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.
Antibiotic Class Overview
| 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: Broad-Spectrum First-Line Agents
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.
4th Generation (Preferred)
- Moxifloxacin 0.5% (Vigamox): no BAK preservative; excellent S. aureus and S. pneumoniae coverage; corneal ulcer standard of care
- Gatifloxacin 0.3%/0.5% (Zymaxid/Zymar): similar spectrum; enhanced gram-positive vs older generations
- Besifloxacin 0.6% (Besivance): first ophthalmic-only fluoroquinolone; no systemic formulation reduces resistance development
- Advantage over older FQ: better MRSP (methicillin-resistant Staphylococcus pseudintermedius) and improved gram+ coverage
2nd–3rd Generation
- Ciprofloxacin 0.3% (Ciloxan): excellent Pseudomonas coverage; white corneal precipitates can form with intensive dosing (not infection)
- Ofloxacin 0.3% (Ocuflox): broad coverage; widely available; effective for bacterial conjunctivitis and keratitis
- Levofloxacin 0.5%/1.5% (Iquix/Quixin): good overall coverage; 1.5% preferred for severe infections
- Lower gram-positive coverage vs 4th gen; still appropriate for conjunctivitis
Dosing for Corneal Ulcer (Intensive Regimen)
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: Tobramycin and Gentamicin
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: Erythromycin and Azithromycin
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.
Erythromycin 0.5% Ointment
- Neonatal prophylaxis: mandatory in most US states for N. gonorrhoeae and C. trachomatis prevention
- Blepharitis: applied to lid margins for S. aureus blepharitis
- Chlamydial conjunctivitis: needs systemic azithromycin for cure (topical alone insufficient)
- Minor abrasions: ointment vehicle + antibiotic protection
- Resistance among staphylococci is common
Azithromycin 1% (AzaSite)
- Longer-lasting tissue concentrations (dosed twice daily × 2 days, then once daily × 5 days)
- Approved for bacterial conjunctivitis
- Enhanced penetration into lid tissue — useful for posterior blepharitis (meibomian gland disease)
- DuraSite vehicle enhances mucoadhesion and prolonged contact time
Polymyxin Combinations and OTC Agents
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.
Anti-MRSA Options: Fortified Vancomycin
MRSA Corneal Infections
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.
Antifungal Agents: Natamycin for Fungal Keratitis
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.
Antiviral Agents: Treating Herpetic Keratitis
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.
Trifluridine 1% (Viroptic)
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.
Ganciclovir 0.15% Ophthalmic Gel (Zirgan)
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.
The Principle: Culture Before Treatment
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.
Practice COA Antibiotic Pharmacology Questions
Opterio includes antibiotic pharmacology questions within the COA exam domains, with AI explanations that connect drug classes to clinical scenarios and resistance principles.