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Swedish University of Agricultural Sciences

Veterinary bacteriology: information about important bacteria
Veterinary bacteriology


Quality control

Quality contro
A control of pure culture should be performed after inoculation onto a non-selective agar plate. In the wells of the microdilution plate the last filled wells do not contain antibiotics and are used as positive growth control (Image: Homayoon Davam & Ingrid Hansson, HBIO)- Click on the image to enlarge it.

Regardless of which method is used for susceptibility testing, it is important that analyses are performed according to standardised methods and quality assurance. It is necessary that the bacteria to be tested are in pure culture. In addition, the result is affected by bacterial density, temperature and atmosphere. Most bacteria are tested under the same conditions, but some require other growth media and conditions (Incubation time, temperature and atmosphere). When changing conditions it may be necessary to validate quality control organisms for these test controls.

Microdilution in commercial microtiter plates, should be performed with a bacterial concentration of 104-105 CFU/ml, which is checked by surface spreading of the inoculation broth on a non-selective agar plate such as blood agar. Furthermore, it is important that the microtiter plate is sealed so that no evaporation occurs when testing, since it can increase the concentration of active substance and falsely low values can be maintained. The MIC value is read as the lowest concentration of antimicrobials that completely inhibit bacterial growth. This can be difficult to determine, especially in the case of bacteriostatic substances where there is often a gradual reduction of growth.

For diffusion methods, the zone of inhibition is measured, which can sometimes be difficult to interpret, as the zone boundary can be difficult to assess. An agar plate that is not horisontal, old (=dried out) or too moist allowing some bacteria to swarm can cause incorrect results. Incubation in an anaerobic or microaerophilic atmosphere changes the pH of the medium and thus the diffusion properties. Furthermore, as mentioned, the regression lines are made for bacteria with a high growth rate, the interpretation can be difficult to assess for weak and slow- growing bacteria.

Updated: 2024-06-17.


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Swedish University of Agricultural Sciences