Persistent lung infections, chronic wounds, and healthcare-associated infections are typically much more difficult to treat than other types of bacterial infections. This is because they are often caused by biofilms, that is, colonies of microbes — mainly bacteria — that grow together in a self-produced matrix that protects and isolates them from the external environment.
Now, a new triple-acting antibiotic has managed to break through the biofilm’s extracellular matrix — a protective structure built by bacteria — and eliminate more than 50% of pathogens at once, according to a study published in the journal npj Biofilms. and Microbiomes. The study was led by Eduard Torrents, lecturer in the Department of Genetics, Microbiology and Statistics of the Faculty of Biology and head of the group on bacterial infections: antimicrobial therapy of the Institute of Bioengineering of Catalonia (IBEC).
This extracellular matrix exacerbates antibiotic resistance, one of the biggest threats to global health, according to the World Health Organization (WHO), as killing the bacteria in the film is up to a thousand times more difficult. Biofilm infections are therefore the main non-specific mechanism of antimicrobial resistance.
Attacking these microbes with antibiotics alone is not enough. There is a need for tools that break down the extracellular matrix to access and kill the bacteria within. The lead author of this study, which has reached this milestone, is Núria Blanco-Cabra, postdoctoral researcher of the group led by Eduard Torrents at the UB and IBEC. The study was conducted together with scientists from CIDETEC (Basque Country).
A triple-acting drug combination
The research focused on the bacterium Pseudomonas areuginosa, a pathogen that often grows in biofilms in the lungs of patients with cystic fibrosis or chronic obstructive pulmonary disease (COPD), which causes persistent infections. “We have grown biofilm cultures in vitro, using a technique similar to the way they exist and grow in nature,” Torrents added. In clinical practice, these infections are usually treated with an antibiotic called tobramycin. However, its effectiveness is limited by its inability to penetrate the film. This happens because tobramycin, which is positively charged, is neutralized by the extracellular matrix (with a negative charge).
The researchers loaded the antibiotic into negatively charged nanoparticle carriers. This could neutralize the positive charge before the drug reached the biofilm, allowing it to break down the extracellular matrix and kill the bacteria within it. More importantly, these carriers — built by single-chain dextran-based nanoparticles — were able to transport up to 40% of the antibiotic’s weight. “Many of the previously studied nanotransporters have been able to tolerate only a small load of the target compound, which has prevented their clinical use. We managed to overcome this obstacle,” Torrents says.
The antibiotic-loaded nanocarriers were also coated in an enzyme called DNase I. One of the compounds that holds bacteria’s biofilms together is structural DNA found throughout the extracellular matrix. DNase I can break down this “glue”, loosening the matrix and allowing the antibiotic to penetrate even further into the biofilm.
“By combining an antibiotic with a number of agents that break up the biofilm, we have developed a drug that is more potent than the antibiotic alone that kills the bacteria living in the film,” said Eduard Torrents, lead researcher on the study.
Using microscopy images, the researchers found that the new drug not only dissolved the structural DNA in the extracellular matrix, but also acted on and killed the bacteria inside. With just a single application, they reduced bacterial biomass by more than half.
“After such significant biofilm removal with a single dose of our drug, we predicted that a full course of antibiotics could significantly reduce the burden of these extremely difficult-to-treat infections.”
New hope for persistent infections
In future clinical applications, this agent could be administered in multiple doses, as is common with antibiotics. The next step is to work on the clinical validation of this system. Its commercialization would represent a decisive advance in the treatment of biofilm infections, the global economic cost of which is currently $4 billion per year.
Blanco-Cabra, N.; Movellan, J.; Marradi, M.; Gracia, R.; Salvador, C.; Dupin, D.; Loinaz, I.; Torrents, E. “Neutralization of ionic interactions by single-chain dextran-based nanoparticles enhances the diffusion of tobramycin in a mature biofilm”. npj Biofilms and Microbiomes, July 2022. DOI: 10.1038/s41522-022-00317-9