Disruption of Bacterial Biofilms by Photothermal Energy to Overcome Antibiotic Resistant Phenotype

There is an unmet need to effectively clear bacterial biofilms, which often cause infections with an antibiotic-resistant phenotype. Biofilms pose an especially acute challenge for combat wound care, as these injuries are often characterized by complex tissue damage that complicates current debridement-based treatment protocols. Targeted localized temperature elevation has been shown to be an effective approach to disrupt cellular membranes. Here, the researchers demonstrated the use of near infrared (NIR) irradiation of gold nanomaterials to elicit the conversion of light to heat energy, and thereby disrupt bacterial biofilms, which rendered them susceptible to antibiotics.

Staphylococcus epidermidis biofilms were established on tissue-culture treated 96-well plates, and incubated with: (1) nothing (control), (2) gold nanorods (Au-NR), (3) Au-NR in conjunction with NIR irradiation, (4) chitosan-coated Au-NR (chitosan-Au-NR), or (5) chitosan-Au-NR in conjunction with NIR irradiation. Following exposure to NIR, biofilms were incubated overnight in tryptic soy broth in the presence of antibiotic linezolid (128 µg/mL) at 37°C. The biofilms then were scraped, resuspended, and plated in order to quantify bacterial recovery under each experimental condition. Crystal violet staining was used to determine any remaining biofilm in each well.

Linezolid, an antibiotic, has been shown to have limited efficacy against S. epidermidis biofilms and its activity was not affected in the presence of Au-NR. Chitosan-Au-NR, with and without NIR irradiation, significantly increased the antibiofilm efficacy of linezolid. Chitosan-Au-NR increased bacterial inactivation by 14-fold, and the combinatorial effect of chitosan-Au-NRs and NIR irradiation was nearly 42-fold (approximately 99% bacterial inactivation). The respective temperatures of wells containing irradiated Au-NR and irradiated chitosan-Au-NR were 46 ± 1°C and 54 ± 2°C, suggesting that significantly more chitosan-Au-NR were bound at the biofilm.

Chitosan-modified Au-NR showed enhanced localized heat following exposure to NIR. This led to an enhancement of antibiofilm antibiotic efficacy, significantly decreasing the remaining number of colony-forming bacteria. It is envisioned that this approach could provide the basis for the development of a more effective antibiofilm therapeutic strategy, especially for treating wound infections.