Secondary infections from bacteria such as Escherichia coli and Staphylococcus aureus are cause for concern within hospitals. Medical devices such as catheters can become coated in biofilms made of these bacteria. Currently, most protocols to combat the biofilms have been reactive, antibiotics and other chemicals used to kill bacteria that have already attached. But a study published in Nature Biotechnology has taken aim at a more proactive approach: surfaces that stop bacteria from attaching in the first place.
How bacteria attach to surfaces is poorly understood, says Morgan Alexander, professor of biomedical surfaces at the University of Nottingham in the United Kingdom. Because the interaction between bacteria and surface is so poorly defined, identifying a bacteria resistant polymer would require screening a vast number combinations, a daunting task which has prevented large-scale investigation in the past. But Alexander and his team developed a high-throughput microarray assay technique with Daniel Anderson and Robert Langer at MIT to screen nearly 600 combinations of acrylate monomers on a single glass slide.
“We didn’t know enough about the way bacteria respond to materials in general to be able to make a [targeted experimental] approach,” says Alexander. “So we wanted to cast our net as widely as possible and use chemicals that we could readily lay our hands on.”
The 22 acrylate monomers chosen were chemically diverse, with representatives exhibiting different ethylene glycol chains, aromatic and amine moieties, fluoro-substituted alkanes and more. These monomers were mixed together at varying ratios, and 300-µm dots of the mixtures were printed on a microscope slide. The slides were then incubated with strains of E.coli, S. aureus and Pseudomonas aeruginosa that had been tagged with fluorescent proteins. When viewed under a confocal microscope, the glowing dots would indicate polymers that bacteria attached to. Alexander and his team were looking for the dark spots on the slide.
Multiple generations of these arrays were utilized using chemical high throughput surface chemical analysis to identify copolymers with strong bacteria resistance properties. Investigation into possible reasons for resistance eliminated common factors such as surface elemental composition, wettability and surface roughness. In the end, surface chemistry seemed to play the largest role in preventing bacteria from attaching. Those monomers with secondary ions from the cyclic carbon groups, ester groups, tertiary butyl moieties, and ions from aliphatic groups prevented bacteria from attaching.
Three of the more successful combinations were tested as coating on catheters to compare against a plain silicone catheter and a state-of-the-art silver-coated catheter. One of polymer coatings led to 67-fold reduction of S. aureus compared to a silicone catheter, and a 30-fold reduction compared to the silver-coated one. Other polymers were also successful at lowering in the incidence of attachment for other strains of bacteria. When the catheters were inserted into mice, the animals with the polymer-coated catheters had an order of magnitude reduction of bacteria in the tissue around the implant, as well as lower numbers in the kidney and spleen.
Now that the proof of concept has been established, says Morgan, this high throughput method can be used to investigate other polymers for fields were biofilm formation is an issue, such as water purification systems, food safety and maritime fouling.