Bacteriological Sensitivity Testing

BACTERIOLOGICAL SENSITIVITY TESTING 

This is generally done by diskagar diffusion method using standardized concentrations of antibiotics based on clinically attained plasma concentrations of these. As such, they serve only as guides and cannot be blindly extrapolated to the clinical situation in every patient and for every organism. Broth cultures with breakpoint concentration (concentration that demarcates between sensitive and resistant bacteria) of antibiotics probably yield more reliable results. Breakpoint concentrations are based on clinically attainable serum concentrations of the antibiotic.

Minimum inhibitory concentration (MIC), i.e the lowest concentration of an antibiotic which prevents visible growth of a bacterium determined in microwell culture plates using serial dilutions of the antibiotic is more informative, but not estimated routinely.

Minimum bactericidal concentration (MBC), of the antibiotic is determined by subculturing from tubes with no visible growth. If the organism is killed, no growth will occur; but if it was only inhibited in the parent culture—it will grow on subculturing in antibioticfree medium. MBC is the concentration of the antibiotic which kills 99.9% of the bacteria. A small difference between MIC and MBC indicates that the antibiotic is primarily bactericidal, while a large difference indicates bacteriostatic action. MBC is not used to guide selection of antibiotics in clinical practice.

Post-antibiotic effect (PAE) After a brief exposure if the organism is placed in antibioticfree medium, it starts multiplying again, but after a lag period which depends on the antibiotic as well as the organism. This lag period in growth resumption is known as ‘post-antibiotic effect’ and is the time required for re-attainment of logarithmic growth. A long PAE has been noted with fluoroquinolones, aminoglycosides and βlactam antibiotics.