Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “UV Disinfection Induces a Vbnc State in Escherichia coli and Pseudomonas aeruginosa” filtration media. The biofilm, as we all know, should have a very high cell density since it is the favorite occurrence pattern for microbial aggregates. So Dr. Gehr’s comments suggest a realistic risk that UV disinfection may be confronted. While, this did not exist in our study since most CFU counts came from planktonic cells. Dr. Gehr thought the UV exposure in our study was extremely long, which suggested the contribution of particles and the poor performance of our UV reactors. However, as he noted that the initial cell density in our study is 109CFU/ml. Although the inactivation efficiency was only 2 logs, the reduction amount was 0.99 × 109CFU/ml. While, in the other studies which had much higher efficiencies as 4−5 logs, the initial cell density was usually around 105CFU/mL, so the reduction amount was much less than that in our studies. In other words, the UV exposure dosage for unit biomass in our study is in a normal level and comparable to other studies. Finally, we really do not know what was wrong in using E. coli and P. aeroginosa in our study. Thousands and thousands of scientists all over the world use these two model microorganisms to study the microbiological safety of drinking water. The former is also the pathogenic indicator in the drinking water quality standards of China, U.S., EU, and WHO. We do not know if we isolate some strains from the real water, Dr. Gehr will say, “Oh, it’s not typical, there is no such species in our water”. In fact, what we cared about originally in our study is the VBNC state of waterborne bacteria. The UV radiation disinfection is only a tool to induce bacteria into this state. To our surprise, this paper has drawn much attention from academia and industries. We are really grateful to all of the people interested in our work including Dr. Gehr. We welcome all to discuss this issue with us via electronic ways or in person. Finally, we want to say that there is no ending for drinking water safety. There is also no best technology. All have two sides; UV is not exceptional. A scientific attitude is to face the possible risk and try to improve the technology.
W
e thank Dr. Ronald Gehr very much for his critical comments on our work about the bacteria VBNC state induced by UV radiation1 though we disagree most, if not all, of them. First, the UV disinfection, as an emerging technology, has a much shorter history and smaller size of application than chlorine, chloramine; even chloride dioxide. The record of no outbreaks in last two decades does not mean no outbreaks forever. In fact, it is very easy to find much more water treatment utilities with chlorine disinfection which have much longer safe running records. However, many scientists and engineers have focused on improving the chlorine disinfection efficiency and lowering the genotoxic effects arose by disinfection byproducts. Why? Because what we pursue in drinking water treatment is absolute security. So any study aiming at finding the defects of a disinfection method should be encouraged. This is especially necessary for UV disinfection since it has some inherent shortcomings such as no residue in the distribution system. Dr. Gehr claimed the bacterial cell density used in our study (109CFU/mL) is too high so that “a large proportion of these are very likely to be in a clumped state.” We agree that the cell density is higher than many papers on UV disinfection technology.2,3 However, the high density is necessary here in order to collect enough mRNA for the molecular biological analysis to reveal the metabolic activity of the VBNC bacteria. It is a common level for molecular analysis. We also agree that some of the bacteria are in the “clumped state”. But “a large proportion” is a rather arbitrary assertion. Dr. Gehr did not provide any literature or other evidence supporting this. As for his state “being 100% clumped bacteria, as is the case in the current study”, it is an emotional expression, which is not suitable for serious academic discussion. Furthermore, even if the amount of clumped state bacteria is substantially large, it can only be reflected in the so-called “tailing”. Since each clump contains “thousands of individual cells” as he said, the survived ones will be detected as the culturable CFUs. The resuscitated VBNC bacteria are defined as the cultured bacteria under a theoretical CFU counts lower than 1. Thus, the culturable clumps are excluded from the very beginning of the experimental design. In addition, many UV tests used an initial bacterial density of 105−6CFU/mL which seemed “normal”. However, they used the diluted one from a 109−10CFU/mL culture.4 The dilution itself will not disperse the clumps. Since the results of those studies were acceptable, we do not think the occurrence of clumped state can shake the main conclusion of our study. With respect to the real application, Dr. Gehr said most of the particles are inorganic or organic colloids (or larger) which may be associated with bacteria on their surface or within them. However, I know that biofiltration, for example, biological sand filtration, O3−BAC, are popular technology in North America and China. A big concern about their security is the microbial leakage which present as the debris of biofilm attached to the © 2015 American Chemical Society
Xin Yu* Shenghua Zhang Chengsong Ye Wenfang Lin Huirong Lin Lu Lv
■
Institute of Urban Environment, Chinese Academy of Science, Xiamen, 361021, P.R. China
AUTHOR INFORMATION
Corresponding Author
*Phone/Fax: +86-592-6190780; e-mail:
[email protected]. Published: May 28, 2015 7502
DOI: 10.1021/acs.est.5b01681 Environ. Sci. Technol. 2015, 49, 7502−7503
Environmental Science & Technology
Correspondence/Rebuttal
Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Zhang, S.; Ye, C.; Lin, H.; Lv, L.; Yu, X. UV disinfection induces a VBNC state in Escherichia coli and Pseudomonas aeruginosa. Environ. Sci. Technol. 2015, 49 (3), 1721−1728. (2) Zimmer, J. L.; Slawson, R. M. Potential repair of E. coli DNA following exposure to UV radiation from both medium- and lowpressure UV sources used in drinking water treatment. Appl. Environ. Microbiol. 2002, 68 (7), 3293−3299. (3) Oguma, K.; Katayama, H.; Ohgaki, S. Photoreactivation of E. coli after low- or medium pressure UV disinfection determined by an endonuclease sensitive site assay. Appl. Environ. Microbiol. 2002, 68 (12), 6029−6035. (4) Tosa, K.; Hirata, T. Photoreactivation of enterohemorrhagic E. coli following UV disinfection. Water Res. 1999, 33 (2), 361−366.
7503
DOI: 10.1021/acs.est.5b01681 Environ. Sci. Technol. 2015, 49, 7502−7503
