Combining Visible Light and Non-Focused Ultrasound Significantly Reduces Propionibacterium acnes Biofilm While Having Limited Effect on Host Cells

Mark E Schafer,Tessie Mcneely

doi:10.3390/microorganisms9050929

Abstract

Bacterial biofilms are highly resistant to antibiotics and have been implicated in the etiology of 60%–80% of chronic microbial infections. We tested a novel combination of low intensity ultrasound and blue light against biofilm and planktonic bacteria. A laboratory prototype was built which produced both energies uniformly and coincidently from a single treatment head, impinging upon a 4.45 cm2 target. To demonstrate proof of concept, Propionibacterium acnes biofilms were cultured on Millicell hanging inserts in 6-well plates. Hanging inserts with biofilms were treated in a custom exposure chamber designed to minimize unwanted ultrasound reflections. Coincident delivery of both energies demonstrated synergy over either alone, killing both stationary planktonic and biofilm cultures of P. acnes. Reduction in biofilm bacteria was dose dependent on exposure time (i.e., energy delivered). P. acnes biofilms were significantly reduced by dual energy treatment (p < 0.0001), with a >1 log10 reduction after a 5 min (9 J/cm2) and >3 log10 reduction after a 30 min (54 J/cm2) treatment (p < 0.05). Mammalian cells were found to be unaffected by the treatment. Both the light and the ultrasound energies are at levels previously cleared by the FDA. Therefore, this combination treatment could be used as a safe, efficacious method to treat biofilm related syndromes.

Highlights

Bacteria in biofilms have a dramatically altered phenotype compared to planktonic bacteria with respect to growth rate and gene transcription
In part, to the damping down of their metabolism, biofilm encased bacteria are highly resistant to the effects of antibiotics, biocides and other drugs which are used to eliminate planktonic bacteria [2]
Low intensity ultrasound did not induce observable bactericidal activity when planktonic bacteria were exposed for 20 min

Summary

Introduction

Bacteria in biofilms have a dramatically altered phenotype compared to planktonic bacteria with respect to growth rate and gene transcription. The biofilm phenotype differs from that of purely sessile cells, such as may be seen growing on agar plates [1]. Many genes necessary for planktonic metabolism are turned off, since the cells in a biofilm are not rapidly dividing [2,3]. Other genes responsible for the biofilm phenotype are upregulated. In part, to the damping down of their metabolism, biofilm encased bacteria are highly resistant to the effects of antibiotics, biocides and other drugs which are used to eliminate planktonic bacteria [2]. Chronic use of antibiotics leads to drug resistance. When considering development of new treatment approaches, there are justifiable reasons to consider treatments that do not rely on drug entities

Methods

Results

Conclusion