Evaluation of Antibiotic Resistance Genes (ARGs) in the Urban Wetland Ecosystem: Las Vegas Wash

Evaluation of Antibiotic Resistance Genes (ARGs) in the Urban Wetland Ecosystem: Las Vegas Wash

Photo: Duane Moser (left) and Xuelian Bai (right) collect filters from the sampling pump to take back to the lab for analysis.

Research on antibiotic resistance genes at DRI


Antibiotic resistance—the ability of bacteria to survive in the presence of antibiotics—is an increasing environmental and public health concern as more antibiotics enter urban waterways and treated wastewater is increasingly used to supplement limited water resources. Current wastewater treatment processes have difficulty removing antibiotics, which also encourages the growth of antibiotic resistance in urban watersheds, such as the Las Vegas Wash.

“Contaminants that are persistent in treated wastewaters that are discarded or reused may lead to health risks for humans,” explains Dr. Xuelian Bai, the principal investigator (PI) of this project that also includes co-PI Dr. Duane Moser and student researcher Rania Eddik-Zein. “The U.S. Centers for Disease Control and Prevention, the World Health Organization, and numerous other global and national agencies recognize antibiotic resistance as a critical challenge.”



The Las Vegas Wash is a unique watershed that is highly affected by anthropogenic activities and flooding during wet seasons.

“A lot of research has been done to monitor chemical contaminants such as nutrients, heavy metals, and organic contaminants, as well as antibiotics in the Las Vegas Wash and Lake Mead,” Bai says. “However, there is still a lack of information on the presence of microbial contaminants and antibiotic resistance genes [ARGs] in the watershed.”

Understanding the presence and abundance of ARGs in this watershed will provide insight into possible antibiotic resistance developing in the wash.

For this project, the researchers will evaluate the occurrence and prevalence of ARGs in the Las Vegas Wash.

“Resistance to antibiotics is encoded in ARGs, which are segments of DNA that enable bacteria to fight antibiotics,” Bai explains. “The major concerns about antibiotic resistance are the tendency of bacteria to share ARGs through horizontal gene transfer and that efforts to kill resistant bacteria, such as UV or chlorine disinfection in wastewater treatment and drinking water facilities, may not remove ARGs.”

The researchers anticipate that the data from this study will provide insight into the prevalence of ARGs in the wash and provide valuable information that can be used to determine water quality and potential human health concerns in southern Nevada.

First, the researchers will take field samples of water and sediment from the Las Vegas Wash to assess the presence of ARGs in an urban wetland ecosystem.

“Municipal wastewater appears to be a significant reservoir of ARGs,” Bai says. “Many studies have detected ARGs at all stages of the municipal wastewater treatment processes.”

Urban water supplies are particularly susceptible to developing antibiotic resistance because of the concentrated quantities of antibiotics that are released when treated municipal wastewater is discharged into the environment.

“Microorganisms in wastewater discharge can transport ARGs to downstream surface waters used for recreation or sources of drinking water, which can lead to human exposure over local, or even global, scales,” Bai explains. “This is a concern in southern Nevada because five major wastewater treatment plants discharge into the Las Vegas Wash. The Las Vegas Wash then discharges into Lake Mead, which is the primary drinking water supply for the Las Vegas Metropolitan Area.”


Researchers carry equipment toward a sampling site at the Las Vegas Wash.

The DRI research team including (from left) Duane Moser, David Basulto, Hai Pham, and Xuelian Bai carry equipment down to the bank of the Lake Mead, one of several sampling sites along the Las Vegas Wash.


Lake Mead supplies water to millions of residents in the southwestern United States, so identifying potential antibiotic resistance is increasingly important, especially with the drastic population growth in the region. Effluent discharged from wastewater treatment plants, urban runoff, and floodwaters during wet seasons carry sediment, nutrients, and other contaminants to Lake Mead. This generates several water-quality concerns, particularly about the effects of contaminants on aquatic habitats.

“The Las Vegas Wash provides the full continuum of major freshwater aquatic habitats, includingwetlands, flowing water, lake water, and sediment,” Bai explains. “Wetlands, flowing water, and lake water are defined by aerobic conditions and exposure to photosphere influence. However, sediments almost always go anoxic very quickly below the surface, usually within millimeters in eutrophic systems. The fate of antibiotics and the microbial genes that mediate changes in anaerobes have been relatively understudied.”

The researchers anticipate that the field sampling and the lab studies conducted for this project—which include microcosm and microbial community experiments, and DNA analysis—will allow them to specifically identify southern Nevada water issues.

“We will detect and quantify target ARGs in water samples collected upstream and downstream along the Las Vegas Wash, as well as target ARGs in sediment samples collected from the Las Vegas Wash wetlands,” Bai says. “We will also determine the fate and spread of ARGs in the aquatic ecosystems, and assess the effects of elevated antibiotic concentrations on the ecosystem.”

Because evaluating ARGs in surface water and sediment has not been fully studied locally or globally, this project will address local water issues in Nevada and provide useful antibiotic resistance data about urban watersheds that can be used worldwide.

This story was originally written for the Nevada Water Resources Research Institute (NWRRI) July 2019 Newsletter. Success and the dedication to quality research have established DRI’s Division of Hydrologic Sciences (DHS) as the Nevada Water Resources Research Institute (NWRRI) under the Water Resources Research Act of 1984 (as amended). The work conducted through the NWRRI program is supported by the U.S. Geological Survey under Grant/Cooperative Agreement No. G16AP00069.

Problem Plastic: Investigating Microplastic Pollution in Nevada’s Waterways

Problem Plastic: Investigating Microplastic Pollution in Nevada’s Waterways

Photo: A collection of marine debris including microplastics. Credit: NOAA Marine Debris Program/Flickr.


Microplastics research at DRI

Even the tiniest pieces of plastic are a big pollution problem.

Microplastics are plastic pieces ranging in size from 5mm to microscopic particles, in other words, the size of a pencil’s eraser or smaller. They come from a variety of sources, including the breakdown of larger products like single-use plastic bottles and from the microbeads in products like facewash and toothpaste.

The extent of microplastic pollution is only just beginning to be understood, with researchers discovering the tiny plastic pieces everywhere from the air we breathe to the deep ocean. Because microplastics are durable, insoluble, and potentially toxic, they could pose threat to natural ecosystems and human health. But to determine the impact of microplastic pollution, researchers must first understand just how much tiny plastic is out there and where it’s coming from.



DRI’s Monica Arienzo, Zoe Harrold, Meghan Collins, Xuelian Bai, and University of Nevada, Reno undergraduate Julia Davidson are exploring these questions in two bodies of freshwater in Nevada: Lake Tahoe and the Las Vegas Wash.

“There has been a lot of work done to understand how much microplastic is in marine environments, but there have been far fewer studies in freshwater, and far fewer even in alpine lakes,” explained Collins, Education Program Manager at DRI. “This study is really well placed to identify what microplastics may be in the water, their sources, and their characteristics.”

The research team is collecting samples from four different sites in Las Vegas—one in Lake Mead and three in the Las Vegas Wash—and six sites in Lake Tahoe. Sites were selected to include areas both high and low human activity, like the Tahoe Keys with significant boat traffic and Emerald Bay State Park where human impact is low. Additional sampling was also conducted at three stormwater outfalls into Lake Tahoe in collaboration with the League to Save Lake Tahoe’s Pipe Keepers citizen science program.

Research team sets up equipment at Lake Tahoe.

The research team sets up the pump and filter system at Lake Tahoe’s Emerald Bay State Park in May 2019.


“The sampling methods we’re using are unique,” said Arienzo, assistant research professor and project lead. “Past studies collected samples by trailing a large net from a boat or standing with it in a moving stream. Our approach is to sample and filter water in the field for microplastics using a pump, which allows us to filter upwards of 15 gallons of water in locations with still water and in places where boat access is limited.”

“Plus, we don’t have to haul netting around or carry the samples back to the lab—everything we need fits into a backpack, which makes sampling in remote and hard to access locations more feasible,” Arienzo added.

To make this novel method work, researchers place a stake with a funnel clipped to it about 20 feet from the water’s edge. The funnel, positioned on the surface of the water, is connected to tubing that runs back to the pump on shore, which draws water through the tubing and over a series of filters which can capture particles of different sizes.

Filter used to capture microplastic particles.

Tubing runs into the column of filters, which capture particles at three different sizes as water flows through.


Tubing runs into the column of filters, which capture particles at three different sizes as water flows through.

Sampling in all locations took place throughout the spring, and now the team is set to process and analyze the samples over the summer.

“To isolate the plastic pieces, we first have to get rid of all the organics, and we’re going to do that by oxidizing them,” explained Harrold, assistant research scientist in DRI’s Division of Earth and Ecosystem Sciences. “It’s a delicate balance between getting rid of the bugs and twigs and whatever else has ended up in there and not dissolving your plastics.”

Once the team oxidizes the organic particles left behind on the filters, they’ll separate the plastics from any remaining sediment using a high-density liquid separation method which will cause the sediments will settle to the bottom while plastics will float to the top.

From there, the team will begin identifying the different kinds of plastic pieces they find. The type of plastic, its size and shape, and the location where it was collected all provide clues about where it may have come from—for example, a nylon fiber may have come from the breakdown of synthetic clothing, and a piece of Styrofoam could have come from a single-use cup.

filter used to sample microplastics

Harrold removes a filter from the sampling instrument to bring it back to the lab for analysis.


However, making determinations about where individual pieces of microplastic originate is far from straightforward.

“We’re only discovering more sources of microplastics,” explained Harrold. “Recent studies have shown that microplastics can be transported through the atmosphere, so though some of what we find might be coming from local sources, the pollution could also be coming from a factory manufacturing plastic on the other side of the world. We just don’t know.”

While it’s daunting that there’s so much still unknown about this increasingly problematic pollutant, the research team also finds it exciting.

“This is the second study ever to be done on microplastics in Lake Tahoe,” said Arienzo. “It’s amazing to be a part of advancing the science in this new area of study.”

The team hopes that this work will contribute to a foundation of scientific information about the extent of microplastics pollution in Nevada freshwater so that scientists will be able to better identify the sources of microplastic, potential harmful effects to plant and animal life, and ways to remove it from the environment.

DRI's microplastics research team at Lake Tahoe

From left: Harrold, Arienzo, Collins, Davidson, and Bai after sampling at Emerald Bay in May 2019.


Funding for this project came from the DRI Foundation’s Innovation Research Program (IRP), which is designed to support DRI faculty and staff as they pursue their very best ideas. The IRP is funded by individual contributions from science enthusiasts like you—if you’d like to donate to the IRP and help make projects like this one possible, please visit: https://www.dri.edu/foundation/innovation-research-program.

DRI researchers successfully remove harmful hormones from Las Vegas wastewater using green algae

DRI researchers successfully remove harmful hormones from Las Vegas wastewater using green algae

Xuelian Bai, Ph.D., Assistant Research Professor of Environmental Sciences, works with an algae sample in the Environmental Engineering Laboratory at the Desert Research Institute in Las Vegas. Credit: Sachiko Sueki.


LAS VEGAS, Nev. (April 8, 2019) – A common species of freshwater green algae is capable of removing certain endocrine disrupting chemicals (EDCs) from wastewater, according to new research from the Desert Research Institute (DRI) in Las Vegas.

EDCs are natural hormones and can also be found in many plastics and pharmaceuticals. They are known to be harmful to wildlife, and to humans in large concentrations, resulting in negative health effects such as lowered fertility and increased incidence of certain cancers. They have been found in trace amounts (parts per trillion to parts per billion) in treated wastewater, and also have been detected in water samples collected from Lake Mead.

In a new study published in the journal Environmental Pollution, DRI researchers Xuelian Bai, Ph.D., and Kumud Acharya, Ph.D., explore the potential for use of a species of freshwater green algae called Nannochloris to remove EDCs from treated wastewater.

“This type of algae is very commonly found in any freshwater ecosystem around the world, but its potential for use in wastewater treatment hadn’t been studied extensively,” explained Bai, lead author and Assistant Research Professor of environmental sciences with the Division of Hydrologic Sciences at DRI. “We wanted to explore whether this species might be a good candidate for use in an algal pond or constructed wetland to help remove wastewater contaminants.”

Samples of Nannochloris grow in the Environmental Engineering Laboratory at DRI. This species of green algae was found to be capable of removing certain types of endocrine disrupting chemicals from treated wastewater. Credit: Xuelian Bai/DRI.

Samples of Nannochloris grow in the Environmental Engineering Laboratory at DRI. This species of green algae was found to be capable of removing certain types of endocrine disrupting chemicals from treated wastewater. Credit: Xuelian Bai/DRI.

During a seven-day laboratory experiment, the researchers grew Nannochloris algal cultures in two types of treated wastewater effluents collected from the Clark County Water Reclamation District in Las Vegas, and measured changes in the concentration of seven common EDCs.

In wastewater samples that had been treated using an ultrafiltration technique, the researchers found that the algae grew rapidly and significantly improved the removal rate of three EDCs (17β-estradiol, 17α-ethinylestradiol and salicylic acid), with approximately 60 percent of each contaminant removed over the course of seven days. In wastewater that had been treated using ozonation, the algae did not grow as well and had no significant impact on EDC concentrations.

One of the EDCs examined in the study, triclosan, disappeared completely from the ultrafiltration water after seven days, and only 38 percent remained in the ozonation water after seven days – but this happened regardless of the presence of algae, and was attributed to breakdown by photolysis (exposure to light).

“Use of algae for removing heavy metals and other inorganic contaminants have been extensively studied in the past, but for removing organic pollutants has just started,” said Acharya, Interim Vice President for Research and Executive Director of Hydrologic Sciences at DRI. “Our research shows both some of the potential and also some of the limitations for using Nannochloris to remove EDCs from wastewater.”

Although these tests took place under laboratory conditions, a previous study by Bai and Acharya that published in November 2018 in the journal Environmental Science and Pollution Research examined the impacts of these same seven EDCs on quagga mussels (Dreissena bugensis) collected from Lake Mead. Their results showed that several of the EDCs (testosterone, bisphenol A, triclosan, and salicylic acid) were accumulating in the body tissues of the mussels.

Researcher examines a sample of quagga mussels collected from Lake Mead. A recent study by Bai and Acharya found that endocrine disrupting chemicals are accumulating in the body tissues of these mussels. Credit: Xuelian Bai.

Researcher examines a sample of quagga mussels collected from Lake Mead. A recent study by Bai and Acharya found that endocrine disrupting chemicals are accumulating in the body tissues of these mussels. Credit: Xuelian Bai.

“Algae sit at the base of the food web, thereby providing food for organisms in higher trophic levels such as quagga mussels and other zooplantkons,” Bai said. “Our study clearly shows that there is potential for these contaminants to biomagnify, or build up at higher levels of the food chain in the aquatic ecosystem.”

Bai is now working on a new study looking for antibiotic resistance in genes collected from the Las Vegas Wash, as well as a study of microplastics in the Las Vegas Wash and Lake Mead. Although Las Vegas’s treated wastewater meets Clean Water Act standards, Bai hopes that her research will draw public attention to the fact that treated wastewater is not 100 percent clean, and will also be helpful to utility managers as they develop new ways to remove untreated contaminants from wastewater prior to release.

“Most wastewater treatment plants are not designed to remove these unregulated contaminants in lower concentrations, but we know they may cause health effects to aquatic species and even humans, in large concentrations,” Bai said. “This is concerning in places where wastewater is recycled for use in agriculture or released back into drinking water sources.”

Bai’s research was funded by the Desert Research Institute Maki Endowment, the U.S. Geological Survey, and the Nevada Water Resources Research Institute. The studies mentioned in this release are available from Environmental Pollution and Environmental Science and Pollution Research journals:

Bai, X. and Kumud Acharya. 2019. Removal of seven endocrine disrupting chemicals (EDCs) from municipal wastewater effluents by a freshwater green alga. Environmental Pollution. 247: 534-540. Available: https://www.sciencedirect.com/science/article/pii/S0269749118347894

Bai, X. and Kumud Acharya. 2018. Uptake of endocrine-disrupting chemicals by quagga mussels (Dreissena bugensis) in an urban-impacted aquatic ecosystem. Environmental Science and Pollution Research. 26: 250-258. Available: https://link.springer.com/article/10.1007/s11356-018-3320-4


The Desert Research Institute (DRI) is a recognized world leader in basic and applied interdisciplinary research. Committed to scientific excellence and integrity, DRI faculty, students, and staff have developed scientific knowledge and innovative technologies in research projects around the globe. Since 1959, DRI’s research has advanced scientific knowledge, supported Nevada’s diversifying economy, provided science-based educational opportunities, and informed policy makers, business leaders, and community members. With campuses in Reno and Las Vegas, DRI serves as one of eight institutions in the Nevada System of Higher Education.