Isolating bacteria from a patient is not enough for determining the proper therapy.
Testing individual pathogens against antibiotics is done by one of three methods:
Inoculate the bacteria to be tested by homogenous streaking across the agar plate.
Apply the chosen antibiotics containing special concentration of the drug at adequate spacing (25 mm apart and 15 mm from the edge of the plate) to the surface of the plate with sterile forceps or disc dispenser.
Transfer the plate to the incubator at 37 degree for 24 hours.
Measure the diameter of the zones of inhibition of bacterial growth.
Refer to tables to determine whether the bacteria is sensitive, moderately sensitive or resistant to a specific antibiotic.
Steps of disc diffusion method
Two fold dilutions of the antibiotic are prepared.
A constant volume of 24 hour broth culture of the bacteria to be tested (diluted 1/100) is added to each tube.
Two control tubes should be included:
One tube containing the antibiotic with no bacteria.
One tube containing the bacteria with no antibiotic.
After incubation at 37 degree for 18-24 hours, all tubes are checked for turbidity.
MIC (minimal inhibitory concentration):
Is the tube with the highest dilution of the antibiotic showing no visible turbidity.
MBC (minimal bactericidal concentration):
Subculturing all tubes showing no visible turbidity.
The tube with the highest dilution that fails to yield growth on subculture plate contains the MBC of the antibiotic for the tested bacteria.
MIC and MBC
Abbreviation of epsilometer
It is a technique for direct determination of MIC.
A gradually increasing concentrations of the antibiotic is fixed a long rectangular plastic test strip which is applied to the surface of the inoculated plate with bacteria.
After overnight incubation, a tear drop shaped zone of inhibition is seen. The zone edge intersects the test strip at the MIC of the antibiotic.