Abstract
BackgroundBacteroides ovatus, a member of the genus Bacteroides, is considered for use in molecular-based methods as a general fecal indicator. However, knowledge on its fate and persistence after a fecal contamination event remains limited. In this study, the persistence of B. ovatus was evaluated under simulated sunlight exposure and in conditions similar to freshwater and seawater. By combining propidium monoazide (PMA) treatment and quantitative polymerase chain reaction (qPCR) detection, the decay rates of B. ovatus were determined in the presence and absence of exogenous photosensitizers and in salinity up to 39.5 parts per thousand at 27°C.ResultsUVB was found to be important for B. ovatus decay, averaging a 4 log10 of decay over 6 h of exposure without the presence of extracellular photosensitizers. The addition of NaNO2, an exogenous sensitizer producing hydroxyl radicals, did not significantly change the decay rate of B. ovatus in both low and high salinity water, while the exogenous sensitizer algae organic matter (AOM) slowed down the decay of B. ovatus in low salinity water. At seawater salinity, the decay rate of B. ovatus was slower than that in low salinity water, except when both NaNO2 and AOM were present.ConclusionThe results of laboratory experiments suggest that if B. ovatus is released into either freshwater or seawater environment in the evening, 50% of it may be intact by the next morning; if it is released at noon, only 50% may be intact after a mere 5 min of full spectrum irradiation on a clear day. This study provides a mechanistic understanding to some of the important environmental relevant factors that influenced the inactivation kinetics of B. ovatus in the presence of sunlight irradiation, and would facilitate the use of B. ovatus to indicate the occurrence of fecal contamination.
Highlights
Bacteroides ovatus, a member of the genus Bacteroides, is considered for use in molecular-based methods as a general fecal indicator
Our observations suggest that in pockets of freshwater directly exposed to discharge from wastewater treatment plants, it is anticipated that the algae organic matter (AOM) would be higher than 1.94 mg C/L and would facilitate a longer persistence of B. ovatus
In this study, solar UVB was important for the persistence of intact B. ovatus in both low and high salinity water
Summary
Bacteroides ovatus, a member of the genus Bacteroides, is considered for use in molecular-based methods as a general fecal indicator. Earlier studies have reported that the occurrence of gastrointestinal illness in both fresh and coastal waters has been found to correlate statistically with the presence of fecal indicator bacteria (FIB) such as coliforms and enterococci [1,2]. The culture-dependent method to enumerate conventional FIBs is commonly used for detecting fecal contamination. This method has always been challenged for various reasons. Most conventional FIBs can proliferate under natural conditions after being released into the environment, and the detection of these FIBs may overestimate the original contamination level. Monitoring the cause of waterborne illness based on conventional FIBs underestimates the actual number of FIBs as it excludes the viable but non-culturable cells [3]. FIBs require incubation time of 24-48 h, posing a possibility for people to be exposed to contaminated water before regulatory announcements can be made [5]
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