Super Surfaces: Pulverizing Polymers Out-Muscle Pesky Pathogens

In other experiments, depicted in Figure 4, active methacrylate-AAC copolymers have been dispersed, at 1 percent concentration, into polyurethane during the production process to yield solid plastic disks possessing inherent antimicrobial properties. These disks are then placed in a vessel, with bacterial-rich broth poured on top. After 24 hours in bacterial broth, the disks are removed, and stained for fluorescence imaging, with the following results. Bacterial colonies, indicated by the green fluorescence, are clearly visible over the entire surface of the control surface, but almost completely absent from the 1 percent AAC surface.

One application of AAC polymers with tremendous potential is their use to combat Stachybortus chartarum, the “black mold,” which causes approximately $90 billion in homes and commercial building damage annually. An AAC polymer can be used as additives to paints, drywall and other construction materials to prevent the growth of this troublesome and unhealthy fungus.

Two of the most promising compounds have shown outstanding activity against a broad range of antibiotic-resistant bacteria. One is a second-generation compound with a benzene ring backbone, and the other is a third-generation molecule with higher rigidity along the backbone.

Activity against most drug-resistant bacteria, as measured by minimum inhibitory concentration, is easily within therapeutic values for most organisms. The compounds are metabolically stable during in vitro studies and blood concentrations well above the therapeutic levels can be achieved in single dose toxicity experiments in rodents.

It is valuable to keep in mind that no antimicrobial surface will completely eliminate pathogenic bacteria from food that comes into contact with it. Rather, AACs can prevent bacteria from surviving and multiplying on contact surfaces, thus greatly reducing the opportunity for cross-contamination or persistent bacterial presence.

A Question of Resistance

Microbial resistance, while principally a phenomenon encountered during antibiotic treatment in humans and animals, is of increasing concern for antimicrobial agents used in consumer and industrial products. Provided working surfaces are maintained hygienically, AACs would probably have little impact on the emergence of resistant bacterial strains on these surfaces. Where these new materials will make a difference is in food that is already contaminated with resistant bacteria, which may have proliferated because of improper usage of veterinary antibiotics. AACs, with their powerful bactericidal activity against resistant and non-resistant bacteria, would lessen the opportunity for such organisms to multiply on food-contact surfaces, where food products could be cross-contaminated and people, ultimately, could suffer foodborne illnesses.

The “broth micro-dilution” method is the industry standard for measuring resistance to common antibiotics. A broth micro-dilution with four of the lead AACs was used against Staphylococcus aureus. In this experiment, the bacterium was exposed serially in the presence of sub-effective concentrations of four AACs. Two common fluoroquinolone antibiotic drugs, ciprofloxacin (Cipro) and norfloxacin, were employed as controls. Bacteria, including S. aureus, readily develop resistance to conventional antibiotics in this model.

Growing bacteria in the presence of increasing concentrations of the drug completes the experiment. The culture tube containing the highest concentration of drug where bacterial growth is seen after 24 hours is selected and the bacteria are re-passaged with a fresh dilution series of drug. This process is repeated every 24 hours for 16 passages and the minimum inhibitory concentration (MIC), the lowest dose required to kill the bacteria) is noted at every passage. Development of resistance is indicated by a progressive increase in the MIC over time (or number of passages).

Figure 5 illustrates results of this experiment. Conventional antibiotic drugs (purple and aqua lines) show significant bacterial resistance developing after three to five passages, and increasing resistance in subsequent passages. The three AACs, whose graphs do not budge from baseline, show no tendency whatsoever, within this experimental protocol, to foster bacterial resistance (interestingly, host defense proteins show no resistance either). These results have been verified by independent investigators and repeated, with nearly identical results.

Broad Applications

Polymeric AACs can be used as additives to materials to create self-sterilizing surfaces and bactericidal products, including paints, plastics, and textiles. Some of these may be developed rapidly at relatively low cost and risk for multi-billion dollar potential market opportunities.