Bacterial responses to ultraviolet irradiation

The UV dose-response behavior of laboratory cultures of waterborne bacteria were examined for UV doses ranging from ca. 0-100 mW .s/cm(2) using a coll imated-beam reactor. Specific physiological responses measured in these tes ts included viability (ability to reproduce) and respiration (oxygen uptake rate). The results of these exposures indicated that resistance to UV-impo sed loss of viability in E. coli cultures can be partially attributed to ag glomeration during the irradiation process. From these results, ii is conje ctured that a bacterial population may be comprised of two sub-populations: one with low resistance (discrete or paired cells) and a second with high resistance (bacterial aggregates). A small fraction of the high-resistance portion of the population appears to be essentially unaffected by UV irradi ation, thereby causing a discontinuity in the measured dose-response behavi or. Moreover, the dose-response behavior of the highly resistant fraction i s variable and difficult to describe quantitatively. The basis of these sta tements and most information in the literature is microbial viability as qu antified by the membrane filtration assay. In contrast to these findings, t he results of analyses for bacterial activity (respiration) suggest that co mparatively little change in the population can be found to result from UV irradiation. This suggests that UV radiation accomplishes inactivation of t he bacteria, but does not "kill" the bacterial cells per se, thereby highli ghting the importance of considering bacterial repair processes in the desi gn of UV disinfection systems.