Doxycycline Bacterial Spectrum of Activity
Doxycycline is a broad-spectrum tetracycline antibiotic that kills or inhibits a wide range of gram-positive and gram-negative bacteria, atypical organisms, rickettsiae, spirochetes, and certain parasites. 1
Primary Bacterial Targets
Sexually Transmitted Infections
- Chlamydia trachomatis - Doxycycline is the recommended first-line treatment 2, 3
- Treponema pallidum (syphilis) - Alternative treatment when penicillin is contraindicated 2
- Neisseria gonorrhoeae - Remains effective against many strains in the United States, though not recommended as first-line due to increasing resistance 2
- Ureaplasma urealyticum - Causes nongonococcal urethritis 1
- Haemophilus ducreyi (chancroid) 1
- Calymmatobacterium granulomatis (granuloma inguinale) 1
Respiratory Pathogens
- Mycoplasma pneumoniae - Highly susceptible 1, 4
- Streptococcus pneumoniae - When susceptible 1
- Haemophilus influenzae - 80.8% of strains susceptible 1, 5
- Chlamydia psittaci (psittacosis) 1
Rickettsial Infections
- Rickettsia species - Causes Rocky Mountain spotted fever, typhus, Q fever, rickettsialpox 1, 6
- All rickettsiae tested show susceptibility to doxycycline 6
Gram-Positive Bacteria
- Staphylococcus aureus - 91% of strains susceptible, though resistance is emerging 5
- Bacillus anthracis (anthrax) - Including inhalational anthrax post-exposure 1
- Listeria monocytogenes 1
- Actinomyces israelii 1
- Clostridium species 1
- Propionibacterium acnes 1
Gram-Negative Bacteria
- Yersinia pestis (plague) 1
- Francisella tularensis (tularemia) 1
- Vibrio cholerae (cholera) 1
- Brucella species - Used with streptomycin 1
- Campylobacter fetus 1
- Bartonella bacilliformis 1
Important caveat: Many strains of common gram-negative organisms show variable susceptibility and require culture testing:
- Escherichia coli - 74% susceptible 5
- Klebsiella species - 81.7% susceptible 5
- Enterobacter aerogenes - Less susceptible 5, 7
- Shigella species 1
- Acinetobacter species 1
Spirochetes
- Borrelia recurrentis (relapsing fever) 1
- Treponema pertenue (yaws) 1
- Fusobacterium fusiforme (Vincent's infection) 1
Parasites
- Plasmodium falciparum - Active against asexual erythrocytic forms for malaria prophylaxis, but not gametocytes 1
- Entamoeba species - Adjunctive therapy 1
- Balantidium coli 1
Mechanism of Action
Doxycycline is primarily bacteriostatic, inhibiting bacterial protein synthesis by binding to the 30S ribosomal subunit 1. However, it demonstrates bactericidal activity against certain organisms like Mycoplasma pneumoniae 4.
Critical Resistance Patterns
Resistance is common and increasing in several important pathogens:
- Streptococcus pyogenes - Up to 44% resistant 1
- Enterococcus - Up to 74% resistant 1
- Proteus mirabilis - Nearly all isolates resistant 5
- Neisseria gonorrhoeae - 24-30% tetracycline resistance in recent studies 2
Clinical implication: Culture and susceptibility testing are recommended for gram-negative organisms, enterococci, and streptococci before using doxycycline 1. The notable exception is Chlamydia trachomatis, where doxycycline remains first-line without routine susceptibility testing 3.