In vitro antimicrobial synergy testing of coagulase-negative staphylococci isolated from prosthetic joint infections using Etest and with a focus on rifampicin and linezolid

In recent years, coagulase-negative staphylococci (CoNS) have been increasingly recognised as causative agents of various infections, especially in immunocompromised patients and related to implanted foreign body materials. CoNS, and especially Staphylococcus epidermidis, transform into a stationary growth phase and produce biofilm when involved in a foreign body infection, making them difficult to eradicate with antimicrobials. Rifampicin has the ability to penetrate biofilm, but resistance may develop rapidly. To reduce the emergence of resistance, rifampicin should be combined with additional antimicrobials, of which several different ones have been proposed, including the relatively new class of antimicrobials, oxazolidinones, represented by linezolid. Thirty-seven CoNS isolates from patients with prosthetic joint infection were investigated by synergy testing using Etest. Nine antimicrobial combinations, based on either rifampicin or linezolid, were tested. For 16 (43%) of the isolates, a synergistic (n = 5), additive (n = 14) and/or antagonistic (n = 11) effect were identified. In conclusion, Etest is an objective and easily performed in vitro method for antimicrobial synergy testing. However, each isolate requires testing for the specific combination considered for treatment.