Respiration, a strategy to avoid oxidative stress in Lactococcus lactis, is regulated by the heme status

Lactic acid bacteria (LAB) species like L. lactis are traditionally considered as obligate fermentative bacteria because even in aerobiosis they use sugar degradation for substrate-level phosphorylation, i.e. ATP production. However, recent studies revealed that this bacterium and some other LAB are capable of activating a heme-dependent cytochrome oxidase (CydAB) and thus undergo a respiration metabolism. Nevertheless, respiratory chain activation is allowed only when cells have access to heme (and additionally menaquinone for some LAB) in the environment because they cannot synthesize these compounds. Respiration increases the biomass yield and extends the long term survival of stored cells. These benefits of respiration are explained in different ways: i) Respiratory chain activity consumes oxygen, limiting the formation of toxic reactive oxygen species. ii) Respiratory chain generates a pH gradient, which potentially increases ATP production via H+-ATPase activity. iii) Respiration metabolism decreases lactic acid production, limiting acid stress. However, LAB have to cope with heme toxicity. Although heme has clear metabolic benefits the intracellular pool of free heme must be stringently controlled to prevent damage to macromolecules like DNA. In L. lactis, a potential efflux pump system, consisting of an ATPase (YgfA) and a permease (YgfB), is specifically highly induced in response to exogenous heme. Interestingly, the ygfA and ygfB genes are in an operon with ygfC, a potential regulator of the TetR family. Our studies implicate the ygfCBA operon is involved in modulating the free heme level and is regulated by YgfC.