Improvement of antimicrobial susceptibility testing in biofilm-growingcoagulase-negative Staphylococcus hominis

Coagulase-negative Staphylococcus hominis causes bloodstream infections and often can form biofilms on medical devices. This study aimed to improve the current methodology for antimicrobial susceptibility testing (AST) in biofilm-growing S. hominis isolates. Biofilm production of S. hominis was assessed using the crystal violet staining method in trypticase soy broth supplemented with 1% glucose (TSB_glu1%), Mueller-Hinton broth (MHB), or MHB_glu1% using flat-bottom plates or the Calgary device. Susceptibility to antibiotics was assessed using the broth microdilution method (MHB and TSB_glu1%) in planktonic cells (round-bottom plates) and biofilm cells (flat-bottom plates and the Calgary device). Biofilm determination using TSB_glu1% yielded better performance over MHB, and flat-bottom plates without agitation were preferred over the Calgary device. Higher fold dilution values between the minimum biofilm eradication concentration (MBEC) and the minimum inhibitory concentration (MIC) were obtained in MHB for almost all antibiotics, except for linezolid. TSB_glu1% and flat-bottom polystyrene plates were preferred over MHB and the Calgary device for biofilm determination. AST in biofilm-growing S. hominis showed better performance using TSB_glu1% compared to MHB. Therefore, when comparing MBEC and MIC values, AST in planktonic cells could also be performed using TSB_glu1% instead of MHB.