Physiological responses of Pseudomonas aeruginosa PAO1 to oxidative stress in controlled microaerobic and aerobic cultures.

Pseudomonas aeruginosa PAO1 was found to exhibit several remarkable physiological responses to oxidative stress upon its growth in a computer-controlled suspension culture. First, it strongly reduced the transfer rate of oxygen from the gas into the liquid phase, causing oxygen-limited or microaerophilic conditions in the culture after a short period of cultivation, even at high aeration rates with pure oxygen. Second, PAO1 that was previously classified as 'non-mucoid' formed a clear polysaccharide capsule on the cell surface (mucoid phenotype) under oxidative-stress conditions. Third, the strain showed a reduced growth rate and a longer lag phase under high oxygen tension. Finally, P. aeruginosa PAO1 released a high amount of proteins into the culture broth. The release of some virulence factors by PAO1, such as elastase, was significantly enhanced or only occurred under microaerobic conditions (i.e. dissolved oxygen tension value around 1% of air saturation). Hence, it is concluded that P. aeruginosa PAO1 prefers microaerobic conditions for growth and for the formation of some of its virulence factors. PAO1 can create such growth conditions by at least two mechanisms: (i) blockage of the transfer of oxygen and (ii) formation of a polysaccharide capsule on the cell surface. It is postulated that the blockage of oxygen transfer may play an important role in the defence of this pathogen against reactive oxygen intermediates.