Evaluation of the in-vitro synergistic potential of vancomycin combined with other antimicrobial agents against methicillin-resistant Staphylococcus aureus isolates

Abstract

Background: Methicillin-resistant Staphylococcus aureus (MRSA) infection is a major public health problem, causing a wide range of infections including bacteremia. Infections caused by MRSA are associated with significant morbidity, mortality, and costs. The present study aimed to determine the frequency of MRSA isolates among bacteremic patients, to determine their antimicrobial susceptibility patterns and to evaluate the in-vitro synergy combinations of vancomycin plus imipenem, cefepime, cefazoline and piperacillin-tazobactam against these isolates. Methods: Fifty confirmed MRSA strains isolated from blood cultures constituted the material of this study. The BD Phoenix was used to determine the susceptibility of these isolates to different antimicrobials. The Two-agent Broth Microdilution checkerboard test was used to evaluate the effect of combinations of two antimicrobial agents on the studied isolates. Results: In the current study the prevalence of MRSA among bacteremic patients was 15.68%, resistance rate was as follow: Gentamicin (80%), erythromycin (68%), ciprofloxacin (64%), norfloxacin (52%), moxifloxacin (36%), trimethoprim-sulfamethoxazole (46%), doxycycline (38%), rifampin (34%), clindamycin (24%), chloramphenicol (8%), linezolid (6%), teicoplanin (2%). All isolates were fully susceptible to daptomycin and vancomycin. Synergy was seen in varying proportions of the MRSA isolates when vancomycin was combined with imipenem, piperacillin-tazobactam, cefepime and cefazolin that was 76%, 66%, 54% and 52% respectively. No antagonism was observed. The mean FIC indices for combination of Vancomycin with Imipenem were significantly inversely correlated with the vancomycin MICs of the isolates using linear regression analysis. Conclusions: The synergistic activity of vancomycin in combination with β- lactam antibiotics offers new insights in treatment options of serious MRSA infections.