Antimicrobial susceptibility of Staphylococcus aureus and Staphylococcus epidermidis biofilms isolated from infected total hip arthroplasty cases.

BACKGROUND: It is difficult to treat implant-related infections because bacteria form biofilms on implants. Biofilms protect the organisms from the host's immune system and prevent penetration of antimicrobial agents. We investigated the susceptibility of six strains isolated from infected total hip arthroplasty (THA) cases to six antimicrobial agents.
METHODS: Six Staphylococcus aureus and S. epidermidis strains isolated from infected THA cases and S. aureus ACTT 25923, positive strains of biofilms, were used in this study. Biofilms were developed on 96-well titer plates. After 7 days the nutrient solution was aspirated, and wells were rinsed three times with sterile water. A series of doubling dilutions of each antimicrobial agent (16, 32, 64, 128, 256, 512, and 1024 microg/ml) in trypticase soy broth were added to the wells. The plates were incubated at 37 degrees C. After 24 h, the contents were aspirated and wells were rinsed three times with sterile water. The surfaces of wells were vigorously scraped with a sterile swab and applied to sheep blood agar plates at 37 degrees C in an aerobic chamber for 24 h. The minimum bactericidal concentrations (MBCs) were determined as the lowest concentration of antibiotic resulting in 100% reduction of viable cells compared to the antibiotic-free control. Biofilms were produced on stainless steel washers, and the time-course changes were observed by scanning electron microscopy (SEM).
RESULTS: Planktonic bacteria were sensitive to the antimicrobial agents, but biofilm bacteria were markedly resistant. SEM revealed that biofilms grew larger with time.
CONCLUSIONS: The MBCs of antimicrobial agents for biofilms bacteria were higher than those of planktonic bacteria. Therefore, it is suggested that implant-related infection is difficult to treat with antimicrobial agents alone.