Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure (Ref2, Ref3) requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 30 µg tetracycline (class disk) or 30 µg minocycline to test the susceptibility of microorganisms to minocycline.

Reports from the laboratory providing results of the standard single-disk susceptibility test with a 30-µg tetracycline or minocycline disk should be interpreted according to the following criteria: For testing aerobic gram-negative microorganisms (Enterobacteriaceae), Acinetobacter species and Staphylococcus aureus

For testing Haemophilus influenzae⁵:

⁵ These zone diameter standards are applicable only to susceptibility testing with Haemophilus influenzae using Haemophilus Test Medium and a 30- g tetracycline disk. Ref2

For testing Neisseria gonorrhoeae⁶:

⁶ These interpretative standards are applicable only to disk diffusion testing using GC agar and 1% growth supplements, and a 30-µg tetracycline disk. Ref2

For testing Streptococcus pneumoniae⁷:

⁷ These interpretative standards are applicable only to disk diffusion testing using Muller-Hinton agar adjusted with 5% sheep blood and a 30-µg tetracycline disk. Ref2 For testing Vibrio cholerae⁸:

⁸ These interpretative standards are applicable only to disk diffusion testing performed with a 30- g tetracycline disk.(Ref2)

Interpretation should be as stated above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for tetracycline.

As with standardized dilution techniques, diffusion methods require the use of laboratory control microorganisms that are used to control the technical aspects of the laboratory procedures. For the diffusion technique, the 30-µg tetracycline or minocycline disk should provide the following zone diameters in these laboratory test quality control strains:

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