Eradication of biofilm cells of Staphylococcus aureus with tobramycin and cephalexin.

The kinetics of growth and formation of biofilm by Staphylococcus aureus were investigated under iron-limited conditions in the chemostat. The population of planktonic cells reached 5.5 x 10(9) cells/mL 24 h after inoculation (D = 0.05 h-1) and remained constant throughout. The number of biofilm cells of S. aureus colonizing the silicone tubing increased exponentially from 6 x 10(4) to 2.7 x 10(7) cells/cm2 (6 days later) and continued to increase at a reduced rate to 2.7 x 10(8) cells/cm2 on day 13. Planktonic cells of S. aureus were susceptible to tobramycin and cephalexin. The planktonic cells could be successfully eradicated with a combination of 5 micrograms tobramycin plus 100 micrograms cephalexin per millilitre. Exposure of young biofilm cells of S. aureus to 5 micrograms tobramycin plus 100 micrograms cephalexin per millilitre resulted in a rapid loss of cell viability. The percentage of survival dropped to less than 0.0001% after exposure to these concentrations of antibiotics for 3 h. Old biofilm cells of S. aureus were found to be extremely resistant to these antibiotics. The cell viability was reduced to 0.09% after exposure to 10 micrograms tobramycin plus 100 micrograms cephalexin per millilitre. The results suggest that it is possible to eradicate S. aureus infection at the early stage with tobramycin plus cephalexin. Any delay in implementing antibiotic therapy is likely to result in the failure of the treatment. It is important to note that the concentrations of antibiotics required for the eradication of young biofilm cells must be determined for the treatment of device-associated infections.