H2S test for water quality? Not so fast
John Feighery

This analysis of the validity and effectiveness of the H2S test to detect fecally contaminated water is inaccurate, misleading and otherwise flawed. The H2S test has been evaluated repeatedly as a fecal indicator bacterium test for water. Many of those studies show that it provides both qualitative (presence or absence) and quantitative data (relative concentrations) that are comparable to and sometimes higher than those of the usual fecal indicator bacteria, such as E. coli and thermotolerant (fecal) coliforms.

Furthermore, field research on small community water supplies in a SE Asia country done by Ku McMahan, me and others on the ability of a quantitative version of the H2S test to predict risks of diarrheal illness from community drinking waters containing different concentrations of E. coli and H2S bacteria showed that the quantitative H2S test was as predictive of risks of diarrheal illness as the E. coli test. There were comparable dose-response relationships, with increasing diarrheal illness rates with increasing concentrations of either E. coli or H2S bacteria in the drinking waters. That is, presence and levels of H2S bacteria in drinking water predict diarrheal illness rates about as well as E. coli does.

Furthermore, other studies we have done on the relationships of H2S bacteria to other “standard” bacterial indicators of fecal contamination like E. coli have shown good agreement or concordance in a variety of different water qualities, including both ground and surface waters from different hydrological and geohydrological settings, including geothermally thermally enriched surface and ground waters.

In our field studies, naturally occurring H2S producing bacteria in these geothermally enriched waters were not detected in the standard H2S test. This is probably because they are such strict anaerobes with profound sensitivity to even traces of oxygen in sampled water, that they quickly die and are not detected when doing a typical H2S bacteria test.

In addition, we have shown that whenever a water sample is positive in the H2S test it is also positive for other bacteria associated with fecal contamination. On the rare occasions when E. coli bacteria are not detected in water samples positive for H2S bacteria, other fecal indicator bacteria are present, such as Enterobacter aerogenes, Klebsiella spp., Citrobacter spp. and Clostridium perfringens. The reality is that E. coli is NOT the ideal indicator bacterium forfecally contaminated water and fecal contamination can be present in water that may not have detectable levels of E. coli but will have detectable level of other fecal indicator bacteria in a given sample. See: McMahan L, Grunden AM, Devine AA, Sobsey MD. (2012) Evaluation of a quantitative H2S MPN test for fecal microbes analysis of water using biochemical and molecular identification. Water Res. 2012 Apr 15;46(6):1693–704. 
McMahan L, Devine AA, Grunden AM, Sobsey MD. (2011) Validation of the H2S method to detect bacteria of fecal origin by cultured and molecular methods. Appl Microbiol Biotechnol. 2011 Dec;92(6) :1287–95.

The real limitation of current H2S tests is that they are presence-absence tests telling you that a particular single sample volume either has H2S bacteria or does not. For most drinking water supplies and sources, especially in the developing world, what is needed is a QUANTITATIVE H2S test that provides data on the concentrations of H2S bacteria. It is best if fecal indicator bacteria are absent from 100-ml samples of water as evidence of safety or low risk, as is recommended by the World Health Organzation Guidelines for Drinking-water Quality (http://apps.who.int/iris/bitstream/10665/44584/1/9789241548151_eng.pdf)

However, WHO also recognizes that it is not always possible to consistently achieve water that is free of fecal indicator bacteria in 100-mL volumes of drinking water for a variety of reasons. Therefore, occasional positive samples, especially with only low levels of fecal indicator bacteria in 100-mL volumes may be tolerable and be of only low or only intermediate risk. Assording to the WHO GDWQ, as the frequency of positive samples and the concentrations of bacteria in positive samples increase, the risks of diarrheal illness and other enteric diseases are considered to increase. This dose-response effect has been documented in studies by my research group as well as others (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2393099/pdf/bullwho00048-0043.pdf).

Therefore, what is really needed are QUANTITATIVE versions of the H2S test to determine the quality of drinking water. This can be done by examining multiple volumes of a water sample at the same time in order to estimate how many of those different volumes of the same water sample are either positive or negative for H2S bacteria. This is what is commonly done to quantify other fecal indicator bacteria such as E. coli and thermotolerant coliforms by the Most Probable Number (MPN) method.

In summary, the assertion that H2S tests are flawed and not reliable indicators of fecal contamination is not supported by a substantial amount of scientific evidence. Quantitative H2S tests appear to provide information on the microbial quality of water that is comparable to the information provided by presence-absence or quantitative tests for other fecal indicator bacteria and to provide comparable information on diarrheal disease risks as evidence of human health effects.

Mark D. Sobsey, PhD

Kenan Distinguished Professor of Environmental Sciences and Engineering, Gillings School of Global Public Health, University of North Carolina; email: mark_sobsey@unc.edu

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