Minocycline hydrochloride microbiology
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mohamed Moubarak, M.D. [2]
Microbiology
The tetracyclines are primarily bacteriostatic and are thought to exert their antimicrobial effect by the inhibition of protein synthesis. The tetracyclines, including minocycline, have a similar antimicrobial spectrum of activity against a wide range of gram-positive and gram-negative organisms. Cross-resistance of these organisms to tetracyclines is common.
Minocycline has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section:
AEROBIC GRAM-POSITIVE MICROORGANISMS
Because many strains of the following gram-positive microorganisms have been shown to be resistant to tetracyclines, culture and susceptibility testing are especially recommended. Tetracycline antibiotics should not be used for streptococcal diseases unless the organism has been demonstrated to be susceptible. Tetracyclines are not the drug of choice in the treatment of any type of staphylococcal infection.
AEROBIC GRAM-NEGATIVE MICROORGANISMS
Calymmatobacterium granulomatis
Because many strains of the following groups of gram-negative microorganisms have been shown to be resistant to tetracyclines, culture and susceptibility tests are especially recommended:
"OTHER" MICROORGANISMS
Fusobacterium nucletum ssp. fusiforme
Treponema pallidum subspecies pallidum
Treponema pallidum subspecies pertenue
Ureaplasma urealyticum
When penicillin is contraindicated, tetracyclines are alternative drugs in the treatment of infections caused by the cited microorganisms.
Susceptibility tests
Susceptibility testing should be performed with tetracycline since it predicts susceptibility to minocycline. However, certain organisms (e.g., some staphylococci, and Acinetobacter ssp.) may be more susceptible to minocycline and doxycycline than to tetracycline.
Dilution techniques
Quantitative methods are used to determine antimicrobial minimal inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized procedure. Standardized procedures are based on a dilution method1,3 (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of tetracycline powder. The MIC values should be interpreted according to the following criteria:
For testing aerobic gram-negative microorganisms (Enterobacteriaceae), Acinetobacter ssp. and Staphylococcus aureus.[1]
A report of "Susceptible" indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable. A report of "Intermediate" indicates that the result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where high dosage of drug can be used. This category also provides a buffer zone which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of "Resistant" indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected.
Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard tetracycline powder should provide the following MIC values:
- Escherichia coli ATCC 25922- 0.5–2.0 µg/mL
- Enterococcus faecalis ATCC 29212- 8.0–32.0 µg/mL
- Staphylococcus aureus ATCC 29213- 0.25–1.0 µg/mL
- Haemophilus influenzae ATCC 49247- 4.0–32.0 µg/mL
- Streptococcus pneumoniae ATCC 49619- 0.12–0.5 µg/mL
- Neisseria gonorrhoeae ATCC 49226- 0.25–1.0 µg/mL
Diffusion techniques
Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure2,3 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 30µg tetracycline (class disk) or 30µg minocycline to test the susceptibility of microorganisms to minocycline. Reports from the laboratory providing results of the standard single-disk susceptibility test with a 30µg tetracycline or minocycline disk should be interpreted according to the following criteria:
For testing aerobic gram-negative microorganisms (Enterobacteriaceae) Acinetobacter ssp. and Staphylococcus aureus:
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 30µg tetracycline or minocycline disk should provide the following zone diameters in these laboratory test quality control strains:
References
- ↑ "DYNACIN (MINOCYCLINE HYDROCHLORIDE) CAPSULE [MEDICIS, THE DERMATOLOGY COMPANY]". Text " accessdate" ignored (help)
Adapted from the FDA Package Insert.