How do bacteria, viruses, fungi, and protozoans cause infectious keratitis?

Infectious keratitis is an inflammation of the cornea caused by bacteria, viruses, fungi, and protozoans. The pathology of these infections typically involves the following mechanisms:1

• Bacteria: Most bacteria cannot penetrate the intact surface of the cornea. Consequently, bacterial infections usually occur following an injury to or a breakdown of the corneal surface.2
• Viruses: Viral infections are more common than bacterial ones. For example, herpesviruses can cause dendritic ulcers, where the virus replicates in the host cell, often leading to recurrent inflammation and potential scarring.12
• Fungi: While the normal cornea is resistant to fungal spores present in the atmosphere, infection can develop after a corneal injury or lesion. The risk of fungal keratitis is notably higher if corticosteroid drugs have been used to treat existing corneal conditions.2
• General Factors: The use of contact lenses significantly increases the risk of serious infectious keratitis. Treatment generally requires culturing the tissue to identify the specific pathogen and administering targeted antimicrobial therapy.1