Comparative susceptibility of in vitro biofilm and planktonic cells of Staphylococcus aureus to antimicrobials

Abstract

This study evaluated the effect of frequently used veterinary wound antimicrobials for their efficacy in killing mature in vitro Staphylococcus aureus biofilms and inhibiting planktonic cells. The predictiveness of the minimum biofilm eradication concentration (MBEC) assay as a tool for antibiotic susceptibility testing was also assessed. The minimum inhibitory concentration (MIC) and MBEC of tetracycline, tetracycline-based commercial wound spray, silver nitrate, gentian violet, iodine tincture, sucrose and a laboratory mixture of sucrose and gentian violet were determined. Whereas low concentrations of all these antimicrobials except sucrose inhibited planktonic S. aureus, only silver nitrate eradicated the biofilm phenotype. Silver nitrate at a Ag+concentration of 4 x MIC showed 100% efficiency of removal or 7.70-log reduction ofS. aureus biofilm cells, 1% gentian violet gave a significant reduction (55% or 0.35-log, P = 0.046) and 120% sucrose in gentian violet also showed a significant percentage reduction of 89.71% (0.98-log, P = 0.001). However, 120% sucrose and 2% iodine tincture reduced biofilms insignificantly (28.26% or 0.14-log, P = 0.098) and (34.78% or 0.18-log, P = 0.065), respectively. Based on the national committee for clinical laboratory brake-points, S. aureus biofilms lacked sensitivity to tetracycline and the tetracycline base wound spray. In conclusion, the antibiofilm properties of Ag+observed in this study may improve the success rate in treating clinical biofilm-associated S. aureus wound infections if the MBEC assay is applied to select appropriate concentrations. Key words: Staphylococcus aureus, biofilms, planktonic, antimicrobials, wound, infections.