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