Ocular Microbiology

Ocular Defenses & Flora - Eye's Tiny Guardians

• Physical Barriers: Eyelids, eyelashes, rapid blink reflex.
• Tear Film Layers:
  • Mechanical flushing action.
  • Antimicrobial: Lysozyme, lactoferrin, secretory IgA (sIgA), β-defensins.
• Epithelial Integrity: Corneal & conjunctival tight junctions prevent microbial entry.
• Normal Commensal Flora:
  • Primarily Gram-positive: Staphylococcus epidermidis (dominant), Corynebacterium spp.
  • Also: Propionibacterium acnes.
  • Function: Competitive inhibition of pathogens.

⭐ Lysozyme in tears hydrolyzes peptidoglycan in bacterial cell walls, a key non-specific defense. oka

Bacterial Eye Villains - Common Culprits

Bacteria are major causes of sight-threatening ocular infections. Identification via culture & Gram stain is crucial.

Key Gram-positive culprits:

• Staphylococcus aureus: Most frequent; keratitis, blepharitis, conjunctivitis, endophthalmitis. Toxin producer.
• Streptococcus pneumoniae: Corneal ulcers (post-trauma), dacryocystitis. Lancet-shaped diplococci.

Key Gram-negative culprits:

• Pseudomonas aeruginosa: Aggressive keratitis (contact lens users!), endophthalmitis. Pyocyanin producer.

  ⭐ P. aeruginosa keratitis: ophthalmic emergency; can perforate cornea in 24-48 hrs.

• Haemophilus influenzae: Conjunctivitis (children), preseptal/orbital cellulitis. Needs X & V factors.
• Neisseria gonorrhoeae: Hyperacute conjunctivitis, ophthalmia neonatorum. Thayer-Martin agar.

Viral Eye Invaders - Stealthy Attackers

• Herpes Simplex Virus (HSV):
  • Primarily HSV-1.
  • Keratitis: Classic dendritic ulcer (fluorescein stain positive).
  • Can cause uveitis, retinitis. Latency in trigeminal ganglion.
• Varicella-Zoster Virus (VZV):
  • Herpes Zoster Ophthalmicus (HZO): CN V1 involvement.
  • 📌 Hutchinson's sign (tip/side of nose vesicles) indicates ↑ ocular risk.
• Adenovirus:
  • Highly contagious. Epidemic Keratoconjunctivitis (EKC), Pharyngoconjunctival Fever (PCF).
  • Keratitis: Subepithelial infiltrates (SEIs) may persist.
• Cytomegalovirus (CMV):
  • Retinitis in immunocompromised (e.g., AIDS with CD4 < 50 cells/µL).
  • "Brushfire" or "pizza-pie" fundus.

⭐ Dendritic ulcer, staining with fluorescein, is pathognomonic for HSV keratitis.

Fungal & Parasitic Foes - Uncommon Intruders

• Fungal Keratitis:
  • Aspergillus, Fusarium: Feathery infiltrates, satellite lesions (trauma with plant matter). Rx: Natamycin.
  • Candida: Creamy infiltrates (immunocompromised). Rx: Amphotericin B/Voriconazole.
  • Dx: KOH, Gram stain, culture (SDA). Confocal microscopy helpful.
• Parasitic Keratitis:
  • Acanthamoeba: Contact lens wear; severe pain disproportionate to signs; ring infiltrate (late); radial keratoneuritis (pathognomonic).
    • Rx: PHMB 0.02%, Chlorhexidine 0.02%. Prolonged therapy.
  • Toxocara canis: Ocular Larva Migrans; unilateral vision loss, strabismus; posterior pole granuloma.
  • ⭐ > Toxoplasma gondii retinochoroiditis is the most common cause of posterior uveitis. Presents as "headlight in fog" with adjacent pigmented scar.

Lab & Stains - Microbe Detectives

• Microscopy & Stains: Key for rapid, initial diagnosis.
  • Gram Stain: Bacteria (Gram +ve/-ve)
  • Giemsa: Chlamydia, Acanthamoeba cysts, Fungi
  • KOH Mount: Fungal elements (dissolves host cells)
  • Ziehl-Neelsen: Mycobacteria (AFB)
  • Calcofluor White: Fungi (fluorescent)
• Culture: Gold standard (Bacteria: Blood/Chocolate Agar; Fungi: SDA)
• PCR: Sensitive for Viruses, Chlamydia, Acanthamoeba.

⭐ Giemsa stain is highly versatile, aiding in the identification of Chlamydial inclusion bodies, Acanthamoeba cysts, and fungal elements in ocular specimens.

High‑Yield Points - ⚡ Biggest Takeaways

• Pseudomonas aeruginosa: Leading cause of bacterial keratitis, especially in contact lens wearers.
• Adenovirus: Most common agent in viral conjunctivitis.
• Fungal keratitis (Aspergillus, Fusarium): Often follows ocular trauma with vegetative matter.
• Acanthamoeba keratitis: Linked to contact lens use and water exposure; causes severe pain.
• Chlamydia trachomatis: Causes trachoma (serotypes A-C) and inclusion conjunctivitis (serotypes D-K).
• Acute post-op endophthalmitis: Commonly due to Staphylococcus epidermidis.
• Herpes Simplex Keratitis: Characterized by dendritic ulcers.