The spread of antibiotic-resistant bacteria is a major threat to public health. These bacteria can reach the environment when wastewater leaves treatment plants, because they often survive conventional treatment processes. A new study shows that these pathogenic bacteria can be killed cheaply and quickly by a combined treatment with sunlight and bacteria-killing viruses called bacteriophages (Environ. Sci. Technol. 2018, DOI: 10.1021/acs.est.8b04501).
Pei-Ying Hong and her colleagues at King Abdullah University of Science and Technology are developing strategies to remove antibiotic-resistant bacteria from wastewater in Saudi Arabia, where water scarcity has led to efforts to reuse wastewater for irrigation and other purposes. Last year, the group explored solar disinfection as a cheap, sustainable way to remove the bacteria from such wastewater (Environ. Sci. Technol. 2017, DOI: 10.1021/acs.est.6b05377). Ultraviolet radiation can kill bacterial cells by damaging their DNA and also reacts with dissolved organic matter in wastewater to produce reactive oxygen species that can harm the cells.
The team tested this method on Escherichia coli carrying the notorious antibiotic-resistance gene NDM-1, which protects bacteria against most β-lactam antibiotics including carbapenems, a class of antibiotics of last resort. The researchers found that it took about three times as long for the sunlight to start killing these bacteria as it took to start killing nonpathogenic bacteria because the pathogenic ones mounted a protective response by expressing certain genes, including those that repair the cell wall. But Hong and her colleagues also noticed that the pathogenic bacteria responded to sunlight by reducing the expression of genes that helped defend them against bacteriophages. Other researchers have shown in lab tests that bacteriophages selectively kill antibiotic-resistant bacteria (Environ. Sci. Technol. Lett. 2017, DOI: 10.1021/acs.estlett.7b00045).
As a result, the scientists thought that combining bacteriophage attack with sunlight might speed up the disinfection. To test their combined strategy, Hong and coworkers first had to identify bacteriophages that would feast on E. coli carrying the NDM-1 gene. They collected wastewater from a local treatment plant, filtered it to remove the majority of bacteria, and then inoculated it with the E. coli to serve as food for bacteriophages naturally present in the wastewater. Then they grew the culture on agar plates and isolated colonies of the E. coli strain that were being eaten by phages. From these, they selected several phages that were least damaged by exposure to simulated sunlight to find those that were most likely to thrive in their combination test. Finally, they treated the antibiotic-resistant bacteria with a cocktail of these resulting phages in the solar simulator.
Under this combined treatment, it took half as long for sunlight to begin killing the pathogenic bacteria—only 2 hours compared with 4 hours of sunlight without phages. The rate of disinfection was similar in both cases; it took about 10 hours to bring bacterial concentrations down by four orders of magnitude to levels naturally present in the environment.
Pedro J.J. Alvarez, an environmental engineer at Rice University who has developed phage treatments in the lab to remove antibiotic-resistant bacteria for wastewater treatment applications, calls the paper “elegant” and “very encouraging” in showing the potential for phages to mitigate the propagation of antibiotic resistance in tandem with sunlight.
Part of the appeal of bacteriophages is that they are specific to their hosts and therefore are unlikely to harm ecosystems they may encounter downstream, the researchers say. At the same time, the bacteriophages cultivated in this work were so highly specific to the single strain of drug-resistant E. coli used that future work may seek strategies to target a broader set of bacteria.