An in vitro
antibiotic susceptibility assay for Staphylococcus aureus biofilms developed on 96-well
polystyrene tissue culture plates was performed to elucidate the activity of citropin 1.1,
rifampin and
minocycline. Efficacy studies were performed in a rat model of staphylococcal CVC
infection.
Silastic catheters were implanted into the superior cava. Twenty-four hours after implantation the
catheters were filled with citropin 1.1 (10 microg/mL). Thirty minutes later the rats were challenged via the CVC with 1.0 x 10(6) CFU of S. aureus strain Smith diffuse. Administration of
antibiotics into the CVC (the
antibiotic lock technique) began 24 h later. The study included: one control group (no CVC
infection), one contaminated group that did not receive any
antibiotic prophylaxis, one contaminated group that received citropin 1.1-treated CVC, two contaminated groups that received citropin 1.1-treated CVC plus
rifampin and
minocycline at concentrations equal to MBCs for adherent cells and 1024 microg/mL in a volume of 0.1 mL that filled the CVC and two contaminated groups that received
rifampin or
minocycline at the same concentrations. All
catheters were explanted 7 days after implantation. Main outcome measures were: minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC), synergy studies, quantitative culture of the biofilm formed on the
catheters and surrounding venous tissues, and quantitative peripheral blood cultures. MICs of conventional
antibiotics against the bacteria in a biofilm were at least four-fold higher than against the freely growing planktonic cells. In contrast, when
antibiotics were used on citropin 1.1 pre-treated cells they showed comparable activity against both biofilm and planktonic organisms. The in vivo studies show that when CVCs were pre-treated with citropin 1.1 or with a high dose of
antibiotics, biofilm bacterial load was reduced from 10(7) to 10(3) CFU/mL and
bacteremia reduced from 10(3) to 10(1) CFU/mL. When CVCs were treated both with citropin 1.1 and
antibiotics, biofilm bacterial load was further reduced to 10(1) CFU/mL and
bacteremia was not detected, suggesting 100% elimination of
bacteremia and a log 6 reduction in biofilm load. Citropin 1.1 significantly reduces bacterial load and enhances the effect of hydrophobic
antibiotics in the treatment of CVC-associated S. aureus
infections.