Insights into effects of algae on decay and distribution of bacterial pathogens in recreational water: Implications for microbial risk management

Abstract

The decay and distribution of bacterial pathogens in water is an important information for the health risk assessment to guide water safety management, and suspended algae might affect bacterial pathogens in water. This study established microcosms to investigate the effects of algae-related factors on the representative indicators and opportunistic pathogen species in water. We found that suspended algae increased the persistence of targeted species by 1-2 orders of magnitude of concentrations compared to microcosms without algae; and the effect of algae on microbial survival was affected by water nutrient levels (i.e., carbon, nitrogen and phosphorus), as the increased microbial persistence were correlated to the increased algae concentrations with more nutrient supplies. Moreover, decay and distribution profiles of representative species were determined. The three opportunistic pathogen species (Pseudomonas aeruginosa, Aeromonas hydrophila and Staphylococcus aureus) showed lower decay rates (0.82–0.98/day, 0.76–0.98/day, 0.63–0.87/day) largely affected by algae-related factors, while the enteric species (Escherichia coli and Enterococcus faecalis) had higher decay rates (0.94–1.31/day, 0.89–1.21/day) with little association with algae, indicating the propensity for attachment to algae is an important parameter in microbial fate. Together results suggest suspended algae played an evident role in the decay and distribution of bacterial pathogens, providing important implications regarding microbial safety in recreational water.

Introduction

Human pathogens in aquatic systems pose a severe threat to public health worldwide (Sherchand, 2012). Recreational waters play vital roles in flood control and receiving treated sewage, and also serve landscaping and recreational functions in a wide range of urban areas. These waterbodies usually harbor various bacterial pathogens, which can pose health risks to nearby communities and people who come into contact with contaminated water. Several studies have reported the persistence and variety of fecal pathogen species or fecal indicator bacteria (e.g., Salmonella spp., Campylobacter spp., Shigella spp., Escherichia coli and Enterococcus spp.) in recreational waters, and some species were presenssted with high concentrations exceeding the local recommended values of infection risks (Fewtrell and Kay, 2015; Sales-Ortells et al., 2015; Chhipi-Shrestha et al., 2017; Murphy, 2017; Fang et al., 2018a). Diverse populations of opportunistic pathogen species (e.g., Pseudomonas aeruginosa, Staphylococcus aureus), which have attracted emerging attention as concerns mount over the increased incidence of opportunistic infections (Falkinham III et al., 2015), also have been reported in recreational waters (Alexander et al., 2015; Fang et al., 2018a). While many studies have investigated the occurrence of pathogenic organisms in water, increasing studies have explored the decay and distribution of bacterial pathogens after entering a waterbody (Mattioli et al., 2017; Murphy, 2017). The decay-related studies that have been performed focus mainly on fecal indicator bacteria or fecal pathogen species, such as E. coli, enterococci and Salmonella spp. (Shapiro et al., 2013; Davidson et al., 2015; Perkins et al., 2016; Gutiérrez-Cacciabue et al., 2016; Wang et al., 2018). To make accurate predictions about the fate of bacterial pathogens and assessments of the corresponding risks to human populations, it is essential to target a greater diversity of bacterial species and investigate how they decay and spread in water.

After entering a waterbody, bacterial pathogens may experience growth, lysis, UV inactivation, sedimentation, etc. (Qian et al., 2016), with their ultimate fate being governed by complex environmental factors (Zimmer-Faust et al., 2017). Among the relevant environmental parameters, suspended particles, which affect the settling-suspension process (Qian et al., 2016), may have a significant impact on the fate of bacterial pathogens in water (Maki and Hicks, 2002). Suspended particles can also provide considerable advantages to pathogen survival by facilitating nutrient assimilation and increasing resistance to adverse conditions (Fang et al., 2018b). In addition, the survival or regrowth of bacterial pathogens caused by particle attachment is particularly problematic because it may lead to falsely positive indications of exogenous pathogen pollution during source monitoring studies (Perkins et al., 2016; Zimmer-Faust et al., 2017). Consequently, the association between bacterial pathogens and suspended particles has drawn much attention, and is a challenging factor in predictions of the fate of bacterial pathogens in water.

Studies evaluating the effects of suspended particles on bacterial pathogens have mainly focused on sediment and resuspended sediment particles in rivers, consistently demonstrating that pathogen survival within particles was greater than in the water (Malham et al., 2014; Perkins et al., 2014; Abia et al., 2016; Zimmer-Faust et al., 2017). While in recreational lakes, suspended particles are primarily biotic, mainly comprising algae (Fang et al., 2018a), and are likely to affect pathogen survival differently than abiotic particles, such as sediment. On one hand, a certain amount of algae could lead to increased oxygenation, which can increase the efficiency of solar inactivation of bacterial pathogens (Ansa et al., 2015), algal toxins can exert a destructive effect on bacterial pathogens (Yang et al., 2017), and competition for nutrients between algae and bacteria could limit pathogen survival. On the other hand, suspended algae could serve as advantageous ecotypes, similar to abiotic particles, for pathogen survival; sporadic field studies have reported that some algae (e.g., Cladophora) are suitable habitats for certain bacterial pathogens (e.g., E. coli, enterococci, Shigella) (Ishii et al., 2006; Oster et al., 2014). Considering the magnitude of the known effects of abiotic particles and the potential range of effects that may be exerted by algae on bacterial pathogens, knowledge about how suspended algae affect pathogen survival in recreational waters needs to be expanded.

In the current study, we provide novel insights into the role of algae-related factors on the decay and distribution of representative pathogen species in recreational water. The aims of this study were to: (1) determine the role of suspended algae on the fate of targeted pathogen species in surface water; (2) further compare the effects of algae on microbial decay in different water nutrient conditions and with abiotic particles of different compositions; and (3) determine decay rates of targeted species and identify the impact of particle-attached/free-floating distribution on survival. Accordingly, three sets of microcosms were established to simulate the effects of algae concentration (set I), water nutrient level (set II) and particle type (set III) on pathogen decay. The effect of bacterial lifestyle on the rate of decay, specifically as either free-floating (FL, 0.22 – 5 μm) or particle-attached (PA, > 5 μm), was also determined. Decay of each species was monitored by qPCR over time. Overall, this study provides insight into the decay and distribution of bacterial pathogens in water that contains algae, which is crucial information for more accurate assessments of microbial risks and can be used to inform management decisions regarding recreational water safety.