Minocin (minocycline hydrochloride 100 mg) Dailymed

Susceptibility Test Methods

When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drugs used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug for treatment.

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 test method (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of tetracycline (class) or minocycline powder^1,2. The MIC values should be interpreted according to the criteria provided in Table 1.

Diffusion techniques:

Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size provides an estimate of the susceptibility of bacteria to antimicrobial compounds. The zone size should be determined using a standardized method^2,3. This procedure uses paper disks impregnated with 30 μg tetracycline (class disk) or 30 μg minocycline to test the susceptibility of microorganisms to minocycline. The disk diffusion interpretive criteria are provided in Table 1.

Table 1: Susceptibility Test Interpretive Criteria for Minocycline and Tetracycline

Species	Minimal Inhibitory Concentration (mcg/mL)			Zone Diameter (mm)			Agar Dilution (mcg/mL)
	S	I	R	S	I	R	S	I	R
Enterobacteriaceae Organisms that are susceptible to tetracycline are also considered susceptible to minocycline. However some organisms that are intermediate or resistant to tetracycline may be susceptible to minocycline.
Minocycline	≤ 4	8	≥16	≥16	13 - 15	≤12
Tetracycline	≤ 4	8	≥16	≥15	12 - 14	≤11
Acinetobacter
Minocycline	≤4	8	≥16	≥16	13 - 15	≤12
Tetracycline	≤4	8	≥16	≥15	12 - 14	≤11
Haemophilus influenzae
Tetracycline	≤2	4	≥8	≥29	26 - 28	≤25
Streptococcus pneumoniae’
Tetracycline	≤1	2	≥4	≥28	25 - 27	≤24
Staphylococcus aureus
Minocycline	≤4	8	≥16	≥19	15 - 18	≤14
Tetracycline	≤4	8	≥16	≥19	15 - 18	≤14
Vibrio cholerae
Minocycline	≤4	8	≥16	≥16	13 - 15	≤12
Tetracycline	≤4	8	≥16	≥19	15 - 18	≤14
Neisseria meningitidis The current absence of resistance isolates precludes defining any result other than “susceptible”. If isolates yielding MIC results other than susceptible, they should be submitted to a reference laboratory for further testing.
Minocycline	--	--	--	> 26	--	--	≤2	--	--
Bacillus anthracis
Tetracycline	≤1	--	--
Francisella tularensis
Tetracycline	≤4	--	--
Yersinia pestis
Tetracycline	≤4	8	≥16

A report of “Susceptible” indicates that the antimicrobial drug is likely to inhibit growth of the microorganism if the antimicrobial drug reaches the concentration usually achievable at the site of infection. 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 antimicrobial drug is not likely to inhibit growth of the microorganism, if the antimicrobial drug reaches the concentrations usually achievable at the infection site; other therapy should be selected.

Quality Control

Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test^1,2,3. Standard tetracycline (class compound) or minocycline powder should provide the following range of MIC values noted in Table 2. For the disc diffusion technique, using the 30 mcg tetracycline or 30 mcg minocycline disk the criteria in Table 2 should be achieved.

Table 2: Acceptable Quality Control Ranges for Minocycline and Tetracycline

Species	Minimal Inhibitory Concentration (mcg/mL)	Zone Diameter (mm)	Agar Dilution (mcg/mL)
Enterococcus faecalis ATCC 29212
Minocycline	1 – 4	--	--
Tetracycline	8 – 32	--	--
Escherichia coli ATCC 25922
Minocycline	0.25 – 1	19 – 25	--
Tetracycline	0.5 – 2	18 – 25	--
Haemophilus influenzae ATCC 49247
Tetracycline	4 – 32	14 – 22	--
Neisseria gonorrhoeae ATCC 49226
Tetracycline	--	30 – 42	0.25 – 1
Staphylococcus aureus ATCC 25923
Minocycline		25 – 30	--
Tetracycline		24 – 30	--
Staphylococcus aureus ATCC 29213
Minocycline	0.06 – 0.5		--
Tetracycline	0.12 – 1		--
Streptococcus pneumoniae ATCC 49619
Tetracycline	0.06 – 0.5	27 – 31	--