Three bacterial strains-Escherichia coli, Acinetobacter calcoaceticus, and the A. calcoaceticus RecA(-) mutant-underwent photosensitization by a low-concentration (0.73 mu mol/L) tetramethyl pyridyl porphine (a cationic hydrophylic photosensitizer) and a 4-J/cm(2) dose of 407 to 420 nm blue light. The viability of the first two strains decreased by approximately 60%. and that of the RecA(-) strain decreased by 90%. Increasing the amount of photosensitizer to 14.6 mu mol/L at the same dose of blue light resulted in a 95% to 98% decrease in viability of the three strains. Very little damage to the bacterial DNA was observed after this treatment. Increasing the concentration photosensitizer under the same illumination conditions also resulted in very little damage to the DNA. Western blotting demonstrated that the low photosensitization procedures enhance RecA production for mending the damaged chromosomal DNA. RecA production as a result of low-dose photosensitization was confirmed and demonstrated by immunofluorescent staining and gold immunolabeling. Although DNA is not the primary target for photosensitization, this process of RecA production may provide a certain degree of DNA mending and may also affect the survival of bacterial cells on low-intensity photosensitization.