Protective effect of low UVA irradiation against the action of lethal UVA on Pseudomonas aeruginosa: role of the relA gene.

The exposure of Pseudomonas aeruginosa cells to very low UVA fluences induces a growth delay, a phenomenon proposed in Escherichia coli as an adaptive mechanism related to protection against lethal and mutagenic effects of UVA. This paper reports that the treatment with low UVA irradiation fluences protects P. aeruginosa PAO1 strain from a subsequent lethal exposure. This phenomenon depends on the relA gene, coding for the main (p)ppGpp synthetase, and is unrelated to the induction of quorum sensing or catalase activity, two essential factors involved in the response of P. aeruginosa to UVA. Cross-protection between osmotic stress and UVA is observed when a great protective response to lethal UVA is caused by the induction of resistance to osmotic stress. The increase in resistance to osmotic shock observed in the pre-irradiated PAO1 strain but not in its relA derivative, unable to show photo-protection, leads us to hypothesize that the photo-protection could be attributed to an adaptive response to osmotic stress. It is concluded that the exposure of P. aeruginosa to low UVA doses induces a relA-dependent adaptive response that protects against cell death induced by high doses and causes an increase in the resistance to osmotic stress.