2004 Abstract : BS 6

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).

    Gentamicin Tobramycin
  No. MIC50/ MIC90 %Resistant MIC50/ MIC90 %Resistant
S. aureus 56 4/>128 38 64/>128 66
Methicillin-resistant 21 32/>128 52 >128/>128 100
Methicillin-susceptible 35 2/128 29 8/64 46
Coagulase negative staphylococci 37 4/128 46 64/>128 65
Methicillin-resistant 29 1/>128 48 64/>128 69
Methicillin-susceptible 8 4/32 38 64/>128 50

Discussion: Staphylococci causing PJI show a high rate of resistance to gentamicin and tobramycin.