Role of transport limitation in the resistance of Pseudomonas aerunginosa biofilms to ciprofloxacin and levofloxacin

Abstract

Bacterial biofilm infections are often far less susceptible to antibiotic therapy than their planktonic, or freely suspended counterparts. Transport limitation of the antibiotic within the biofilm has been proposed as one explanation for this resistance. To explore the possibility that transport limitation contributes to a decreased antibiotic efficacy, a Comparison was made between the efficacies of two fluoroquinolone antibiotics, ciprofloxacin and levofloxacin, against freely suspended and biofilm bacteria. Transport of the antibiotics to the biofilm-substratum interface was monitored using ATR-FTIR spectroscopy techniques. Both biofilms and planktonic organisms were treated with antibiotic concentrations of 100, 250, and 500 μg ml-1 for 30 min and rinsed for 1 h with fresh media. Antibiotic efficacy was determined by the ratio of culturable bacteria to total cell counts. The experimental ATR-FTIR transport data was then simulated with a mathematical computer model containing the processes of molecular diffusion, adsorption, and desorption to provide possible explanations for differences in antibiotic delivery. Levofloxacin was found to be more efficacious against planktonic organisms than biofilm cells. However, no difference in efficacy was seen when planktonic and biofilm bacteria were treated with ciprofloxacin. ATR-FTIR results showed that the biofilm provided very little transport limitation for each antibiotic, thus suggesting that the reduced susceptibility of a biofilm to antimicrobial treatment may be due to factors other than transport.