March 2, 2016
Linnea K. Ista, Research Assistant Professor's article, "Reusable nanoengineered surfaces for bacterial recruitment and decontamination" made the Publicaton Highlight section of the March AVS Newsletter.
Researchers at the University of New Mexico (Ista), Duke University and the University of Florida describe their work in a new paper in the journal Biointerphases.
In nature, most bacteria grow attached to surfaces in communities known as biofilms. Some, including the microorganisms that decompose garbage in landfills, are helpful but others, such as the plaque that forms on teeth or the coating often found on contact lenses, can serve as reservoirs for infection. Scientific studies have even shown that genes for resisting antibiotics can be transferred between bacteria within a biofilm leading to potentially lethal "superbug" strains of disease. Materials that either avoid attachment by biofilms ("fouling resistant") or directly kill attached microbes ("biocidal") have been considered as remedies but neither has individually lived up to its promise: the former eventually foul and the latter accumulate debris that reduce their efficiency.
"Combining fouling release and biocidal materials in a single surface offers the ability to perform both functions efficiently," says biomedical engineer Linnea K. Ista, lead author of the Biointerphases paper. "To achieve this, we turned to long-chain molecules called stimuli responsive polymers, or SRPs, that attract bacteria and hold them in place, and partnered them with another class of polymers known as poly(phenylene ethynylenes), or PPEs, that kill bacteria by destroying their cell walls. When the killing is done, we can then change conditions so that the SRPs release the dead bacteria and debris."
Using fabrication techniques developed in the 1990s to rapidly manufacture computer chips, Ista and her colleagues can grow a series of parallel rows of an SRP called poly(N-isopropylacrylamide), or PNIPAAm, atop a silicon substrate. Next, they deposit alternating layers of positively charged and negatively charged PPEs in the "channels" between the SRP rows until they reach the height of the "channel walls."
"We control the SRP so that it attracts bacteria and exposes them to the positively charged PPE layer [known to be highly bactericidal] atop the filled channels," Ista says. "Once the microbes are dead and biofilm formation is avoided, we can direct the SRP to release them to yield a surface that is ready once again to perform its mission on a new population of bacteria."
In their trials, the researchers found that they could effectively use their nanoengineered antimicrobial surface at least three times. Interestingly, Ista says, they also learned that a top layer of negatively charged PPE is just as effective in killing bacteria as a positive one. "The standard wisdom is that, since the overall charge of the bacterial cell wall is negative, a positively charged biocide will interact with it," she explains. "However, we have shown that this is not necessarily the case if a negatively charged compound can be brought into close proximity."
Another finding was that the size and shape of the target cell mattered for release. "For example, a small, round Staphylococcus epidermidis released better than a large, cylindrical Escherichia coli," Ista says.
Now that the nanoengineered antimicrobial surface concept has proven viable, Ista says that the research team plans to refine the technology for more direct applications. "First off, we want to see if changing the features and spacing of the micropattern on our surface enables us to selectively retain and kill specific bacterial species such as MRSA [Methicillin-resistant Staphylococcus aureus] and other antibiotic-resistant strains," she says.