title: Double-disk synergy test for detection of synergistic effect between antibiotics against nosocomial strains of staphylococcus aureus.
abstract: OBJECTIVE
  Synergistic effect between commonly used antibiotics against nosocomial multidrug-resistant strains of Staphylococcus aureus, if present, could provide a viable option as an alternative therapy for infections due to this pathogen. The aim of this study was searching for any synergistic effect between several antibiotics against drug-resistant strains of S. aureus with nosocomial origin using double-disk synergy test and to determine the applicability of this test for such a purpose.
  
  
  METHODS
  Over a 6-month period, strains of S. aureus isolated from clinical specimens of hospitalized patients with documented nosocomial infection underwent disk diffusion test using antibiotic disks of oxacillin, cephalothin, clindamycin, ciprofloxacin, vancomycin, cotrimoxazole, rifampin, erythromycin, gentamicin and meropenem. Double-disk synergy test was performed for all isolates resistant to at least two of applied antibiotics. Combinations of all possible pairs of antibiotics (to which the microorganism was resistant) were tested by placing antibiotic disks at distance of 20 mm from each other (center to center). After 16- 20 hours of incubation, if synergistic effect was present among two antibiotics, an inhibition zone was formed between their disks.
  
  
  RESULTS
  Among all of possible two-antibiotic combinations tested for 41 resistant isolates, only two cases of synergistic effect were detected; both effects were among rifampin and cotrimoxazole.
  
  
  CONCLUSIONS
  The combination of rifampin and cotrimoxazole could provide a viable option for treatment of infections due to resistant strains of S. aureus; however, clinical trials are needed before any new recommendation. Also, double-disk synergy test seems to be capable of detecting the synergistic effect between antibiotics at in vitro level.
authors.affiliation: Department of Clinical Pharmacy and Pharmacy Practice, Isfahan University of Medical Sciences, Isfahan, Iran.
authors.forename: Rasool
authors.initials: R
authors.lastname: Soltani
authors.affiliation: Department of Clinical Pharmacy, Tehran University of Medical Sciences, Tehran, Iran.
authors.forename: Hossein
authors.initials: H
authors.lastname: Khalili
authors.affiliation: Department of Clinical Pharmacy and Pharmacy Practice, Isfahan University of Medical Sciences, Isfahan, Iran.
authors.forename: Fateme
authors.initials: F
authors.lastname: Shafiee
keywords: Antibiotic; double-disk test; staphylococcus aureus; synergism