Pseudomonas aeruginosa UV-A-induced lethal effect: influence of salts, nutritional stress and pyocyanine

Abstract The presence of NaCl in plating media srlows an important protection against the Pseudomonas aeruginosa UV-A-iduced Tethal effect, contrasting with the known sensitizing action of salts on UV-A-irradiated Escherichia coli cells. MgSO 4 exhibits a similar protection, but lower concentrations than for NaCl are needed to achieve the same effect. NaCl protection from lethal effects involves an osmotic mechanism, while MgSO 4 could act by a different process. On the other hand, when cells grown in a complete medium are then incubated for 20 min in a synthetic medium and irradiated with UV-A, a very marked protection is obtained. This protection is dependent on protein synthesis, since treatment with tetracycline, during the nutritional stress, blocks its induction. These results offer a new example of cross-protection among different stressing agents. In our experimental conditions, natural phenazines of P. aeruginosa are not present in the cells, ruling out the possibility that these pigments act as photosensitizers. Conversely, pyocyanine (the major phenazine produced by this microorganism) prevents the UV-A killing effect in a concentration-dependent way when present in the irradiation media. Finally, UV-A irradiation induces, as in E. coli , the accumulation of guanosine tetraphosphate and guanosine pentaphosphate, although the physiological meaning of this finding has yet to be determined.