Susceptibility Test of Methicillin-resistant Staphylococcus aureus against Vancomycin Using Triphenyltetrazolium Chloride

Han, Seok Joo; Seong, Inwha

Abstract

BACKGROUND: Abuse and inappropriate use of antibiotics have resulted in emergence of antibiotic-resistant microorganisms, increased nosocomial infections and urgency of developing new drugs effectively coping the resistant microorganisms. Recently infections caused by methicillin-resistant Staphylococcus aureus (MRSA) strains with reduced susceptibility to vancomycin (MIC=8 microgram/mL) have been reported in both Japan and the United States. Micro-broth dilution method and population analysis method revealed that those strains in Japan were heterogeneously resistant to vancomycin. The aim of this study was to determine the susceptibility and minimum inhibitory concentrations (MIC) of 99 MRSA strains to vancomycin by modified microplate-broth dilution method with triphenyltetrazolium chloride. METHODS: Susceptibility and minimum inhibitory concentration of stains of MRSA collected to vancomycin were determined by micro-plate broth dilution method with triphenyltetrazolium chloride solution, one of the growth indicator of microorganisms. RESULTS: MICs of 99 strains of MRSA isolated from otorrhea, pus, blood, respiratory tract, tissue fluid, and sputum to vancomycin were determined with microplate-broth dilution method with triphenyltetrazolium chloride. All the 99 strains showed MIC under 8 microgram/mL, there was no strain with reduced susceptibility to vancomycin. Microplate-broth dilution method with triphenyltetrazolium chloride was simple, easy to interpret the results. CONCLUSION: All the strains of MRSA tested for the susceptibility to vancomycin were susceptible to vancomycin. The microplate-broth dilution method with triphenyltetrazolium chloride was simple, easy to interpret the results and economic, and it is suggested that this method could be applied to the antimicrobial susceptibility tests of the other bacteria and fungi.