UV-induced self-aggregation of E. coli after low and medium pressure ultraviolet irradiation

•Ultraviolet irradiation induces self-aggregation of E. coli.•Self-aggregation of bacteria was observed after exposure to LP-UV at 20, 40, 60, and 80 mJ/cm2.•For MP-UV, only a dose of 80 mJ/cm2 had a significant impact on the formation of aggregates.•At the same LP-UV dose, changing the light intensity changed the extent of UV-induced aggregation.•UV light intensity and wavelength appear to play a role in aggregation of bacteria.

Presence of aggregated bacteria has been shown to decrease the efficacy of ultraviolet (UV) disinfection and there is some indication that UV irradiation may promote aggregation of bacteria among themselves. This study aims to provide an in-depth understanding of the effect of UV light on inducing self-aggregation of Escherichia coli bacteria by using microscopy and particle counter analysis techniques. The bacteria were observed and quantified before and after UV irradiation by employing size and concentration parameters. Four doses of low-pressure (LP) UV irradiation, 20, 40, 60 and 80 mJ/cm2, and two doses of medium-pressure (MP) UV irradiation, 40 and 80 mJ/cm2, were tested. At all LP UV doses tested, a significant increase in particle size was observed following UV exposure, indicating UV-induced self-aggregation. However, the magnitude of UV dose did not seem to have an impact. In the MP UV experiments, only a dose of 80 mJ/cm2 had a significant impact on the formation of aggregates upon UV exposure. Changing the light intensity and exposure time to deliver the same LP UV dose resulted in different levels of aggregation. The results indicated that UV light intensity and wavelength may play a role in aggregation of bacteria.