Doxycycline Susceptibility Testing 

Doxycycline is a tetracycline-class antibiotic with broad-spectrum activity against Gram-positive, Gram-negative, atypical bacteria, and intracellular pathogens. It is widely used for treating respiratory tract infections, tick-borne diseases, skin infections, and sexually transmitted infections. Due to the increasing prevalence of tetracycline-resistant bacteria, susceptibility testing is crucial to guide therapy and monitor resistance trends.

Testing Methods

Doxycycline susceptibility is determined using standardized phenotypic and molecular techniques to ensure accurate resistance detection:

• Broth Microdilution (BMD): CLSI and EUCAST gold-standard method for determining MIC values in Staphylococcus spp., Enterobacterales, Acinetobacter spp., and Streptococcus spp.
• Agar Dilution: Used for precise MIC determination but less frequently employed in routine diagnostics.
• E-test (Epsilometer Test): Gradient diffusion method providing quantitative MIC values in clinical microbiology labs.
• Disk Diffusion Assay: Provides qualitative susceptibility interpretation based on CLSI/EUCAST breakpoints.
• Molecular Methods (PCR, Whole Genome Sequencing): Detects resistance genes such as tet(M), tet(O), tet(X), and tet(A-E), responsible for tetracycline resistance.

Clinical Applications

Doxycycline susceptibility testing is essential in the management of various bacterial infections, particularly in the following cases:

• Community-Acquired Pneumonia (CAP) and Atypical Respiratory Infections: Ensures efficacy against Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella spp.
• Methicillin-Resistant Staphylococcus aureus (MRSA) Infections: Evaluates susceptibility of MRSA in skin and soft tissue infections (SSTIs).
• Tick-Borne Diseases (Lyme Disease, Anaplasmosis, Ehrlichiosis, and Rocky Mountain Spotted Fever): Confirms susceptibility of Borrelia burgdorferi, Anaplasma phagocytophilum, and Rickettsia spp.
• Urinary Tract Infections (UTIs): Guides therapy for doxycycline-susceptible Enterobacterales strains.
• Sexually Transmitted Infections (STIs): Monitors susceptibility of Chlamydia trachomatis and Ureaplasma urealyticum.

Doxycycline susceptibility testing is essential for managing respiratory, vector-borne, and sexually transmitted infections, particularly where tetracycline resistance genes are prevalent. Broth microdilution remains the gold standard, while molecular assays provide rapid resistance detection, supporting antimicrobial stewardship and precision therapy.