Evaluating the Transport of Bacillus subtilis Spores as a Potential Surrogate for Cryptosporidium parvum Oocysts

Surrogates of foodborne and waterborne protozoan parasites: A review

The protozoan parasites Cryptosporidium parvum, Cyclospora cayetanensis, and Toxoplasma gondii are major causes of waterborne and foodborne diseases worldwide. The assessment of their removal or inactivation during water treatment and food processing remains challenging, partly because research on these parasites is hindered by various economical, ethical, methodological, and biological constraints. To address public health concerns and gain new knowledge, researchers are increasingly seeking alternatives to the use of such pathogenic parasites. Over the past few decades, several non-pathogenic microorganisms and manufactured microparticles have been evaluated as potential surrogates of waterborne and foodborne protozoan parasites. Here, we review the surrogates that have been reported for C. parvum, C. cayetanensis, and T. gondii oocysts, and discuss their use and relevance to assess the transport, removal, and inactivation of these parasites in food and water matrices. Biological surrogates including non-human pathogenic Eimeria parasites, microorganisms found in water sources (anaerobic and aerobic spore-forming bacteria, algae), and non-biological surrogates (i.e. manufactured microparticles) have been identified. We emphasize that such surrogates have to be carefully selected and implemented depending on the parasite and the targeted application. Eimeria oocysts appear as promising surrogates to investigate in the future the pathogenic coccidian parasites C. cayetanensis and T. gondii that are the most challenging to work with.

Transport and removal of spores of Bacillus subtilis in an alluvial gravel aquifer at varying flow rates and implications for setback distances

To guarantee proper protection from fecally transmitted pathogen infections, drinking water wells should have a sufficiently large setback distance from potential sources of contamination, e.g. a nearby river. The aim of this study was to provide insight in regards to microbial contamination of groundwater under different flow velocities, which can vary over time due to changes in river stage, season or pumping rate. The effects of these changes, and how they affect removal parameters, are not completely understood. In this study, field tracer tests were carried out in a sandy gravel aquifer near Vienna, Austria to evaluate the ability of subsurface media to attenuate Bacillus subtilis spores, used as a surrogate for Cryptosporidium and Campylobacter. The hydraulic gradient between injection and extraction was controlled by changing the pumping rate (1, 10 l/s) of a pumping well at the test site, building upon previously published work in which tracer tests with a 5 l/s pumping rate were carried out. Attachment and detachment rate coefficients were determined using a HYDRUS-3D model and ranged from 0.12 to 0.76 and 0-0.0013 h^-1, respectively. Setback distances were calculated based on the 60-day travel time, as well as a quantitative microbial risk assessment (QMRA) approach, which showed similar results at this site; around 700 m at the highest pumping rate. Removal rates (λ) in the field tests ranged from 0.2 to 0.3 log/m, with lower pumping rates leading to higher removal. It was shown that scale must be taken into consideration when determining λ for the calculation of safe setback distances.

A unified parameter model based on machine learning for describing microbial transport in porous media

The transport and retention of microorganisms are typically described using attachment/detachment and straining/liberation models. However, the parameters in the models varied significantly, posing a significant challenge to describe microbial transport under different environmental conditions. A neural network (ANN) model was developed in this study to link the parameters in the model with the factors influencing microbial transport including the properties of microorganisms such as size and surface potentials, and the properties of porous media such as grain size and porosity, and flow conditions. Exhaustive search of literature renders 420 sets of experimental data of microbial transport, which were fitted using the microbial transport model to obtain model parameters. The model parameters, together with the factors influencing microbial transport, were then used to train an ANN model to search for their relationship. An ANN-based parameter relationship was derived and was then used to simulate microbial transport. The simulated results using the relationship roughly matched with the experimental data under different environmental conditions, indicating that a unified relationship was established between the parameters of the microbial transport model and the factors influencing microbial transport, and that microbial transport can be described using the microbial transport model with the ANN-based unified relationship for model parameters.

Host-Endosymbiont Relationship Impacts the Retention of Bacteria-Containing Amoeba Spores in Porous Media

Amoebae are protists that are commonly found in water, soil, and other habitats around the world and have complex interactions with other microorganisms. In this work, we investigated how host-endosymbiont interactions between amoebae and bacteria impacted the retention behavior of amoeba spores in porous media. A model amoeba species, Dictyostelium discoideum, and a representative bacterium, Burkholderia agricolaris B1qs70, were used to prepare amoeba spores that carried bacteria. After interacting with B. agricolaris, the retention of D. discoideum spores was enhanced compared to noninfected spores. Diverse proteins, especially proteins contributing to the looser exosporium structure and cell adhesion functionality, are secreted in higher quantities on the exosporium surface of infected spores compared to that of noninfected ones. Comprehensive examinations using a quartz crystal microbalance with dissipation (QCM-D), a parallel plate chamber, and a single-cell force microscope present coherent evidence that changes in the exosporium of D. discoideum spores due to infection by B. agricolaris enhance the connections between spores in the suspension and the spores that were previously deposited on the collector surface, thus resulting in more retention compared to the uninfected ones in porous media. This work provides novel insight into the retention of amoeba spores after bacterial infection in porous media and suggests that the host-endosymbiont relationship regulates the fate of biocolloids in drinking water systems, groundwater, and other porous environments.

Development of cell-imprinted polymer surfaces for Cryptosporidium capture and detection

Cryptosporidium parvum is waterborne parasite that can cause potentially life-threatening gastrointestinal disease and is resistant to conventional water treatment processes, including chlorine disinfection. The current Environmental Protection Agency-approved method for oocyst detection and quantification is expensive, limiting the ability of water utilities to monitor complex watersheds thoroughly to understand the fate and transport of C. parvum oocysts. In this work, whole cell imprinting was used to create selective and sensitive surfaces for the capture of C. parvum oocysts in water. Cell-imprinted Polydimethylsiloxane (PDMS) was manufactured using a modified stamping approach, and sensitivity and selectivity were analyzed using different water chemistries and different surrogate biological and non-biological particles. The overall binding affinity was determined to be less than that of highly specific antibodies, but on par with standard antibodies and immune-enabled technologies. These initial results demonstrate the potential for developing devices using cell-imprinting for use in waterborne pathogen analysis.

Transport and Retention of Free-Living Amoeba Spores in Porous Media: Effects of Operational Parameters and Extracellular Polymeric Substances

Amoebas are protists that are widespread in water and soil environments. Some species are pathogenic, inducing potentially lethal effects on humans, making them a major threat to public health. Nonpathogenic amoebas are also of concern because they have the potential to carry a mini-microbiome of bacteria, either transiently or via more long-term stable transport. Due to their resistance to disinfection processes, the physical removal of amoeba by filtration is necessary to prevent their propagation throughout drinking water distribution networks and occurrence in tap water. In this study, a model amoeba species Dictyostelium discoideum was used to study the transport and retention behavior of amoeba spores in porous media. The key factors affecting the transport behavior of amoeba spores in fully saturated media were comprehensively evaluated, with experiments performed using a quartz crystal microbalance with dissipation monitoring (QCM-D) and parallel plate chamber system. The effects of ionic strength (IS) on the deposition of spores were found to be in contrast to the predicted Derjaguin-Landau-Verwey-Overbeek (DLVO) theory that more deposition is observed under lower-IS conditions. The presence of extracellular polymeric substances (EPS) was found to be the main contributor to deposition behavior. Overall, these results provide plausible evidence for the presence of amoeba in tap water. Furthermore, this is one of the first studies to examine the mechanisms affecting the fate of amoeba spores in porous media, providing a significant baseline for future research to minimize the safety risk presented by amoeba in drinking water systems.

Virus transport from drywells under constant head conditions: A modeling study

Many arid and semi-arid regions of the world face challenges in maintaining the water quantity and quality needs of growing populations. A drywell is an engineered vadose zone infiltration device widely used for stormwater capture and managed aquifer recharge. To our knowledge, no prior studies have quantitatively examined virus transport from a drywell, especially in the presence of subsurface heterogeneity. Axisymmetric numerical experiments were conducted to systematically study virus fate from a drywell for various virus removal and subsurface heterogeneity scenarios under steady-state flow conditions from a constant head reservoir. Subsurface domains were homogeneous or had stochastic heterogeneity with selected standard deviation (σ) of lognormal distribution in saturated hydraulic conductivity and horizontal (X) and vertical (Z) correlation lengths. Low levels of virus concentration tailing can occur even at a separation distance of 22 m from the bottom of the drywell, and 6-log₁₀ virus removal was not achieved when a small detachment rate (k_d1=1 × 10⁻⁵ min⁻¹) is present in a homogeneous domain. Improved virus removal was achieved at a depth of 22 m in the presence of horizontal lenses (e.g., X=10 m, Z=0.1 m, σ=1) that enhanced the lateral movement and distribution of the virus. In contrast, faster downward movement of the virus with an early arrival time at a depth of 22 m occurred when considering a vertical correlation in permeability (X=1 m, Z=2 m, σ=1). Therefore, the general assumption of a 1.5-12 m separation distance to protect water quality may not be adequate in some instances, and site-specific microbial risk assessment is essential to minimize risk. Microbial water quality can potentially be improved by using an in situ soil treatment with iron oxides to increase irreversible attachment and solid-phase inactivation.

Inactivation of indicator organisms on different surfaces after urban floods

The high frequency and intensity of urban floods caused by climate change, urbanisation and infrastructure failures increase public health risks when the flood water contaminated from combined sewer overflows (CSOs) or other sources of faecal contamination remains on urban surfaces. This study contributes to a better understanding of the effects of urban and recreational surfaces on the occurrence of waterborne pathogens. The inactivation of selected indicator organisms was studied under controlled exposure to artificial sunlight for 6 h followed by 18 h in dark conditions. Concrete, asphalt, pavement blocks and glass as control were inoculated with artificial floodwater containing, as indicator organisms, Escherichia coli bacteria, which are common faecal indicator bacteria (FIB) for water quality assessment, Bacillus subtilis spores chosen as surrogates for Cryptosporidium parvum oocysts and Giardia cysts, and bacteriophages MS2 as indicators for viral contamination. On practically all the surfaces in this study, E. coli had the highest inactivation under light conditions followed by MS2 and B. subtilis, except asphalt where MS2 was inactivated faster. The highest inactivation under light conditions was seen with E. coli on a concrete surface (pH 9.6) with an inactivation rate of 1.85 h^-1. However, the pH of the surfaces (varying between 7.0 and 9.6) did not have any influence on inactivation rates under dark conditions. MS2 bacteriophage had the highest inactivation under light conditions on asphalt with a rate of 1.29 h^-1. No die-off of B. subtilis spores was observed on any of the surfaces during the experiment, neither in light nor in dark conditions. This study underpins the need to use different indicator organisms to test their inactivation after flooding. It also suggests that given the sunlight conditions, concentration of indicator organisms and type of surface, the fate of waterborne pathogens after a flood could be estimated.

Lessons from 10 Years of Experience with Australia’s Risk-Based Guidelines for Managed Aquifer Recharge

The Australian Managed Aquifer Recharge Guidelines, published in 2009, were the world’s first Managed Aquifer Recharge (MAR) Guidelines based on risk-management principles that also underpin the World Health Organisation’s Water Safety Plans. In 2015, a survey of Australian MAR project proponents, consultants and regulators revealed that in those states advancing MAR, the Guidelines were lauded for giving certainty on approval processes. They were also considered to be pragmatic to use, but there was feedback on onerous data requirements. The rate of uptake of MAR has varied widely among Australian state jurisdictions, for reasons that are not explained by the drivers for and feasibility of MAR. The states where MAR has progressed are those that have adopted the Guidelines into state regulations or policy. It was originally intended that these Guidelines would be revised after five to ten years, informed by experience of any hazards not considered in the guidelines, and by new scientific developments including advances in monitoring and control methods for risk management. As such revision has not yet occurred, this paper was prepared to give a precis of these Guidelines and review ten years of experience in their application and to identify issues and suggest improvements for consideration in their revision by Australian water regulators. This paper also discusses the factors affecting their potential international applicability, including the capabilities required for implementation, and we use India as an example for which an intermediate level water quality guideline for MAR was developed. This paper is intended to be useful information for regulators in other countries considering adopting or developing their own guidelines. Note that the purpose of these Guidelines is to protect human health and the environment. It is not a guide to how to site, design, build and operate a managed aquifer recharge project, for which there are many other sources of information.

On the use of total aerobic spore bacteria to make treatment decisions due to Cryptosporidium risk at public water system wells

Spore reduction can be used as a surrogate measure of Cryptosporidium natural filtration efficiency. Estimates of log10 (log) reduction were derived from spore measurements in paired surface and well water samples in Casper Wyoming and Kearney Nebraska. We found that these data were suitable for testing the hypothesis (H₀) that the average reduction at each site was 2 log or less, using a one-sided Student's t-test. After establishing data quality objectives for the test (expressed as tolerable Type I and Type II error rates), we evaluated the test's performance as a function of the (a) true log reduction, (b) number of paired samples assayed and (c) variance of observed log reductions. We found that 36 paired spore samples are sufficient to achieve the objectives over a wide range of variance, including the variances observed in the two data sets. We also explored the feasibility of using smaller numbers of paired spore samples to supplement bioparticle counts for screening purposes in alluvial aquifers, to differentiate wells with large volume surface water induced recharge from wells with negligible surface water induced recharge. With key assumptions, we propose a normal statistical test of the same hypothesis (H₀), but with different performance objectives. As few as six paired spore samples appear adequate as a screening metric to supplement bioparticle counts to differentiate wells in alluvial aquifers with large volume surface water induced recharge. For the case when all available information (including failure to reject H₀ based on the limited paired spore data) leads to the conclusion that wells have large surface water induced recharge, we recommend further evaluation using additional paired biweekly spore samples.