In recent years, increasing numbers of patients worldwide have contracted severe bacterial infections that are untreatable by most available antibiotics. Some of the gravest of these infections are caused by bacteria carrying genes that confer resistance to a broad class of antibiotics called β-lactams, many of which are treatments of last resort. Now a research team reports that some wastewater treatment plants in China discharge one of these potent resistance genes into the environment (Environ. Sci. Technol. Lett. 2013, DOI: 10.1021/ez400152e). Environmental and public health experts worry that this discharge could promote the spread of resistance.
When Pedro J. J. Alvarez of Rice University and his collaborators detected abundant antibiotic resistance genes in China’s Haihe River several years ago (Environ. Sci. Technol. 2010, DOI: 10.1021/es100233w), they wanted to find out how the genes were reaching the environment. So they decided to examine wastewater treatment plants in the region, because antibiotic resistance genes are widespread in fecal bacteria. The treatment plants cultivate bacteria in the incoming water to break down sewage and industrial wastewater. The water is then disinfected with chlorine and released into local waterways. Farmers sometimes use treated sludge, the solid by-product of the treatment process, as a fertilizer on their fields.
The researchers, including Daqing Mao of Tianjin University, in China, extracted bacterial DNA from water at various treatment stages in two wastewater treatment plants in northern China. They also took samples of treated sludge. Using polymerase chain reaction (PCR), the team measured the abundance of a potent antibiotic resistance gene known as New Delhi metallo-β-lactamase, or NDM-1, and a gene carried by all bacteria called 16S rRNA. NDM-1 first appeared in 2008 and has already spread to every continent except Antarctica. It allows bacteria to withstand almost all β-lactam drugs.
The PCR revealed that the treated water released by both plants contained thousands of copies of the resistance gene per milliliter, and the treated sludge contained much higher concentrations of about 10 million copies per gram of dry weight. In one of the treatment plants, the total number of NDM-1 genes leaving the plant was greater than that coming in. Alvarez says the NDM-1 genes measured in the study could be carried by living bacteria or represent extracellular DNA expunged by dead bacteria.
To test whether the resistance gene could be passed on to other bacteria species in the environment, the researchers added NDM-1-carrying bacteria from the treatment plant to sediments collected from the Haihe River. Bacteria in these sediments did not harbor the gene.
After incubating the samples for nine days, the researchers isolated sediment bacteria carrying NDM-1, indicating that the gene had been transferred. The NDM-1 gene sits on a circular strand of DNA called a plasmid, which makes it easy for it to jump to other bacteria.
The study “sounds a huge alarm,” says Jeffrey Duchin, chair of the public health committee of the Infectious Diseases Society of America. The more prevalent these β-lactam resistance genes are in the environment, he says, the more opportunity there is for them to spread to other organisms.
Duchin points out that serious β-lactam-resistant infections, such as the so-called carbapenem-resistant infections, are primarily caused by species of bacteria found in hospitals. So, at the moment, hospital patients are the most at risk of contracting one of these infections. But if NDM-1 were to spread to bacteria outside hospitals that cause common ailments such as urinary tract infections and diarrhea, these conditions could become untreatable with routine antibiotics. “It would be a disaster,” he says.
Alvarez says treatment plants could reduce the risk of environmental contamination by disinfecting water more efficiently before discharge. The chlorine disinfection step at the Chinese plants could be too weak to significantly reduce the release of NDM-1. The researchers found that while the chlorination step removed the majority of the common 16S rRNA gene, it was half as effective in eliminating NDM-1. This suggests that the NDM-1 carriers may also have a way to withstand chlorine. He and his team are developing an alternative and possibly more effective disinfection method that involves ultraviolet radiation and photocatalysts.