What should be the direction of microbiological investigation for a patient with a greyish-white corneal ulcer and feathery infiltration?

Microbiological Investigation for Fungal Keratitis

The microbiological investigation should be directed against fungal pathogens, specifically filamentous fungi, as the clinical presentation of feathery, finger-like infiltration with indistinct elevated margins is pathognomonic for fungal keratitis. 1

Clinical Features Indicating Fungal Etiology

The key diagnostic features in this case that point toward fungal infection include:

• Feathery, finger-like infiltration into adjacent corneal stroma is a characteristic finding that distinguishes fungal from bacterial keratitis 1
• Indistinct, elevated margins of the ulcer (rather than the well-demarcated suppurative margins typical of bacterial infections) 1
• The greyish-white appearance with stromal involvement and hypopyon formation 1

Even experienced cornea specialists correctly differentiate bacterial from fungal keratitis in less than 70% of cases based on clinical appearance alone, making microbiological confirmation essential 1

Required Diagnostic Procedures

Corneal Scraping with Specific Stains

Obtain corneal scrapings and perform:

• Calcofluor-KOH stain (most sensitive for fungal elements) 1
• Potassium hydroxide (KOH) preparation 1
• Gram stain (to rule out bacterial co-infection) 1
• Giemsa stain 1

Fungal Culture

• Direct bedside inoculation of culture plates with corneal scraping material is critical for optimal yield 1
• Use at least one culture plate or slant containing nonselective fungal growth medium 1
• Cultures should be maintained for extended periods as fungi are slow-growing organisms 2

Additional Diagnostic Considerations

• Confocal microscopy can be helpful for identifying fungal elements in vivo, particularly for filamentous fungi 1
• PCR testing may complement traditional methods for rapid specific identification, though it cannot distinguish between viable organisms and antigenic material 1, 2

Common Fungal Pathogens to Consider

The most frequent filamentous fungi causing this presentation include:

• Fusarium species 1, 2
• Aspergillus species 1, 2
• Dematiaceous fungi 1
• Colletotrichum species (coelomycetous fungi) 3

Critical Pitfalls to Avoid

• Do not delay fungal-specific testing based solely on clinical appearance, as bacterial and fungal keratitis have overlapping features 1
• Avoid empiric corticosteroid use until fungal infection is definitively ruled out, as steroids can worsen fungal keratitis outcomes 4
• Do not perform aqueous tap for the hypopyon alone, as it represents sterile inflammatory response in uncomplicated keratitis 5
• Ensure adequate sample collection from the leading edge of the infiltrate where viable organisms are most likely present 1

When Bacterial Investigation is Also Warranted

While the clinical picture strongly suggests fungal etiology, concurrent bacterial cultures should be obtained because:

• Atypical organisms like Nocardia can produce similar feathery infiltrates and require acid-fast staining 1
• Mycobacterial infections (particularly atypical mycobacteria) can present with chronic, slowly progressive ulcers 6
• Bacterial superinfection can occur 1