Document Type : Original Article


1 Department of Clinical Pharmacy and Pharmacy Practice, Isfahan University of Medical Sciences, Isfahan, Iran

2 Department of Clinical Pharmacy, Tehran University of Medical Sciences, Tehran, Iran


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.
Findings: 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 
Conclusion: 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.


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