A model for testing drug susceptibility of Pseudomonas aeruginosa and Staphylococcus aureus grown in biofilms on medical devices.

UV-sterilized polyethylene rings infected with Pseudomonas aeruginosa or with Staphylococcus aureus were implanted into artificial wounds of mice. During 5 days incubation biofilm was formed on the plastic surfaces. To determine the Minimal Bactericidal Dose on sessile bacteria, rings with biofilm were removed and incubated in Luria broth containing serial dilutions of different antibiotics. Parallel, the sensitivities of planktonic phase organisms were also determined using cells grown in broth. The biofilm mode of growth strongly reduced the sensitivity of the strains against most of the antibiotics used, especially against polymyxin B. On the other hand, beta-lactam type antibiotics were equally effective against bacteria both in the sessile and planktonic phase of growth.