Microbiology

The ocular surface is not sterile. A healthy lid margin and conjunctival sac carry a stable resident flora dominated by coagulase-negative staphylococci (chiefly Staphylococcus epidermidis), with Corynebacterium species (diphtheroids), smaller numbers of Staphylococcus aureus, and anaerobic Cutibacterium (formerly Propionibacterium) acnes in the lash follicles and meibomian glands. Tears defend that surface mechanically by flushing, and chemically through lysozyme, lactoferrin and secretory IgA, while an intact corneal epithelium is the physical barrier that keeps the flora outside.

Almost every clinically important eye infection is a story about one of those three things failing: the barrier is broken (abrasion, contact lens, surgery, trauma), the flora overgrows (blepharitis, hordeolum), or an organism arrives that does not need a break at all. That last group is short and worth memorizing, because it explains why hyperacute conjunctivitis is an emergency: Neisseria gonorrhoeae, Corynebacterium diphtheriae, Haemophilus and Listeria are the classically cited organisms able to penetrate an intact corneal epithelium.