Therefore, tetracyclines should not be used for streptococcal disease unless the organism has been demonstrated to be susceptible. The use of antibiotic disk susceptibility test methods which measure zone diameter give an accurate estimation of susceptibility of microorganisms to minocycline hydrochloride. One such standard procedure has been recommended for use with disks for testing antimicrobials. Either the 30 mcg tetracycline-class disk or the 30 mcg minocycline disk should be used for the determination of the susceptibility of microorganisms to minocycline. With either the tetracycline-class disk or the minocycline disk, zone sizes of 19 mm or greater indicate susceptibility , zone sizes of 14 mm or less indicate resistance, and zone sizes of 15 to 18 mm indicate intermediate susceptibility.
Standardized procedures require the use of laboratory control organisms.
The 30 mcg tetracycline disk should give zone diameters between 19 and 28 mm for Staphylococcus aureus A.C. 25923 and between 18 and 25 mm for Escherichia coli A.C. 25922. The 30 mcg minocycline disk should give zone diameters between 25 and 30 mm for S. aureus A.C. 25923 and between 19 and 25 mm for E. coli A.C. 25922. As with standard diffusion methods, dilution procedures require the use of laboratory control organisms. In the rat, chronic treatment with minocycline hydrochloride has resulted in goiter accompanied by elevated radioactive iodine uptake and evidence of thyroid tumor production. Minocycline hydrochloride has also been found to produce thyroid hyperplasia in rats and dogs.