Impact of biofilm on the in vitro activity of vancomycin alone and in combination with tigecycline and rifampicin against Staphylococcus aureus

OBJECTIVES: This study evaluated vancomycin susceptibility and activity alone and in combination with rifampicin and tigecycline against low-biofilm- and high-biofilm-producing methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates. METHODS: Forty MRSA isolates recovered from bloodstream infections were analysed. Susceptibilities were performed in planktonic and biofilm cultures by microbroth dilution. Biofilm production was determined using an adherent plate assay. Time-kill analysis was performed on six low- and six high-biofilm-producing isolates with 15 mg/L vancomycin alone and in combination with rifampicin or tigecycline at 4x MIC. RESULTS: Vancomycin susceptibility displayed a 4-fold and an 8-fold increase in the MIC(50) and MIC(90), respectively, in the presence of biofilm. Rifampicin and tigecycline susceptibilities also increased in biofilms, but still remained within the susceptibility breakpoints except for a tigecycline MIC(90) of 1 mg/L. High biofilm production was detected in 60% of the isolates. In time-kill analysis, 15 mg/L vancomycin achieved bactericidal activity against only low-biofilm-producing strains with a 1.8 log(10) cfu/mL difference in bacterial kill compared with high-biofilm-producing strains (P < 0.001). Rifampicin alone had minimal activity, resulting in resistance. Tigecycline was minimally effective and was not bactericidal, but no difference was observed in the comparison of biofilm-producing strains. Vancomycin in combination with rifampicin or tigecycline was bactericidal against all strains (mean kill 4.5 +/- 0.5 log(10) cfu/mL), regardless of biofilm production. CONCLUSIONS: Vancomycin exposures at 15 mg/L may not be adequate in eradicating biofilm-producing S. aureus. Alternative treatments or combination therapy should be explored to optimize outcomes in biofilm-associated infections.