Detection of antibiotic susceptibility by colorimetric minimum inhibitory concentration in staphylococcal isolates

AIM: Assess performance of broth microdilution (BMD) as well as agar dilution methods for antimicrobial susceptibility testing of Staphylococci using tetrazolium salt. METHODS AND RESULTS: Minimum inhibitory concentration (MIC) of eight antimicrobials; vancomycin (VA), linezolid, oxacillin, gentamicin (CN), tetracycline, ciprofloxacin, erythromycin and clindamycin was investigated for 80 isolates of Staphylococci by BMD with the addition of dimethyl thiazole diphenyl tetrazolium bromide (MTT), agar dilution with the addition of MTT and triphenyl tetrazolium chloride at the standard bacterial concentration together with addition of MTT at an experimental bacterial concentration. BMD (MTT) showed the highest agreement in comparison with the standard BMD. CONCLUSIONS: Colorimetric BMD was rapid and easy to interpret. Colorimetric agar dilution (MTT) was less tedious than BMD. SIGNIFICANCE AND IMPACT OF THE STUDY: Colorimetric antibiotic susceptibility is a good option to provide rapid reliable results for critically ill patients. In addition, agar dilution (MTT) helps to investigate outbreaks of methicillin-resistant Staphylococcus aureus (MRSA), VISA or VRSA. BMD (MTT) can be performed routinely to detect VA MIC in MRSA blood stream infections and hospital acquired pneumonia, where, high VA MIC is associated with a higher mortality rate.