Macromolecules: Weapons in the Fight Against Antibiotic Resistance

By targeting the bacterial membrane and potentiating the activity of conventional antibiotics, two macromolecules have demonstrated a novel approach to treating topical Gram-negative bacterial infections, according to a study recently published in PLoS One.

“Bacterial biofilms are the root cause of recurring infections,” wrote the study’s authors. Bacteria within the biofilms are metabolically dormant, rendering conventional antibiotics against Gram-negative bacteria ineffective. Moreover, this dormant bacterial population reactivates post-antibiotic treatment. New strategies are therefore needed, the authors noted, to treat chronic bacterial infections.

One strategy is to disrupt the bacterial membrane, which cationic and hydrophobic macromolecules have demonstrated in previous studies. These molecules also reportedly delay or prevent development of bacterial resistance. Given these characteristics, the current study’s authors hypothesized that such compounds could also disrupt Gram-negative bacterial biofilms.

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The authors performed a series of in vitro and in vivo experiments to evaluate the activity of two macromolecules (Qn-prAP, QCybuAP) against several Gram-negative bacterial species (E. coli, Acinetobacter baumannii, and Klebsiella pneumoniae). Antibacterial activity of conventional antibiotics, alone and in combination with the macromolecules, was also evaluated.

In Vitro Studies

Activity against dormant, antibiotic-tolerant, and actively growing bacteria

Compared with conventional antibiotics like ampicillin, both macromolecules significantly reduced counts of E. coli antibiotic-tolerant cells, with complete killing occurring at very low concentrations. Both compounds also completely killed dormant and actively growing A. baumannii. Notably, at low concentrations, QCybuAP killed all dormant E. coli.

Interestingly, colistin, a last-resort treatment for multidrug resistant Gram-negative bacterial infections, significantly reduced counts of actively growing, but not antibiotic-tolerant or dormant, E. coli.

Membrane disruption

At very low concentrations, both macromolecules disrupted the bacterial membrane of E. coli antibiotic-tolerant cells, indicated by reduced membrane potential and increased membrane permeability. QCybuAP also demonstrated membrane permeability against actively growing E. coli. These findings, the authors noted, suggested the macromolecules’ membrane-active mechanism of action.

Biofilm Disruption

Using laser scanning microscopy, authors observed marked disruption of E. coli and A. baumannii biofilms. This disruption was most effective with macromolecule + erythromycin treatment, rather than either treatment alone.

With bacterial biofilm disruption, the once-enclosed bacteria can disperse, replicate, and cause major tissue damage. In this study, both macromolecules, alone and combined with erythromycin, effectively prevented this replication and killed the dispersed cells.

Antibiotic Potentiation

Gram-negative bacteria use efflux pumps to pump out tetracyclines, thereby acquiring resistance. Given the improved biofilm disruption and bacteria-killing with combination treatment in this study, authors supposed that the macromolecules potentiate antibiotic activity by reducing efflux through membrane depolarization. This potentiation hypothesis will need further investigation, the authors acknowledged.

Resistance

QCybuAP either prevented or delayed resistance to erythromycin and rifampicin by E. coli and A. baumannii.

In Vivo Studies in Mice

Safety

Injection of macromolecules, alone and in combination with antibiotics, caused minimal toxicity.

Efficacy

Authors created small burn wounds and infected them with A. baumannii. Although bacterial count was reduced with each macromolecule and antibiotic alone, macromolecule + erythromycin or rifampicin treatment reduced bacterial counts to near or below minimal detection limits. Similarly, in surgical wounds infected with K. pneumoniae, combination therapy most markedly reduced bacterial counts.

Notably, colistin was as effective as the combination therapy. However, the authors noted, bacterial resistance to colistin, compared with minimal resistance to the macromolecules, is a drawback.

Histopathology revealed variable healing according to treatment. For example, burn wounds receiving combination therapy had squamous epithelial cell regeneration and neutrophil infiltration beneath fibrous tissue.

Bringing it Together

Given the study’s findings, the authors concluded that combination therapy with membrane-active macromolecules and conventional antibiotics shows great therapeutic potential for topical Gram-negative bacterial infections.

Dr. JoAnna Pendergrass received her Doctor of Veterinary Medicine degree from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, a medical communications company.