Title: Gentamicin and Tobramycin Susceptibility of Staphylococci Causing Prosthetic Joint Infection
Authors: Paloma Anguita-Alonso, Arlen D. Hanssen, Douglas R. Osmon, Andrej Trampuz, Kerryl E. Piper, James M. Steckelberg, Robin Patel.
Addresses: Mayo Clinic, Rochester, MN, USA.
Purpose: Prosthetic joint infection (PJI) is an important complication in orthopedic surgery and is frequently caused by staphylococci. In patients with prosthetic joints, antibiotic-impregnated polymethylmethacrylate (PMMA) is often used for prevention and treatment of infection. We determined the in vitro activity of gentamicin and tobramycin, two antibiotics commonly incorporated into PMMA, against staphylococci causing PJI.
Methods: 93 staphylococcal isolates from patients with PJI (57 knee and 36 hip) diagnosed from January 1999 - October 2003 were tested. Minimal inhibitory concentration (MIC) values of gentamicin and tobramycin were determined according to NCCSL guidelines, using the following MIC breakpoints (µg/ml): susceptible, =4; intermediate, 8; and resistant, =16. Statistic non-parametric tests for paired or non-paired data were used for data analysis.
Results :MIC90 and MIC50 values for gentamicin and tobramycin and the percentage of resistant isolates are summarized in the table. All MIC90 values were greater than NCCLS susceptible or intermediate breakpoints. Tobramycin MIC50 values were greater than the NCCLS susceptible breakpoint. Overall, gentamicin MIC values were significantly lower than tobramycin MIC values (p<0.001). Differences in aminoglycoside MIC values between Staphylococcus aureus and coagulase negative staphylococci were not significant. Amongst S. aureus gentamicin and tobramycin resistances were more frequently present in methicillin-resistant versus methicillin-susceptible isolates (p=0.026 and p<0.001, respectively).
|No.||MIC50/ MIC90||%Resistant||MIC50/ MIC90||%Resistant|
|Coagulase negative staphylococci||37||4/128||46||64/>128||65|
Discussion: Staphylococci causing PJI show a high rate of resistance to gentamicin and tobramycin.