Correspondence/Rebuttal pubs.acs.org/est
Comment on “UV Disinfection Induces a Vbnc State in Escherichia coli and Pseudomonas aeruginosa”
Z
hang et al.1 make the rather startling claim, based on tests using standard plate culturing techniques as well as advanced microbial methods, that pure strains of Escherichia coli and Pseudomonas aeruginosa exposed to UV doses much higher than typically applied in water and wastewater treatment processes, enter the VBNC state, and could subsequently be pathogenic to human hosts. Many thousands of full-scale UV disinfection systems have operated successfully worldwide for over two decades without any adverse health outbreaks being associated with them; this alone should raise doubts regarding the veracity and strength of their claim. The following will identify areas of the study which are likely responsible for the erroneous conclusions having been drawn. The cell density used, as reported under the UV materials and methods (109 CFU/mL) is very high. A large proportion of these are very likely to be in a clumped state. It is well-known that UV cannot easily penetrate such clumps, leading to a tailing of the dose−response curve at higher doses. Such behavior is indeed observed in Figure 1. Each particle will lead to a single colony, yet may house many thousands of individual cells. In this case, it is not at all surprising that regrowth, interpreted as coming from the VBNC state, would occur, if the particles were subsequently dispersed by successive dilutions, as described under the resuscitation experiments. In real-world applications, most particles are first filtered out by various upstream processes prior to UV disinfection, and cell counts applied to UV units are several orders of magnitude lower than 109 CFU/mL. Furthermore, those particles that do remain and that do enter the UV reactor are inorganic or organic colloids (or larger) which may be associated with bacteria on their surface or within them, but the particles themselves are far from being 100% clumped bacteria, as is the case in the current study. The duration of UV exposure is extremely long3 to 18 mincompared to typical laboratory, pilot-plant and full-scale exposures of only a few seconds, and the actual performance of the UV system is poor compared to well-established values in the literature. As reported in the results and discussion section, at a dose of 50 mJ/cm2, the E. coli count was reduced by only 2 logs. In contrast, typical doses compiled from the literature2 for a wide variety of free-swimming E. coli strains for 2-log reductions range from 0.7−8.0 mJ/cm2. This again points to particles being the major contributor to the plate counts. Furthermore, the authors do not report the UVT (UV254 transmission), nor do they indicate what value of intensity I in Figure 1 was used. I suggest that they used the surface UV intensity, rather than the average UV intensity, and that their actual UV doses were much lower than stated. This would explain why such long exposure times were needed, and why their UV reactor performed so poorly. Finally, the natural water environment and wastewater effluents contain a wide array of wild-type microbes, the virulence characteristics of which may not correspond to those of the pure E. coli (CMCC 44103) and P. aeruginosa (ATCC 15692) strains used by these authors. Strains derived from real © 2015 American Chemical Society
waters and wastewater effluents would more accurately respond to the UV doses and resuscitation procedures used in the paper. In summary, I suggest that the authors have severely underestimated the UV doses applied to their pure cultures. Furthermore, an artifact of the experimental conditions was the presence of bacterial clumps which showed up as single colonies on culture media, but which dispersed during subsequent dilutions to yield apparent resuscitation. This study does not change the safety profile of UV disinfection systems.
Ronald Gehr*
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Department of Civil Engineering McGill University 817 Sherbrooke St W Montreal, Quebec H3A 0C3 Canada
AUTHOR INFORMATION
Corresponding Author
*Phone: +1 514 398 6861; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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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) Chevrefils, G.; Caron, É.; Wright, H.; Sakamoto, G. UV dose required to achieve incremental log inactivation of bacteria, protozoa and viruses. IUVA News 2006, 8 (1), 38−45.
Published: May 28, 2015 7501
DOI: 10.1021/acs.est.5b00769 Environ. Sci. Technol. 2015, 49, 7501−7501
