Microbial source tracking, pathogen detection, QMRA

A key question in public health and recreational water quality is how pathogens reach the urban environment (their transport routes) and what determines their survival in the waterways (their fate). The following projects investigate the role of biofilms as repositories of pathogens and the effect of shear force to ascertain potential ramifications that might need consideration in restructured waterways.

Biofilm/water interactions

We are using hydrodynamic modelling and metagenomic sequencing to study the fate and transport of common waterborne pathogens in freshwater systems, and their effect on mature biofilm communities. Four surrogates are used to represent pathogens: Pseudomonas aeruginosa, Enterococcus faecalis and the bacteriophages P22 and GA. Two flow regimes representing conditions in urban canals during dry weather and rainfall periods are assessed. Replicated (n = 4) experiments use laminar flow flumes (n = 4), two of which are spiked once with the surrogates, and surrogate populations, with nucleic acids in water and sediment biofilms being tracked for 21 d. Additionally, microbial community assembly is monitored through metagenomic sequencing of the biofilms collected from flumes with and without surrogates.

Currently, we are extending our approach to study the fate and effects of human and fish pathogens in coastal waters, specifically, when they reach the sediment-water interface. The main hypothesis is based on the a priori assumption that any one biological agent does not exist in isolation but rather tends to adhere to or form aggregates.

 

Microbial source tracking

The occurrence of waterborne diseases among recreational water users in developed countries and high annual deaths associated with faecal pollution of water sources in developing countries underscore the need for comprehensive microbial water quality assessment to protect public health. We validated host-associated Bacteroidales genetic markers for microbial source tracking in Singapore and determined sediment-associated decay rates of the best-performing human-associated markers, B. theta and BacHum (previously developed by our group at UC Davis), and those of faecal indicator bacteria (FIB) in the presence and absence of grazing protozoa. Two human- (B. theta and BacHum) and one canine-associated (BacCan) Bacteroidales markers, and one universal Bacteroidales marker (BacUni) were validated for microbial source tracking (MST) in Southeast Asia using 295 faecal samples from animals. These findings provided insights into the utility of widely used genetic markers for microbial water quality assessment in the urban tropical environments of Singapore and Southeast Asia. The validated markers are now ready for use in urban freshwater environments.

 

Drinking water biofilms

Singapore has a tropical rainforest climate with no distinct seasons and high uniform temperature and humidity. These environmental conditions expedite the decay of monochloramine by auto-decomposition producing free ammonia. Thus, the ammonia is available as an energy source for ammonia oxidising bacteria and archaea by oxidation to nitrite. Nitrite can react with chloramine, reinforcing its decay, and/or be oxidised to nitrate by nitrite oxidising bacteria, promoting the development of biofilms and leading to nitrification issues in the pipeline network. Consequently, drinking water stability is compromised and problems arise such as microbially influenced corrosion and unpleasant odour and taste, and there is a potential risk that biofilms act as a refuge for waterborne pathogens.

A comprehensive and in-depth study of biofilms from full-scale monochloraminated DWDS was performed using next generation sequencing techniques and a multiparametric approach to characterise the microbial community. Emphasis was placed on nitrifiers and bacteria of hygienic relevance to understand their effect on drinking water safety in a tropical country. Information of this type is extremely scarce because of the cost, inconvenience and limited access to water mains