Contributor: Gordon K. Klintworth
Infectious keratitis (microbial keratitis) includes all cases of keratitis caused by microbial organisms. The eye is constantly exposed to numerous bacteria, but relatively few microorganisms result in an infectious keratitis. Microbial keratitis can be subdivided into keratitis due to bacteria [keratitis - bacterial], fungi [keratitis - fungal], viruses [keratitis - viral], amebae [keratitis - amebic] and other parasites [keratitis - parasitic]. Conditions predisposing to bacterial ulcerative keratitis include conjunctivitis, contact lenses, topical corticosteroids, accidental trauma, dry eyes, trichiasis, and a poor blink reflex. Risk factors for microbial keratitis following a penetrating keratoplasty include broken or exposed sutures, treatment with corticosteroids, use of antibiotics, epithelial defects, trauma and contact lens use. Causes of acute bacterial keratitis include Neisseria gonorrhoeae, Neisseria meningitidis, H. influenzae, staphylococcus, and streptococcus. Complications of bacterial keratitis include endophthalmitis [endophthalmitis - bacterial]. 

 Most bacteria cannot penetrate the corneal surface directly and only enter the tissue following accidental or surgical trauma. A loss of epithelial cells on the surface of the cornea following physical trauma or other injuries predisposes to infectious keratitis. Often the precipitating injury is minor or unnoticed and may be forgotten by the time that the keratitis develops. Contact lens wear is associated with an increased risk of infectious keratitis. Although the overall incidence of ulcerative keratitis occurring in contact lens patients is low, numerous cases with complications occur because of the large number of persons using contact lenses. Infections tend to be more common in patients who use soft lenses (both day-wear and exended-wear) than hard or gas permeable lenses. Infectious ulcers associated with disposable lenses have also been reported.
Marginal keratitis is common in persons with functionally abnormal leukocytes.
Some highly virulent bacteria (Neisseria gonorrhoeae [gonorrhea], Neisseria meningiditis [meningococcal infection], and Corynebacterium diphtheriae [diphtheria]) can invade an intact corneal epithelium. After disruption of the corneal epithelium, bacteria vary in their ability to adhere to the the underlying corneal stroma. Factors influencing this adherence include structural components of the cell wall of the pathogen (such as pili or fibriae of Pseudomonas). Pseudomonas [Pseudomonas Aeruginosa infection] is one of the most common bacteria causing ulcerative keratitis in persons wearing contact lenses. Several lectins on the surface of Pseudomonas may promote adherance by binding to carbohydrate moieties in the cornea. Exotoxin A, produced by most strains of Pseudomonas, is toxic to corneal endothelium and epithelium. Pseudomonas also elaborates proteases which are capable of destroying corneal and scleral collagen and endotoxins discussed below. Keratitis due to Moraxella occurs mainly in alcoholics and debilitated persons. Gram-negative bacteria also cause ulcerative keratitis, but primary ocular infection rarely results in sepsis or endotoxic shock. Conditions predisposing to bacterial ulcerative keratitis include contact lenses, topical corticosteroids, accidental trauma, dry eyes, trichiasis, and a poor blink reflex. Diseases of the host that predispose to bacterial infection in general include conditions with defective neutrophils (Chédiak-Higashi syndrome, chronic granulomatous disease of childhood). Other factors that predipose to ocular infection are contact lenses [contact lens - complications].
The use of topical corticosteroids on the eye dramatically dampens the effects of neutrophil-mediated inflammation. The lack of inflammatory response to invading bacteria in pseudocrystalline keratopathy is the result of topical corticosteroids. Bacteria proliferate relatively unimpeded between collagen lamellae in the virtual absence of inflammation. Infectious pseudocrystalline keratopathy is frequently caused by the growth of minimally virulent bacteria, such as Streptococcus viridans, and usually occurs in eyes having had previous accidental or surgical trauma.
Bacterial endophthalmitis [endophthalmitis - bacterial] may follow bacterial keratitis.