Endogenously induced oxidative stress effects on Azotobacter vinelandii biofilm genesis

The generation of reactive oxygen species (ROS) is an inevitable aspect of life under aerobic conditions.The equilibrium between the production and the scavenging of ROS can be disturbed leading to enhanced ROS level to cellular constituents called “oxidative stress”. Most of scientific works focused the attention on the effects of oxidative stress towards planctonic cells, but we have to keep in mind that cells exist, in both natural and engineered ecosystems, in a biofilm state enabling bacteria to develop coordinated survival strategies. As biofilm constitutes the dominant mode of microbial life in most natural and artificial ecosystems, it is important to focus on the sessile point of view. The rhizobacterium Azotobacter vinelandii is of interest for application in agriculture as biofertiliser, for bioremediation processes of cyanide-containing waste and for the industrial production of biopolymers like alginate. The present study was designed to explore for the first time the effects of endogenously-induced oxidative stress on A. vinelandii biofilm genesis and characteristics. To generate an endogenous source of ROS, an oxidant sensitive strain was used, that is a mutant in the gene coding for the rhodanese-like protein Rhda. The oxidant sensitive strain (MV474) showed enhanced ability to develop biofilm. In line with the oxidant events monitored during biofilm formation, both the activity of the hydrogen peroxide scavenger catalase and the levels of the ahpC transcript were higher in MV474 biofilm and decreased along with the biofilm development. Taken together, these data suggested a sensitive growth stage in biofilm development, corresponding to the early stage of biofilm formation. Likely, the elevated oxidative stress level observed in the most vulnerable biofilm growth step, the early stage, might provide the selective pressure to increase MV474 biofilm forming capacity.