Responses Of Clostridia To Oxygen: From Detoxification To Adaptive Strategies

Clostridia comprise bacteria of environmental, biotechnological and medical interest and many commensals of the gut microbiota. Because of their strictly anaerobic lifestyle, oxygen is a major stress for Clostridia. However, recent data showed that these bacteria can cope with O-2 better than expected for obligate anaerobes through their ability to scavenge, detoxify and consume O-2. Upon O-2 exposure, Clostridia redirect their central metabolism onto pathways less O-2-sensitive and induce the expression of genes encoding enzymes involved in O-2-reduction and in the repair of oxidized damaged molecules. While Faecalibacterium prausnitzii efficiently consumes O-2 through a specific extracellular electron shuttling system requiring riboflavin, enzymes such as rubrerythrins and flavodiiron proteins with NAD(P)H-dependent O-2- and/or H2O2-reductase activities are usually encoded in other Clostridia. These two classes of enzymes play indeed a pivotal role in O-2 tolerance in Clostridioides difficile and Clostridium acetobutylicum. Two main signalling pathways triggering O-2-induced responses have been described so far in Clostridia. PerR acts as a key regulator of the O-2- and/or reactive oxygen species-defence machinery while in C. difficile, sigma(B), the sigma factor of the general stress response also plays a crucial role in O-2 tolerance by controlling the expression of genes involved in O-2 scavenging and repair systems.