Relative decay of Bacteroidales microbial source tracking markers and cultivated Escherichia coli in freshwater microcosms

Dick LK, Stelzer EA, Bertke EE, Fong DL, Stoeckel DM. (2010). “Relative decay of Bacteroidales microbial source tracking markers and cultivated Escherichia coli in freshwater microcosms” Appl Environ Microbiol 76(10), 3255-3262.

by Yolanda Brooks& Asli Aslan

The objective of the study was to determine if the cultivated Escherichia coli decay rate has a constant ratio to three 16S rRNA Bacteroidales gene markers’ decay rate; AllBac (total Bacteroidales), qHF183 and BacHum (two human Bacteroidales markers). The authors tested their objective using recreational water microcosms with four different environmental stress treatments over the period of 11 days. The four treatments were (a) artificial sunlight 12hr/day, (b) reduced temperature (at 15°C instead of 25°C for the other treatments), (c) reduced predation (the sample seeded with autoclaved river water) and (d) addition of river sediment.

The authors hypothesized that standardized ratios of qHF183 and/or BacHum:AllBac:cultivated E. coli over time can help determine the proportion of human associated fecal pollution in recreational water. They also hypothesized that from these constant ratios, it would be possible to estimate the recreational water’s E. coli concentration by quantifying molecular marker and determining when the E.coli concentration will be at a permissible range by the molecular marker’s decay rate.

The microcosm treatments of sunlight, sediment and reduced predation had decay rates that were higher in the qHF183 and BacHum markers than cultivated E. coli. However, all the Bacteroidales markers and cultivated E. coli t99 values in days (time to achieve a 99% reduction of the markers’ original concentrations) in the control and reduced temperature microcosom weren’t significantly different. The control microcosm had a t99 range between 1.74-3.28 days, and 2.35-3.01 days for reduced temperature. The reduced predation treatment showed a significant difference between the molecular markers’ t99 values (2.78days- qHF13, 3.03days- BacHum, 3.75 days- Allbac) compared to the cultivated E. coli t99 value (7.14 days). Even within the markers’ t99 values, there was a significant difference between the Allbac marker compared to the others. This was due to a biphasic decay rate from a persistent Bacteroidales subpopulation. These results correlate with the Walters et al,. 2009) study in which they compared the persistence of the human and ruminant specific Bacteroidales markers in light and dark microcosms.

Overall, the results determined that only qHF183 and BacHum may be used to give a conservative estimate of the cultivated E. coli concentration from human sources in all of the microcosm treatments. AllBac can’t be used as a fecal indicator to replace and/or complement cultivated E. coli and BacHum and qHF183 in reduced predation systems like drinking water. Further analysis is needed to confirm that all the markers’ decay rates have accurately represented possible environmental conditions and will be able to accurately represent many other complicated abiotic and biotic environmental conditions in this investigated recreational water. Similarly, certain subgroups of E. coli can persist and even propagate in certain environmental conditions, while other die-off. This is evident with E. coli O157:H7 because it better tolerate acidic environments than other E. coli subgroups.

If further testing confirms that the decay rates accurately depicted by this study’s microcosms, then models can be created to correctly calculate the decay rates of the above molecular markers and cultivated E. coli concentrations in many environmental conditions. The creation of effective models for this recreational water will trigger the production of other models to describe fecal pollution decay rates for other recreational waters. This will hopefully allow recreational water quality criteria policy changes to allow for a faster and cultivation-free method using molecular markers to determine the total and source specific E. coli concentration in recreational waters. These changes will give recreational water monitoring agencies a faster, cost-effective method to test for fecal pollution markers. These models will also be one of the first models to not only identify a pollution source, but also quantify it in recreational waters.

While the initial step have been taken to improve microbial source tracking of fecal contaminators, there are still many steps to further understand how target molecular markers from a multitude of species, especially human specific, decay in distinct environmental conditions and how their decay can be estimated. This goal is furthered complicated by the species distribution and decay rates in target Bacteroidales populations and cultivated E. coli concentrations from both human and non-human sources may fluctuate globally in recreation waters due to many biotic and abiotic environmental factors. Some of these include environmental physicochemical processes, nonpoint pollution sources and even different types of human nutrition.

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