Mechanism of Action
Doxycycline inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit. Doxycycline has bacteriostatic activity against a broad range of Gram-positive and Gram-negative bacteria.
Resistance
Cross resistance with other tetracyclines is common.
Antimicrobial Activity
Doxycycline has been shown to be active against most isolates of the following microorganisms, both in vitro and in clinical infections [see Indications and Usage (1)].
Gram-negative bacteria
Acinetobacter species
Bartonella bacilliformis
Brucella species
Campylobacter fetus
Enterobacter aerogenes
Escherichia coli
Francisella tularensis
Haemophilus ducreyi
Haemophilus influenzae
Klebsiella granulomatis
Klebsiella species
Neisseria gonorrhoeae
Shigella species
Vibrio cholerae
Yersinia pestis
Gram-positive bacteria
Bacillus anthracis
Listeria monocytogenes
Streptococcus pneumoniae
Anaerobic bacteria
Clostridium species
Fusobacterium fusiforme
Propionibacterium acnes
Other bacteria
Nocardiae and other aerobic Actinomyces species
Borrelia recurrentis
Chlamydophila psittaci
Chlamydia trachomatis
Mycoplasma pneumoniae
Rickettsiae species
Treponema pallidum
Treponema pallidum subspecies pertenue
Ureaplasma urealyticum
Parasites
Balantidium coli
Entamoeba species
Plasmodium falciparum*
*Doxycycline has been found to be active against the asexual erythrocytic forms of Plasmodium falciparum, but not against the gametocytes of P. falciparum. The precise mechanism of action of the drug is not known.
Susceptibility Testing Methods
When available, the clinical microbiology laboratory should provide cumulative reports of in vitro susceptibility test results for antimicrobial drugs used in local hospitals and practice as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid in the selection of an appropriate antibacterial drug for treatment.
Dilution techniques:
Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs values should be determined using a standardized test method5,6,7,8,9 (broth and/or agar). The MIC values should be interpreted according to criteria provided in Table 1.
Diffusion techniques:
Quantitative methods that require measurement of zone diameters can also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds.
The zone size should be determined using a standardized test method5,7,10. This procedure uses paper disks impregnated with 30 mcg doxycycline to test the susceptibility of bacteria to doxycycline. The disk diffusion interpretive criteria are provided in Table 1.
Anaerobic Techniques:
For anaerobic bacteria, the susceptibility to doxycycline can be determined by a standardized test method5,11. The MIC values obtained should be interpreted according to the criteria provided in Table 1.
a Organisms susceptible to tetracycline are also considered susceptible to doxycycline. However, some organisms that are intermediate or resistant to tetracycline may be susceptible to doxycycline.
b The current absence of resistance isolates precludes defining any results other than "Susceptible". If isolates yielding MIC results other than susceptible, they should be submitted to a reference laboratory for further testing.
c Gonococci with 30 mcg tetracycline disk zone diameters of less than 19 mm usually indicate a plasmid-mediated tetracycline resistant Neisseria gonorrhoeae isolate. Resistance in these strains should be confirmed by a dilution test (MIC ≥ 16 mcg per mL). |
| Table 1: Susceptibility Test Interpretive Criteria for Doxycycline and Tetracycline |
| Pathogena
| Minimal Inhibitory Concentrations (mcg/mL) | Disk Diffusion Zone Diameters (mm) | Agar Dilution (mcg/mL) |
| S | I | R | S | I | R | S | I | R |
| Acinetobacter spp. | | | | | | | | | |
| Doxycycline | ≤4 | 8 | ≥16 | ≥13 | 10 - 12 | ≤9 | - | - | - |
| Tetracycline | ≤4 | 8 | ≥16 | ≥15 | 12-14 | ≤11 | - | - | - |
| Anaerobes | | | | | | | | | |
| Tetracycline | - | - | - | - | - | - | ≤4 | 8 | ≥16 |
| Bacillus anthracisb
| | | | | | | | | |
| Doxycycline | ≤1 | - | - | - | - | - | - | - | - |
| Tetracycline | ≤1 | - | - | - | - | - | - | - | - |
| Brucella speciesb
| | | | | | | | | |
| Doxycycline | ≤1 | - | - | - | - | - | - | - | - |
| Tetracycline | ≤1 | - | - | - | - | - | - | - | - |
| Enterobacteriaceae | | | | | | | | | |
| Doxycycline | ≤4 | 8 | ≥16 | ≥14 | 11-13 | ≤10 | - | - | - |
| Tetracycline | ≤4 | 8 | ≥16 | ≥15 | 12-14 | ≤11 | - | - | - |
| Franciscella tularensisc
| | | | | | | | | |
| Doxycycline | ≤4 | - | - | - | - | - | - | - | - |
| Tetracycline | ≤4 | - | - | - | - | - | - | - | - |
| Haemophilus influenzae | | | | | | | | | |
| Tetracycline | ≤2 | 4 | ≥8 | ≥29 | 26-28 | ≤25 | - | - | - |
| Mycoplasma pneumoniaeb
| | | | | | | | | |
| Tetracycline | | - | - | - | - | - | ≤2 | - | - |
| Nocardiae and other aerobic Actinomyces speciesb
| | | | | | | | | |
| Doxycycline | ≤1 | 2-4 | ≥8 | - | - | - | | - | - |
| Neisseria gonorrhoeae speciesc
| | | | | | | | | |
| Tetracycline | - | - | - | ≥38 | 31-37 | ≤30 | ≤0.25 | 0.5-1 | ≥2 |
| Streptococcus pneumoniae | | | | | | | | | |
| Doxycycline | ≤0.25 | 0.5 | ≥1 | ≥28 | 25-27 | ≤24 | - | - | - |
| Tetracycline | ≤1 | 2 | ≥4 | ≥28 | 25-27 | ≤24 | - | - | - |
| Vibrio cholerae | | | | | | | | | |
| Doxycycline | ≤4 | 8 | ≥16 | - | - | - | - | - | - |
| Tetracycline | ≤4 | 8 | ≥16 | - | - | - | - | - | - |
| Yersinia pestis | | | | | | | | | |
| Doxycycline | ≤4 | 8 | ≥16 | - | - | - | - | - | - |
| Tetracycline | ≤4 | 8 | ≥16 | - | - | - | - | - | - |
| Ureaplasma urealyticum | | | | | | | | | |
| Tetracycline | - | - | - | - | - | - | ≤1 | - | ≥2 |
Doxycycline susceptibility testing interpretive criteria for anaerobes, Haemophilus influenzae, Mycoplasma pneumoniae, Neisseria gonorrhoeae, and Ureaplasma urealyticum have not been established. Isolates of these species that are susceptible to tetracycline are also considered susceptible to doxycycline.5
A report of Susceptible (S) indicates that the antimicrobial drug is likely to inhibit growth of the pathogen if the antimicrobial drug reaches the concentration usually achievable at the site of infection. A report of Intermediate (I) 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 product is physiologically concentrated or in situations where high dosage of drug can be used. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of Resistant (R) indicates that the antimicrobial drug is not likely to inhibit growth of the pathogen if the antimicrobial drug reaches the concentration 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 the supplies and reagents used in the assay, and the techniques of the individuals performing the test5,6,7,8,9,10,11. Standard doxycycline powders should provide the following range of MIC values noted in Table 2. For the diffusion technique using the 30 mcg doxycycline disk, the criteria noted in Table 2 should be achieved.
Table 2: Acceptable Quality Control Ranges for Doxycycline and Tetracycline| a ATCC is the American Type Culture Collection |
| QC Strain | Minimal Inhibitory Concentration (mcg per mL) | Disk Diffusion (zone diameter mm) | Agar Dilution (mcg/ml) |
| Enterococcus faecalis ATCCa 29212 | | | |
| Doxycycline | 2 – 8 | - | - |
| Tetracycline | 8-32 | - | - |
| Escherichia coli ATCC 25922 | | | |
| Doxycycline | 0.5 – 2 | 18 – 24 | - |
| Tetracycline | 0.5-2 | 18-25 | - |
| Eggerthella lenta ATCC 43055 | | | |
| Doxycycline | 2 – 16 | - | - |
| Haemophilus influenza ATCC 49247 | | | |
| Tetracycline | 4-32 | 14-22 | - |
| Neisseria gonorrhoeae ATCC 49226 | | | |
| Tetracycline | - | 30-42 | 0.25-1 |
| Staphylococcus aureus ATCC 25923 | | | |
| Doxycycline | - | 23 – 29 | - |
| Tetracycline | - | 24-30 | - |
| Staphylococcus aureus ATCC 29213 | | | |
| Doxycycline | 0.12 - 0.5 | - | - |
| Tetracycline | 0.12-1 | - | - |
| Streptococcus pneumoniae ATCC 49619 | | | |
| Doxycycline | 0.016 - 0.12 | 25-34 | - |
| Tetracycline | 0.06-0.5 | 27-31 | - |
| Bacteroides frugilis ATCC 25285 | | | |
| Tetracycline | - | - | 0.125-0.5 |
| Bacteroides thetaiotaomicron ATCC 29741 | | | |
| Doxycycline | 2 – 8 | - | - |
| Tetracycline | - | - | 8-32 |
| Mycoplasma hominis ATCC 23114 | | | |
| Tetracycline | - | - | 0.12-1 |
| Mycoplasma pneumoniae ATCC 29342 | | | |
| Tetracycline | 0.06-0.5 | - | 0.06-0.5 |
| Ureaplasma urealyticum ATCC 33175 | | | |
| Tetracycline | - | - | ≥8 |