Culturing the Cornea for the Primary Care Optometrist
AT A GLANCE
- Corneal infections occur when there is a break in the epithelium, and they result in discomfort and reduced vision.
- Most corneal infections can be managed empirically based on patient history and clinical characteristics.
- On rare occasions, uncommon or unlikely pathogens may call for culturing the cornea.
Corneal infections, including abrasions and ulcers, occur when there is a break in the epithelium, and they result in discomfort and reduced vision.
The corneal surface is protected by the physical, mechanical, and immunoactive barriers of the lids, the blink mechanism, conjunctival epithelial cells, and the tear film. The immune barrier of the tear film is composed of lactoferrin, lysozyme, beta lysin, tear-specific albumin, and immunoglobulin A (IgA).1 An ulceration of the cornea manifests as an excavation of tissue associated with a defect in the corneal epithelium. Annually in the United States, 1 million clinic and emergency room visits are attributed to infectious keratitis and ulcers.2
When a patient presents to the office with pain, photophobia, discharge, and reduced vision, the initial response is to carefully examine the cornea, cautiously looking for epithelial defects, stromal edema, folds in Descemet membrane, and so on: that is, any clinical signs that may indicate that the immune-privileged barrier has been infiltrated. Prompt evaluation of corneal infections empirically based on patient history and clinical characteristics will often suffice.3,4 However, there are rare occasions when uncommon or unlikely pathogens can create superinfections or mask the true etiology of an infection. In these instances, culturing the cornea may be indicated.
WHEN AND WHY TO CULTURE
Cornea specialists agree that large, sight-threatening infections that are unresponsive to treatment, where atypical organisms may be suspected, should be cultured. Atypical organisms can be suspected in cases involving contact lens overwear or abuse, injury or trauma, or an unusual case history.5,6 Clinical signs that may indicate corneal infections that are not bacterial in nature include feathery edges, satellite lesions (Figure), ring-shaped ulcers, slow or poor healing, and pain disproportionate to presentation.7 It is especially important to pay close attention to higher risk patients, including individuals who are immune-compromised, monocular, or postsurgery (see Clues for When to Culture an Ulcer.)
Clues for When to Culture an Ulcer
Indications To Culture
- Ulcer is >2 mm diameter
- Central vision is affected
- Stroma is involved
- Ulcer is unresponsive to treatment
Signs Indicating Nonbacterial Cause
- Feathery edges
- Satellite lesions
- Raised or gray ulcers
- Ring-shaped ulcer
- Descemetocele
HOW TO CULTURE
When we culture the cornea, we begin by discussing risks and benefits with the patient in addition to obtaining informed consent. Before culturing, we collect and label culture kits, media plates, and/or glass slides. The media plates are allowed to come to room temperature. Subsequently, topical anesthetic is instilled. Proparacaine is the preferred anesthetic because it possesses the least bactericidal properties.5,8 Although tetracaine is a better anesthetic, it contains antimicrobial properties that may interfere with organism recovery when cultures are performed.
Many tools can be used to culture the cornea. Some pathology labs may provide standard culture kits or quick culture kits that contain swabs with sterile tubes. Common instruments to use include golf spud, blade, Kimura spatula, or a culturette swab. With the selected instrument, we gently scrape or swab the cornea with mild pressure at the base of an ulcer and/or the leading edge of an infiltrate. The leading edge of an infiltrate and base of an ulcer contain the highest microbe concentration.8
GETTING STARTED
Not familiar with culturing? Here are some guidelines to help get you started.
- Establish differential diagnosis (fungal, bacterial, viral, parasitic).
- Call the pathology lab at a local hospital or private lab.
- The pathologist may make suggestions on media or slides based on turnaround time (eg, Giemsa stain for identifying Chlamydia will take about 1 week versus basic wet mount gram stain, which will be ready as early as 24 hours from culture time).
- Labs will have basic quick culture kits to keep in your office (including sterile swab and broth to transport organisms). Pathology labs will typically handle processing, plating, and wet mounts.
- At the very least, getting a quick culture kit to send to a local pathology lab for bacteria identification can be helpful in targeting treatment, is accessible, and covers you legally.
The edge of the instrument should be moving in one direction only and should be held tangential to the cornea to minimize risk of corneal perforation. We avoid contacting the eyelids or eyelashes with the instrument to prevent possible cross-contamination by skin flora.
After culturing, the specimen should be transferred onto media in a C-shaped streak pattern to avoid breaking the surface of solid agars on the culture plates.8,9 Quick culture kits contain sterile swabs and prepared broth that are ready to be sent back to pathology labs.
When fungus is suspected, Sabouraud agar and potato dextrose agar are appropriate media in addition to Giemsa and potassium hydroxide wet mount slides. With suspicion of aerobic bacteria, chocolate agar is appropriate. For aerobic and anaerobic bacteria, blood agar and thioglycolate broth are good places to start. In nonhealing bacterial infections, evaluation for the gram-negative diplococci aerobic bacteria Neisseria, gram-negative rod-shaped aerobic bacteria Chlamydia, and gram-negative rod-shaped facultative anaerobe Pseudomonas should all be considered. The presence of Chlamydia, however, can be confirmed only with Giemsa staining. Tables 1 and 2 offer further details.
GET COMFORTABLE WITH CULTURING
A thorough patient history and detailed clinical exam will often suffice in managing corneal ulcers. However, when faced with an unresponsive or unusual case, culturing the cornea can help guide a clinician in formulating an ideal treatment plan. It is wise to become familiar with your local pathology lab and to keep a few standard or quick culture kits on hand, should the need to culture a cornea arise. Corneal culturing can help to ensure optimal visual outcomes in patients with complex corneal infectious pathologies.
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