Lactococcus lactis is a facultative anaerobe and has long been used for its capacity to ferment foods to produce fermented consumables. Interestingly L. lactis cells grown in the presence of oxygen and a heme-source actually thrive. This phenotype includes industrially important traits such as increased growth efficiency, resistance to oxidative, acid stress and improved long-term survival. Heme is a cofactor of terminal (quinol) oxidases in electron transport chains of respiring cells. Furthermore L. lactis possesses genes which, if expressed, could form a simple Electron Transport Chain (if supplied with heme). These observations suggest that aerated heme grown cells of L.lactis respire.

We proved the existence of a functional electron transport chain in L. lactis, by measuring formation of proton motive force, using a probe sensitive to changes in membrane potential. We observed formation of membrane potential in (aerobically grown heme-supplemented) wild-type cells even when the F1-F0 ATPase is inhibited, which does not occur in a cytochrome-negative mutant.

Few Lactic Acid Bacteria are known to demonstrate respiratory-like behaviour, when grown aerobically with heme. We looked at the occurrence of this behaviour among the different Lactic Acid Bacteria species and in L. lactis strains in detail.

A latest development is the discovery of a branched electron transport chain of Lactobacillus plantarum WCFS1, which may be capable of both nitrate and aerobic respiration when supplied, both with heme and vitamin K2 (a menaquinone source). This discovery indicates that the respiratory capabilities of lactic acid bacteria are highly underestimated and warrant further research.