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Ophthalmic antimicrobial agents are among the most prescribed medications in eye care. The COA encounters these drugs daily in the context of conjunctivitis, corneal ulcers, post-surgical prophylaxis, and lid disease. Knowing the classes, their spectrums of activity, their appropriate indications, and the critical principle of culturing before treating is essential for both the COA exam and for supporting safe patient care.
The landscape of ophthalmic antimicrobials extends beyond bacterial antibiotics to include antifungal, antiviral, and antiprotozoal agents — each targeting specific pathogens that bacterial antibiotics will not address. Understanding the clinical context that suggests each organism (and therefore each agent class) is part of the COA Pharmacology domain.
| Class | Examples | Spectrum | Mechanism | Primary Indication |
|---|---|---|---|---|
| Fluoroquinolones (4th gen) | Moxifloxacin, gatifloxacin | Broad (gram+/gram−) | DNA gyrase/topoisomerase IV inhibition | Corneal ulcer, conjunctivitis |
| Fluoroquinolones (2nd–3rd gen) | Ciprofloxacin, ofloxacin | Broad (especially gram−) | DNA gyrase inhibition | Conjunctivitis, corneal ulcer |
| Aminoglycosides | Tobramycin, gentamicin | Gram− (Pseudomonas) | 30S ribosome binding | Gram-negative conjunctivitis/keratitis |
| Macrolides | Erythromycin, azithromycin | Gram+ and Chlamydia | 50S ribosome binding | Neonatal prophylaxis, blepharitis, chlamydial |
| Polymyxin combinations | Polytrim (polymyxin B + trimethoprim) | Gram+ and gram− | Cell membrane disruption (polymyxin B) + DHFR inhibition | Bacterial conjunctivitis (OTC) |
| Glycopeptides (fortified) | Vancomycin (compounded) | Gram+ (MRSA) | Cell wall synthesis (transpeptidase inhibition) | MRSA corneal ulcer |
| Antifungal | Natamycin 5% | Fusarium, Aspergillus | Ergosterol binding (membrane disruption) | Fungal keratitis |
| Antivirals | Trifluridine (viroptic), ganciclovir 0.15% gel | HSV-1, HSV-2, CMV | Thymidine kinase-mediated viral DNA polymerase inhibition | Herpetic keratitis |
Fluoroquinolones are the workhorse antibiotic class in ophthalmology. They work by inhibiting bacterial DNA gyrase and topoisomerase IV — enzymes essential for DNA replication and repair — causing bacterial cell death. They are bactericidal and provide broad-spectrum coverage, making them ideal for empirical treatment when the causative organism is unknown.
Corneal ulcers require much more aggressive dosing than conjunctivitis. A typical intensive regimen: every 15–30 minutes for the first 6 hours, then every 30–60 minutes while awake, then tapering over 2–4 weeks based on clinical response. The high frequency is needed to maintain bactericidal concentrations in the corneal stroma. This intensive schedule requires that the patient and caregiver understand the importance of compliance with an alarm-based schedule, including overnight dosing for severe ulcers.
Aminoglycosides are particularly effective against gram-negative organisms, including Pseudomonas aeruginosa (the most common causative organism in contact lens-related keratitis). They work by binding the 30S ribosomal subunit, causing misreading of the genetic code and synthesis of defective proteins, leading to bacterial cell death.
Tobramycin 0.3% (Tobrex)
Less corneal epithelial toxicity than gentamicin with prolonged use. Available in drop and ointment formulations. Also available in combination with dexamethasone (Tobradex) for post-surgical or inflammatory/infectious conditions. Used for bacterial conjunctivitis and as adjunct in corneal ulcers.
Gentamicin 0.3% (Garamycin)
Broader gram-negative coverage than tobramycin (some authorities say gentamicin has slightly less Pseudomonas activity). More corneal epithelial toxicity with intensive use. Fortified gentamicin (14 mg/mL, 0.3% = 3 mg/mL) is used for severe gram-negative corneal ulcers. Can cause punctate epithelial erosions with prolonged use.
Macrolides bind the 50S ribosomal subunit, inhibiting translocation of the ribosome along mRNA and thereby blocking bacterial protein synthesis. They are bacteriostatic at standard concentrations. Their coverage of gram-positive organisms and Chlamydia trachomatis makes them particularly valuable for neonatal prophylaxis, blepharitis, and chlamydial conjunctivitis.
Polymyxin B + trimethoprim (Polytrim) is one of the most commonly prescribed antibiotic preparations for bacterial conjunctivitis. Polymyxin B disrupts the bacterial outer membrane by binding to lipopolysaccharide (gram-negative cell walls), causing membrane permeability and cell death. Trimethoprim inhibits dihydrofolate reductase (DHFR), blocking bacterial folic acid synthesis. Together they provide broad-spectrum coverage of the most common conjunctivitis pathogens: S. aureus, S. pneumoniae, H. influenzae, E. coli, and Pseudomonas. The combination has a favorable safety profile and is available as a generic, making it cost-effective.
Methicillin-resistant Staphylococcus aureus (MRSA) is an increasing concern in ocular infections, particularly post-surgical endophthalmitis and corneal ulcers in immunocompromised patients or those with ocular surface disease. MRSA is resistant to all beta-lactam antibiotics (penicillins, cephalosporins) and often resistant to fluoroquinolones.
Treatment options for MRSA ocular infections: vancomycin (compounded fortified drops 25–50 mg/mL), topical linezolid (compounded), or systemic anti-MRSA agents (IV vancomycin for endophthalmitis). Cultures with sensitivities are essential for guiding MRSA treatment. The COA should be familiar with the concept of antibiotic resistance and the importance of sensitivity-guided therapy.
Fungal keratitis (fungal corneal ulcer) is less common than bacterial keratitis but potentially vision-threatening and requires specific antifungal therapy. Natamycin 5% suspension is the only commercially available topical ophthalmic antifungal in the US. It binds ergosterol (unique to fungal cell membranes) and disrupts membrane integrity.
When to Suspect Fungal Keratitis
Fungal keratitis classically follows corneal trauma with vegetable matter (plant material, thorn, tree branch) or in agricultural workers and immunocompromised patients. The ulcer has a feathery, dry, raised edge; satellite lesions; and progresses more slowly than bacterial ulcers. Fungal keratitis does NOT respond to antibiotics and will worsen with steroids — which can be catastrophic if misdiagnosed as bacterial.
Natamycin Dosing and Limitations
Natamycin is dosed every 1–2 hours initially for several days, then tapered over 4–6 weeks. Shake the white suspension before use (it settles). Limitations: poor penetration into the corneal stroma (it works primarily at the epithelial level), which is why deep fungal ulcers often require systemic antifungal therapy (voriconazole) in addition to topical natamycin.
Herpes simplex virus (HSV) keratitis is the most common cause of infectious corneal blindness in developed countries. It requires antiviral treatment — antibiotics have no activity against viruses. The classic finding is a dendritic corneal ulcer (branching, linear epithelial ulcer with terminal end bulbs), visible under fluorescein staining at the slit lamp.
The original topical antiviral for HSV keratitis. Incorporated into viral DNA, preventing replication. Dosed 9 times daily (every 2 hours while awake) for 14 days. Significant epithelial toxicity with prolonged use (toxic keratopathy) limits duration. Now largely replaced by ganciclovir gel in many settings.
Newer antiviral gel with better tolerability than trifluridine. Dosed 5 times daily until ulcer heals, then 3 times daily for 7 days. Lower toxicity profile. Also active against CMV. Ganciclovir is a guanosine nucleoside analog — it is phosphorylated by viral thymidine kinase and inhibits viral DNA polymerase.
For any significant ocular infection (corneal ulcer, severe conjunctivitis, endophthalmitis), obtaining microbiological specimens before starting antimicrobial therapy is the standard of care. This allows culture and sensitivity testing to direct targeted therapy. The exception is minor bacterial conjunctivitis in a community setting, where empirical therapy with a broad-spectrum agent is reasonable.
Corneal Scraping Technique
After topical anesthesia (use preservative-free if possible to avoid inhibiting culture growth), a sterile Kimura spatula, 21-gauge needle, or Dacron swab is used to scrape the base and edges of the ulcer under slit lamp magnification. Specimens are immediately plated onto blood agar (gram-positive), chocolate agar (Haemophilus, N. gonorrhoeae), Sabouraud agar (fungi), and thioglycolate broth (anaerobes and enrichment). Gram stain smears are made simultaneously for immediate preliminary identification.
Antibiotic Resistance Warning
Fluoroquinolone resistance is increasing, particularly among Staphylococcus epidermidis, MRSA, and Pseudomonas aeruginosa. Empirical fluoroquinolone monotherapy may fail for these organisms. Prior antibiotic use (especially fluoroquinolone) is a risk factor for resistant organisms. Sensitivity-guided therapy adjustments should be made as culture results return, typically within 24–72 hours.
Opterio includes antibiotic pharmacology questions within the COA exam domains, with AI explanations that connect drug classes to clinical scenarios and resistance principles.
Conjunctivitis, keratitis, corneal ulcers, and other anterior segment diseases.
Antibiotic toxicity, aminoglycoside corneal effects, and allergy vs toxic conjunctivitis.
Anesthetic use before corneal procedures including culture and foreign body removal.
Exam format, content domains, eligibility, pass rates, and registration guide.
Fourth-generation fluoroquinolones — specifically moxifloxacin (Vigamox) and gatifloxacin (Zymaxid) — are the first-line choice for bacterial corneal ulcers. They provide broad-spectrum coverage including Pseudomonas aeruginosa (common in contact lens-related ulcers) and gram-positive organisms. They penetrate the corneal stroma well at high concentrations without requiring preservatives. For severe ulcers, fortified antibiotic drops (tobramycin 14 mg/mL + cefazolin 50 mg/mL, or vancomycin for MRSA) prepared by compounding pharmacies may be needed. Before starting treatment, corneal scrapings should be obtained for Gram stain and culture to guide targeted therapy.
Erythromycin 0.5% ophthalmic ointment is a macrolide antibiotic with several key uses: (1) Neonatal ocular prophylaxis — mandatory in most US states for all newborns within 1 hour of birth to prevent ophthalmia neonatorum (gonococcal and chlamydial conjunctivitis). (2) Bacterial conjunctivitis — mild cases, particularly in children; also active against Chlamydia trachomatis causing inclusion conjunctivitis. (3) Blepharitis — applied to the lid margins for staphylococcal blepharitis. (4) Mild corneal abrasions — the ointment vehicle protects the healing epithelium and the antibiotic prevents secondary infection. The ointment formulation blurs vision temporarily after instillation, making drops preferred during daytime. Resistance among staphylococci is common.
Aminoglycosides (tobramycin, gentamicin) have excellent gram-negative coverage (Pseudomonas, Klebsiella, E. coli) but poor gram-positive and anaerobic coverage. They work by irreversibly binding the 30S ribosomal subunit, causing misreading of mRNA and defective bacterial protein synthesis. They are bactericidal. Fluoroquinolones have broader spectrum coverage (both gram-positive and gram-negative), better corneal penetration, and are generally preferred for corneal ulcers. Aminoglycosides are often used in combination (e.g., with erythromycin ointment or a gram-positive agent for corneal ulcers requiring broad-spectrum coverage), or as primary treatment for gram-negative conjunctivitis. Tobramycin 0.3% (Tobrex) has less corneal epithelial toxicity than gentamicin with longer use. Neither aminoglycoside is effective against Chlamydia, MRSA, or viruses.
Natamycin 5% suspension (Natacyn) is the only topical antifungal commercially available in the US specifically formulated for ophthalmic use. It is the treatment of choice for fungal keratitis (fungal corneal ulcer), particularly Fusarium and Aspergillus species. Natamycin works by binding ergosterol in the fungal cell membrane, increasing permeability and causing cell death. It requires frequent instillation (every 1–2 hours initially, then tapered over weeks), poor corneal penetration into the stroma is a known limitation. Voriconazole (compounded ophthalmic or systemic) is used for Aspergillus and other molds resistant to natamycin. Fungal keratitis typically follows corneal trauma with plant or soil material (e.g., a thorn scratch), which distinguishes it from bacterial ulcers. Diagnosis requires corneal scraping and specific fungal stains/cultures.
Obtaining corneal scrapings for Gram stain, culture, and sensitivity before initiating antibiotic therapy is the standard of care for corneal ulcers for several reasons: (1) Identifies the specific pathogen so treatment can be targeted rather than empirical. (2) Determines antibiotic sensitivity, which guides switching if the initial agent fails. (3) Identifies organisms requiring specific therapy that broad-spectrum antibiotics may miss, including fungi (require antifungals, not antibiotics), Acanthamoeba (requires specific agents like PHMB, propamidine), and HSV (requires antivirals). (4) Prevents unnecessary antibiotic escalation — knowing the organism avoids adding more agents when one targeted drug is sufficient. In clinical practice, empirical broad-spectrum fluoroquinolone therapy is started immediately while awaiting culture results, with adjustment based on sensitivity data.