containing membrane-associated oxidoreductase (Cj1585c). Both enzymes contribute to growth on L-lactate, as single mutants in each system grew as well as wild-type on this substrate, while a cj0075c cj1585c double mutant showed no L-lactate oxidase activity and did not utilize or grow on L-lactate; D-lactate-dependent growth was unaffected. Orthologues of Cj0075c-73c (LldEFG/LutABC) and Cj1585c (Dld-II) were recently shown to represent two novel families of L- and D-lactate oxidases; this is the first report of a bacterium where both enzymes are involved in L-lactate utilization only. The cj0075c-73c genes are located directly downstream of a putative lactate transporter gene (cj0076c, lctP), which was also shown to be specific for L-lactate. The avian and mammalian gut environment contains dense populations of obligate anaerobes that excrete lactate; our data indicate that C. jejuni is well equipped to use L- and D-lactate as both electron-donor and carbon source.