Methicillin-Resistant Staphylococcus Strains Isolated from Adult Intensive Care Units with E-test MIC Values of Different Antibiotic Research

Objective: Nasocomial infections are major health problems due to their high morbidity and mortality, prolonged hospital duration and higher treatment costs. Methicillin-resistant staphylococcus species became one of the leading bacteria causing nasocomial infections especially in intensive care units, recently.

The minimum inhibitory concentration value of an antibiotic gives the concentration of antibiotic needed to inhibit the bacteria in the infection area. Careful monitoring of minimal inhibitory concentration (MIC) values is necessary especially during long-term treatments of meticillin-resistant Staphylococcus aureus (MRSA) and meticillin-resistant coagulase-negative staphylococci (MRCoNS) infections^1,2. Increasing antibiotic resistance in methicillin-resistant staphylococci, has led to the need for different antibiotics.

Methods: A total of 60 meticillin-resistant staphylococci strains isolated in Microbiology Laboratory of Dicle University Hospital, from clinical specimens of patients in adult İntensive Care Units (ICUs) between April 2013 and March 2014 were included in this study. After identification with conventional and automated system, the antibiotic susceptibility rates of vancomycin, teicoplanin, daptomycin, linezolid, quinupristin/dalfopristin, tigecycline, ceftaroline were determined by E-test method.

Results: The majority of the samples (26.7%) were sent from Pulmonary Diseases and Tuberculosis intensive care unit and the blood samples were the most common materials (80%) . All staphylococcal strains in our study were determined as susceptible to vancomycin, daptomycin, linezolid, teicoplanin and tigecycline. One (1.6%) MRCoNS isolate was resistant to quinupristin/dalfopristin while 11 (36.6%) of the MRSA isolates were resistant to ceftaroline. In comparison with the MIC values of MRSA and MRCoNS, only tigecycline was significantly different. Thirty MRSA strains were evaluated in terms of vancomycin-intermediate Staphylococcus aureus/heteroresistant vancomycin-intermediate Staphylococcus aureus (VISA/hVISA) with macro E-test method; any VISA/hVISA isolate was not detected.

Antibiotic concentrations below the MIC level, not only leads to treatment failure but also causes mutant bacteria to appear. In order to control the resistance to antibiotics in the treatment of infections due to MRSA and MRCoNS agents, the clinician should be notified of the MIC values of the drugs and the treatment should be planned accordingly.

VISA/hVISA isolates should be considered in treatment failures of infections due to MRSA which are in vitro susceptible to vancomycin. Further testing is needed to detect these isolates.

Despite the fact that ceftaroline is not a drug used in our country, the high resistance rate in our study is remarkable. This situation may be due to the intensive use of other beta-lactam antibiotics. Therefore, antibiotic susceptibility results should be taken into consideration during planning the treatment of infections.

The high average MIC values of tigecycline in MRCoNS infections should also be monitored carefully