According to Dr. David Wendell, professor of Environmental Engineering at the University of Cincinnati, and corresponding author for the study, “the immediate application of the technology is to remove antibiotics that end up in lakes and rivers. However, the proteoliposome platform is versatile. It can also be adapted to remove other contaminants including hormones and heavy metals.”
Every year, large quantities of antibiotics are released into lakes and rivers as a byproduct of their use in farming and medicine. Antibiotics in the environment can select for antibiotic-resistant bacterial populations, which can cause severe infections if they come into contact with humans or animals. Recent evidence also suggests that some types of antibiotics may lead to infertility. In addition, antibiotics can disrupt the natural bacterial flora that plays a vital role in maintaining the balance of ecosystems.
“Activated carbon is the most common substrate used to remove organic contaminants, including antibiotics, from water” explains Wendell. “But, the problem with activated carbon is that is it not specific, meaning that it binds many different types of organic molecules.” Antibiotics, which are usually present at low concentrations, can escape capture by activated carbon, which is ‘blocked’ by the abundant organic compounds.
As an alternative to activated carbon, Wendell and his team developed lipid vesicles embedded with ‘AcrB’. AcrB is a membrane transport protein that is produced in some strains of bacteria including e. coli and salmonella. It has evolved to protect these bacteria from toxins, including antibiotics, by actively pumping them out of the bacterial cytosol. By embedding AcrB in the membrane of lipid vesicles so it faces opposite its natural orientation, AcrB pumps antibiotic molecules into the vesicle, which then acts as a reservoir.
AcrB is an ‘active’ membrane pump that uses a proton gradient as an energy source. Usually, this proton gradient is established by bacterial cell metabolism. To power AcrB using a natural energy source, Wendell and his team fused it with ‘delta-rhodopsin’ (dR) which uses sunlight to establish a proton gradient. dR allows the proteoliposomes to extract antibiotics from contaminated water without an additional source of chemical energy.
To see how well their constructs performed under realistic conditions, Wendell and his team spiked two model antibiotics: ampicillin and vancomycin into natural river water. In the presence of sunlight, the proteoliposomes removed nearly twice a many antibiotic molecules as activated carbon.
“The upper limit on the storage capacity of the proteoliposomes is set by leakage of the AcrB pump. As the concentration of antibiotic molecules in the proteoliposome increases, the molecules start to diffuse back out through AcrB” explains Wendell. The 200nm proteoliposomes have an antibiotic capacity of about 9ug/ml.
Recovery of the proteoliposomes from contaminated water is a potential challenge. Wendell points out that, in practice, they could be included within floating hydrogels or semipermeable membranes. “By siting on the surface of the water, the proteoliposomes would have access to sunlight. It would also make them easy to collect after the purification is finished.”
Antibiotic molecules could be recovered from the proteoliposomes by solubilizing them with mild detergents. Recovered antibiotics could be re-used, which would partially off-set the cost of implementing the technology. If the solubilization treatment is mild enough, the proteoliposomes could be re-used multiple times. In contrast, once organic molecules are adsorbed to activated carbon, they must be “burned off” at high temperatures. This takes energy, which usually comes from burning fossil fuels.
Innovative manufacturing strategies may also help to reduce the costs of the technology. For example, specially-designed bacteria could be used to both produce the AcrB fusion protein and to serve as a storage system.
The AcrB proteoliposomes could also be used to capture other contaminant molecules. For example, estradiol hormones, which are the active ingredient in birth control pills, can also be pumped by AcrB. By substituting other transport proteins for AcrB, chemically-diverse pollutants, including heavy metals, could be extracted.
Moving forward, it is likely that the amounts and diversity of pollutants entering natural water supplies will increase in the coming years. Technological innovations such as proteoliposomes offer an efficient and environmentally-friendly way to counteract the problem.
By Carl Walkey, Integrated Nanotechnology & Biomedical Sciences Laboratory, University of Toronto, Canada.