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Antibiotic resistance poses a growing threat to human health around the world, as drugs that were once humanity’s frontline defense against common infections are proving useless against virulent new strains of superbugs.
Overuse and misuse of antibiotics in agricultural and health care settings are the root causes of this resistance. But there is an environmental dimension to the problem as well, says Tala Navab, assistant professor of environmental engineering at Oregon State University.
Navab is a contributing author to a landmark paper, published in the May 2018 issue of Environmental Engineering Science, that aims to establish parameters and priorities for the discipline as it designs its approach to antibiotic resistance.
Navab’s own research investigates the growth and persistence of enteric pathogens as they move through the environment — from wastewater systems to waterways, into soils, onto fields, into crops and, potentially, onto our dinner plates.
“Septic systems are one of the main reservoirs of antibiotic resistance,” Navab said. “When people take antibiotic drugs, they don’t get completely metabolized. So bacteria start to develop resistance in the gut. Then these bacteria, along with the unmetabolized antibiotics, are passed from the body into wastewater systems, such as septic tanks, where prolonged exposure creates additional selective pressure making the bacteria more resistant.”
In the United States, most homes are connected to municipal sewers that carry wastewater directly to centralized treatment plants. Even in homes that rely on septic tanks (roughly one-fifth), the resulting sludge is commonly disposed of in wastewater treatment facilities, or in well-isolated landfills. However, in less industrially developed parts of the world, sanitation practices are typically not as rigorous.
“Studies have shown that in Vietnam the majority of the septic sludge never makes it to a landfill,” Navab said. “Instead, it is dumped directly into some sort of receiving environment — rivers, canals, other types of waterways, parks, fields — all of that. Sometimes it is applied directly to agricultural soil as fertilizer. These are all environmental reservoirs that people have access to. They’re swimming in them, fishing in them, playing in them, growing food in them — and they all have fecal contamination.”
One of Navab’s current projects is a first-of-its-kind study examining the prevalence of multiple-antibiotic-resistant enteric bacteria in septic systems and contaminated soils. She is collaborating with Mi Nguyen, research group leader at NTT Hi- Tech Institute, Nguyen Tat Thanh University, in Vietnam.
Vietnam is an ideal location for this type of study. The country has one of the highest rates of antibiotic resistance in the world. Antibiotics are easy to obtain, and improper use is rampant. Additionally, about 80 percent of the households rely on septic tanks for sanitation, even in developed urban areas like Ho Chi Minh City (also known as Saigon), the country’s largest metropolis, with some 12 million residents.
Genevieve Schutzius, an environmental engineering graduate student working with Navab, spent three months in Ho Chi Minh City during the winter of 2018. She collected samples of canal water and adjacent soils from 55 areas in 12 districts of the city. Bacteria from these samples were characterized, genotypically and phenotypically, on their antibiotic resistance content and compared to samples taken directly from septic tanks.
“We have data on the susceptibility of enteric bacteria collected from reservoirs containing nine different antibiotics that span four different mechanisms of action,” Schutzius said. “Our preliminary results found that the majority of our environmental samples contained bacteria that are resistant to two or more antibiotics, with some resistant to as many as eight antibiotics. The prevalence of resistance was somewhat higher in the septic sludge, which was expected, but the difference was not that significant.”
Navab says the purpose of this research is twofold.
“First, we need to get a comprehensive overview of the problem so we can clearly state what it is: Where and how are the prevailing practices creating conditions that favor antibiotic resistance?” Navab said. “Then we can start looking at solutions, like improving septic system design and incorporating better treatment methods, such as anaerobic digesters and other technologies.”