Antimicrobial Susceptibility Pattern of Staphylococcus aureus Isolates Against Newly Marketed Antibiotics: A Report From Imam Reza Hospital of Mashhad, Iran

Infections due to Staphylococcus aureus have long been considered as a big challenge to clinicians. The innate ability of this microorganism to develop resistance to different antibiotics, has led to the appearance of MRSA (methicillin-resistant Staphylococcus aureus) and lately VRSA (vancomycin-resistant Staphylococcus aureus) strains, which are considered as major problems for both patients and clinicians. In this study, we tried to evaluate susceptibility pattern of S. aureus isolates against some prevalent antibiotics as well as some infrequent ones. This inquiry was performed on 238 clinical samples, collected from different wards of Imam Reza Hospital of Mashhad between 2011 and 2012, which were previously defined as S. aureus and stocked in -70°C. Kirby-Bauer’s disc diffusion was performed for the following antibiotics: quinupristin-dalfopristin, linezolid, cefoxitin and mupirocin according to EUCAST 2014 (v. 4), cotrimoxazole, doxycycline, tigecycline, oxacillin based on CLSI 2012 (M100-S22) and vancomycin according to CLSI 2007 guidelines. Out of 238 samples, 5.88% were resistant to quinupristin-dalfopristin; 5.46% to linezolid; 60.92% to Co-trimoxazole; 31.93% to doxycycline; 18.90% to tigecycline; 5.04% to vancomycin; 9.24% to mupirocin; 43% to oxacillin and 46.21% of our isolates were resistant to cefoxitin. Coming across isolates with reduced susceptibility to quinupristin-dalfopristin and resistant to linezolid in this study are worrisome although these antibiotics are not used in our hospital. This might be a new challenge in the treatment of MRSA.