HYPERTONIC TREATMENTS ENHANCE THE EFFICACY OF ANTIBIOTICS AGAINST BACTERIAL BIOFILM COMMUNITIES

Abstract

Biofilm is a structured community of bacterial cells enclosed in a self-produced matrix of extracellular polymeric substance (EPS). Biofilms have long been implicated in bacterial infections that are mostly untreatable. The bacterial communities within biofilm can be up to 1,000 times more resistant to antimicrobial agents compared to planktonic cultures. The mechanisms of protection are varied, but the EPS surrounding bacterial communities likely reduces penetration of antibiotics to the bacteria. Disrupting this protective barrier is key to addressing this problem. Hypertonic concentrations of osmotic compounds could play an important role in biofilm treatment by shrinking biofilm volume and thereby reducing diffusion distances. In this study we evaluated how osmotic compounds [maltodextrin, sucrose, and polyethylene glycol (PEG)] enhance antibiotic efficacy against Acinetobacter baumannii biofilm communities. Established biofilms (24 h) were treated with osmotic compounds in the presence or absence of 10X the minimum inhibitory concentration of different antibiotics. Combining antibiotics and hypertonic concentrations of osmotic compounds reduced the cell count of biofilm communities by 5-7 log (P <0.05). The effect was improved with increasing concentrations of osmotic compounds, but only for relatively small mass compounds. PEG (400 Da) (small mass compound) with tobramycin demonstrated generalizable effect against eleven different A. baumannii strains. Multivariate regression models showed that antibiotic mass and lipophilicity (r2 > 0. 0.82; P <0.002) are important predictors for reduced cell recovery. A similar relationship was evident for biofilm formed by E.coli K-12. Biofilms were further treated with a hypertonic solution followed by a hypotonic solution (distilled water) containing antibiotic to draw antibiotic into the biofilm matrix. Depending on the treatment combination, this sequential treatment reduced the cell counts by 2-7 log (P < 0.05). Relative to tobramycin treatment alone, the efficacy of sequential treatment was evident for all osmotic compounds (P < 0.05). Sequential treatment with erythromycin or chloramphenicol worked, but only when the concentration of antibiotic was increased from 10X to 20X of the minimum inhibitory concentration. This study supports the clinical evaluation of the combinatorial and sequential strategies of osmotic treatments and antibiotics against wound biofilm infections.