Considerable discrepancies among HPC, ATP, and FCM detection methods in evaluating the disinfection efficiency of Gram-positive and -negative bacterium by ultraviolet radiation and chlorination

Abstract

Up to now, the effects of ultraviolet (UV) and chlorine disinfection on microbes have been evaluated by heterotrophic plate counting (HPC). Recently, with the application of flow cytometry (FCM) in aquatic environment, it is used to analyze the membrane integrity, DNA damage and enzymatic activity of bacteria. The present work investigated the disinfection efficacy of UV-C and chlorine on E. coli (Gram negative) and Bacillus subtilis (Gram positive) and compared the detection methods for disinfection efficacy with HPC, Adenosine triphosphate (ATP), and FCM. The results show that there are considerable discrepancies among HPC, ATP, and FCM detection for E. coli and B. subtilis when disinfected by UV and chlorine. Specifically, the bacterium was sharply inactivated when evaluated by HPC, whereas the more gentle inactivation trend was observed when detected by ATP and FCM during both UV and chlorine disinfections for E. coli. In addition, for B. subtilis disinfection, the results between ATP and HPC detection were of little difference, especially with chlorine disinfection; thus, ATP measurement could be a replacement of HPC as it was much more fast and convenient. The results of FCM demonstrated that most of the bacteria undergo viable but non-culturable (VBNC) states which could not be detected by HPC but had the infection ability. Considering the risk of VBNC cells and the limitation of HPC, the following was suggested: when applied UV-C disinfection, HPC should combine ATP or FCM to evaluate the microbial viability, whereas FCM was a powerful tool to distinguish viable, but non-culturable cells (VBNC) applied for chlorine disinfection.