Methods

Dilution method used to determine the minimum inhibitory concentrations (MICs) of antimicrobial agents. Considered the reference method for antimicrobial susceptibility testing. Incubating bacteria in broth with dilutions of test agent (antibiotic, inhibitor etc.) to determine MIC that inhibits growth. Reference broth microdilution (rBMD) as well as commercially available panels will be offered.

Broth Microdilution MBC determination : The minimum bactericidal concentration (MBC) is the lowest concentration of  an antibacterial agent required to kill a particular bacterium and is determined from broth dilution MIC tests by subculturing to agar plates determining the lowest concentration of antibacterial agent that reduces the viability of the initial bacterial inoculum by ≥99.9%.

E-test (Epsilometer test): Exponential gradient method of determination of antimicrobial resistance using a predefined gradient of antibiotic concentrations on a plastic strip to determine the Minimum Inhibitory Concentration (MIC) of antibiotics, antifungal agents and antimycobacterial agents. Synergy studies of combination of antibiotics can also be performed by E-test.

Disk Diffusion (Kirby-Bauer): Test of antibiotic susceptibility using antibiotic-impregnated disks, placed on agar previously inoculated with the test bacterium. The antibiotic diffuse radially outward through the agar medium producing an antibiotic concentration gradient. The zone around an antibiotic disk that has no growth is referred to as the zone of inhibition and approximates the MIC, and classifies the isolate as susceptible, intermediate or resistant. Synergy studies of combination of antibiotics can also be performed by disk diffusion.

Broth microdilution bactericidal checkerboard method: In vitro method of detecting synergy between combinations of antibiotics using broth microdilution to calculate the fractional inhibitory concentration index (comparison of the MIC of each agent alone with the combination-derived MIC)

Time-kill test (time-kill curve): Time-kill assays allow one to assess the rate of bactericidal activity at varying antibiotic concentrations over time. The results of combinations of antimicrobial agents can also be assessed for evaluating synergy.

Resistance may also be established through tests that directly detect the presence of a particular resistance mechanism. For example, phenotypic tests to determine ESBLs (extended spectrum beta-lactamases) or modified-Hodge test or mCIM to determine the production of carbapenemases.