Antibiotic susceptibility of planktonic-and biofilm-grown staphylococci isolated from implant-associated infections: should MBEC and nature of biofilm formation replace MIC?

Purpose. The purpose of this study was to develop an alternative, more clinically relevant approach to susceptibility reporting for implant-associated infections. Using 20 staphylococcal isolates, isolated from clinical implant infections, the majority (85 %) demonstrated biofilm-forming capabilities. A significantly increased minimum biofilm eradication concentration (MBEC) compared to minimum inhibitory concentration (MIC) breakpoint was obtained, with MBEC values greater than 256 mu g ml(-1) for the majority of bacteria. Such a vast increase was also demonstrated for isolates defined as negligible biofilm formers via crystal violet staining, likely due to the high protein content of biofilms, as confirmed by proteinase-K treatment. Methodology. This study employed a variety of techniques to assess MIC and MBEC of the isolates tested. In addition, the nature of bacterial biofilm across a range of clinical isolates was investigated using crystal violet staining, sodium metaperiodate and proteinase-K treatment, and PCR analysis. Results/Key findings. Infection of medical implants is associated with increased rates of infection and increased bacterial tolerance to antibiotic strategies. Clinical significance is due to the presence of pathogens attached to biomaterial surfaces enclosed in an extracellular polymeric matrix termed the biofilm. This article highlights the importance of defining the clinical susceptibility of implant-associated infections in vitro using methods that are relevant to the biofilm phenotype in vivo, and highlights how current planktonic-based antimicrobial susceptibility tests are often misleading. Conclusion. The use of biofilm-relevant susceptibility tests would improve patient outcomes by enabling correct antimicrobial regimens to be rapidly identified, reducing treatment failure and halting the spread of antimicrobial-resistant strains.