Introduction
Infectious keratitis is one of the leading causes of blindness in the world. Because, in most cases, these infections represent preventable or treatable ophthalmic diseases, a thorough understanding of the epidemiology, diagnosis, and treatment of the various forms of infectious keratitis is essential for eye-care practitioners and public health officials.
Epidemiology and Pathogenesis
The estimated incidence of ulcerative keratitis in the United States is 28 per 100 000 person years, with a higher incidence among contact lens wearers (130 per 100 000 person years). The incidence of infectious keratitis in the developing world is even higher, with estimated incidence rates ranging from 100 to 800 per 100 000 person years. Given the potential blinding complications of severe bacterial keratitis, these infections are a significant public health issue. A host of bacterial organisms can cause infectious keratitis. The incidence of infection by specific organisms varies by region. Practitioners should be aware of the local epidemiological patterns of corneal infection. Whereas staphylococcal species are most commonly seen in Canada and the eastern and northeastern United States, Pseudomonas infection is more common in the southern United States. Streptococcus pneumoniae was once the most common pathogen isolated from bacterial corneal ulcers, but as contact lens wear and related infectious keratitis have increased, the relative incidence of pseudomonal and staphylococcal infection has increased. These two organisms account for the majority of infections associated with contact lens wear, followed by Serratia marcescens. Corneal infections that occur in patients with systemic debilitating conditions, such as alcohol abuse, malnutrition, or diabetes often are associated with Moraxella. In the developing world, streptococcal corneal infection remains the most common, followed by staphylococcal and pseudomonal keratitis.
The corneal surface is normally well protected by a variety of mechanisms. The eyelids and eyelashes form a physical outer barrier to foreign material, and the blink reflex sweeps away debris trapped in tears. A second line of defense is the tear film, which contains a variety of antimicrobial and anti-inflammatory factors, such as lactoferrin, lysozyme, beta-lysin, tear-specific albumin, and immunoglobulin A (IgA). Finally, the corneal and conjunctival epithelial cells provide a barrier via their tight junctions, express molecules important for innate immunity (e.g., toll-like receptors), and produce a variety of antimicrobial peptides.
The conjunctiva provides additional protection from infection. The conjunctiva contains mast cells, which, when activated, induce vascular dilatation and increased vascular permeability, which results in the production of an antimicrobial transudate. The conjunctiva also contains conjunctiva-associated lymphoid tissue (CALT), which consists of nodules of small and medium-sized lymphocytes responsible for local antigen processing. Plasma cells, macrophages, and a variety of T cells also are present, as well as IgG, IgA, and IgM, which are brought in by the conjunctival vasculature.
The natural microbial environment of the ocular surface consists of both sessile and free-floating bacteria. This population is kept in check by the antimicrobial features of the tear film, and by the products of resident microbes; these products, called bacteriocins , are high-molecular-weight proteins that inhibit the growth of pathogens, such as pneumococci and Gram-negative bacilli.
In the majority of cases where bacterial keratitis develops, at least one risk factor that represents a compromise of one or more of these defense mechanisms can be identified. In developed countries, soft contact lens wear is the most important risk factor. Contact lenses likely cause physiological and traumatic changes to the ocular surface, additionally providing a scaffold for bacterial biofilm formation. Extended or overnight contact lens wear and poor lens hygiene substantially increase the risk; unfortunately, daily disposable lenses may not reduce this risk. Corneal trauma, as well as keratorefractive procedures, such as laser in situ keratomileusis (LASIK), can disrupt the epithelial barrier and allow invasion of infectious organisms into the stroma ( Fig. 4.12.1 ). Lid abnormalities, such as entropion or ectropion, exposure of the corneal surface, or trichiasis, can cause breakdown of the protective corneal epithelium. Poor tear production can lead to a reduction of antimicrobial tear components and epithelial desiccation and damage. Epithelial problems, such as bullous keratopathy, medication toxicity, and prior herpetic infection, can allow microbial adherence and invasion. Drugs that may be smoked, such as cocaine and methamphetamine, have been associated with microbial keratitis, probably because of a direct toxic effect, exposure keratopathy, neurotrophic changes, or mechanical trauma ( Fig. 4.12.2 ). Local or systemic immune compromise can lead to impairment of local immune defenses. This is most commonly caused by the use of topical corticosteroids, but immunosuppression, malignancy, malnutrition, or extensive burns also can cause it. Occasionally, keratitis can be established via the corneoscleral limbus by hematogenous spread.
Clinical Features
The clinical signs and symptoms of bacterial keratitis depend greatly on the virulence of the organism and the duration of infection. Other influential factors include the previous status of the cornea and the use of corticosteroids. Patients may describe decreased vision, pain, and photophobia. The cardinal corneal sign is a localized or diffuse infiltration of the epithelium or stroma ( Fig. 4.12.3 ). Commonly, epithelial absence over a gray-white necrotic stromal infiltrate occurs. Less commonly, a stromal abscess can appear beneath an intact epithelium. Infiltration and edema of the cornea can appear distant to the primary site of infection. Occasionally, bacterial keratitis can present with predominantly multifocal epithelial infiltration, especially in the setting of soft contact lens wear.
Other ocular structures usually demonstrate associated inflammation. Some degree of lid erythema and edema, conjunctival injection and chemosis, tearing, and discharge often occur. A nonspecific conjunctival papillary response might be seen. Anterior chamber inflammation often is present, ranging from cells and flare in milder cases to hypopyon in more severe cases. The aqueous might become dense and fibrinoid, and fibrinous endothelial plaques may develop. The hypopyon usually is sterile unless accompanied by a full-thickness corneal perforation.
Gram-Positive Cocci
Staphylococcus
Staphylococci are Gram-positive cocci, which, on stained smears, tend to appear singly or in pairs, although clusters of organisms can be seen. The two most common species that cause keratitis are Staphylococcus aureus and Staphylococcus epidermidis , both of which are commonly found on skin, eyelids, and the conjunctiva. Although non- aureus strains are usually less virulent, antibiotic resistance tends to be more common, and aggressive keratitis occasionally occurs.
Infection tends to occur in compromised corneas, such as those with bullous keratopathy, herpetic disease, and persistent epithelial defect. Usually a well-defined, cream-colored or gray-white stromal infiltrate with an overlying epithelial defect is seen. Sometimes multiple foci of abscesses can develop that resemble fungal satellite lesions. S. aureus tends to cause more severe infiltration and necrosis than S. epidermidis. Over time, the former can extend deep into the stroma, and necrosis of this abscess can lead to perforation. A hypopyon and endothelial plaque can be seen.
Streptococcus
Streptococci are Gram-positive cocci. On stained smears, most species tend to appear in chains, but they also can be arranged singly, in pairs, or in loose clusters. The most common species causing keratitis is Streptococcus pneumoniae ( Pneumococcus ), which appears as lancet-shaped diplococci arranged with the flattened ends together. Streptococcal species are distinguished by their ability to hemolyze red blood cells. Streptococcus viridans and S. pneumoniae do so partially (alpha-hemolysis), S. pyogenes completely (beta hemolysis), and gamma-hemolytic species do not hemolyze red blood cells at all.
Pneumococcal infections can readily spread, producing a deep stromal abscess, fibrin deposition, plaque formation, severe anterior chamber reaction, hypopyon, and iris synechiae ( Fig. 4.12.4 ). The advancing necrosis often produces an undermined leading edge with overhanging tissue. Infection by Streptococcus pyogenes occurs less frequently but has a similar, severe presentation and course. Group viridans Streptococcus spp . tend to cause less aggressive disease and are associated with a more indolent course, as is seen in infectious crystalline keratopathy.
Infectious crystalline keratopathy describes a particular pattern of corneal infiltration characterized by needle-like opacities that can be found at all levels of the corneal stroma ( Fig. 4.12.5 ). The crystalline opacities range from fine, feathery, and white to thick, brown, arborizing aggregations, without apparent cellular infiltrate or ocular inflammation. The entity most frequently occurs in corneal grafts but has also been associated with other conditions, such as incisional keratotomy, epikeratophakia, contact lens wear, chemical burns, and topical anesthetic abuse. Long-term corticosteroid therapy is thought to play a role in the pathogenesis. Although the viridans group of streptococci accounts for most cases, other organisms associated with infectious crystalline keratopathy include S. pneumoniae , Haemophilus aphrophilus , Peptostreptococcus , Pseudomonas aeruginosa , and a number of other bacteria, as well as Candida and Alternaria fungal species.
Gram-Positive Bacilli
Bacillus
Bacillus cereus is an aerobic, spore-forming, typically Gram-positive rod, although considerable variability in staining characteristics has been noted. They are ubiquitous and are found in water, in soil, and on vegetation. Corneal infection, therefore, can be seen after penetrating injury, especially when soil contamination occurs. Bacillus infection also has been reported in contact lens–related keratitis. Posttraumatic infection characteristically develops within 24 hours of injury and is associated with chemosis, profound lid edema, and proptosis. A diffuse or a peripheral ring of microcystic edema often occurs, followed by a circumferential corneal abscess. This is an extremely virulent organism, and perforation of the cornea can develop within hours.
Corynebacterium
The corynebacteria, which include C. diphtheriae , are Gram-positive, club-shaped or pleomorphic rods arranged in the so-called Chinese-letter formation, Y’s, or palisades. An infrequent cause of keratitis, the clinical picture characteristically begins with diffuse epithelial haze, followed by stromal necrosis and melting.
Listeria
Listeria monocytogenes is a Gram-positive, short, rod-shaped facultative anaerobe. Infection usually occurs in animal handlers. It can colonize persistent epithelial defects and lead to a necrotizing keratitis. Typically, a ring ulcer and an exuberant anterior chamber reaction with fibrinous exudate and a hypopyon occur.
Clostridium
Clostridia are anaerobic, spore-forming, Gram-positive bacilli. Infrequently, clostridial conjunctivitis can be associated with the development of a marginal keratitis. Direct corneal infection is associated with marked edema and a frothy, bullous keratitis caused by trapped intraepithelial, subepithelial, and intrastromal gas produced by the organism. Gas might also be seen in the anterior chamber.
Propionibacterium acnes
Propionibacterium acnes is an anaerobic, nonspore-forming, Gram-positive rod. It forms part of the normal flora of the eyelid and conjunctiva. Keratitis can be established in the setting of corneal disease, trauma, surgery, contact lens wear, or chronic topical corticosteroid use. Although keratitis caused by P. acnes can assume the appearance of typical infectious keratitis, the infection can be indolent, with a stromal abscess covered by an intact epithelium.
Filamentous Bacteria
Actinomyces and Nocardia
Actinomyces and Nocardia are Gram-positive, filamentous bacteria. Actinomyces is obligatorily anaerobic and nonacid-fast, whereas Nocardia is obligatorily aerobic and variably acid-fast. On Gram staining, the filaments of these organisms are seen as branching and intertwined; some might display terminal clubs, and the filaments often fragment into bacillary and coccoid forms.
Actinomycotic keratitis is usually part of a mixed infection with other organisms that might have different antibiotic sensitivities. Infection is rare and usually follows trauma. Typically, the ulcer bed appears dry and necrotic and is surrounded by a yellow demarcating gutter. Alternatively, ring abscesses have been described. Inflammation can be severe, with iritis and a hypopyon.
Infections by Nocardia also tend to follow trauma, especially if soil contamination occurs. The ulcer is characteristically superficial, with a wreath-shaped gray-white infiltrate and an undermined necrotic edge ( Fig. 4.12.6 ). The base might assume a cracked windshield appearance. Nocardia keratitis often resembles fungal infection, with a filamentous-appearing border and satellite lesions.
Gram-Negative Rods
Pseudomonas
Pseudomonas aeruginosa is the most common Gram-negative organism isolated from corneal ulcers and is a frequent cause of contact lens–associated keratitis. These aerobic bacilli are found in moist environments and frequently contaminate inadequately chlorinated swimming pools and hot tubs, ventilators, nebulizer and vaporizer solutions, and ophthalmic solution bottles. The organism readily adheres to damaged epithelium. Stromal invasion is rapid.
Pseudomonas keratitis tends to progress rapidly if inadequately treated. Most commonly, the organism produces destructive enzymes, such as protease, lipase, elastase, and exotoxin, which result in necrotic, soupy ulceration. The organism’s surface glycocalyx protects it against phagocytosis and complement attack. The ulcer often extends peripherally and deeply within hours and rapidly can involve the entire cornea ( Fig. 4.12.7 ). Ring ulcers can develop. The corneal epithelium peripheral to the primary ulcer typically develops a diffuse gray, ground-glass appearance. The corneal stroma appears to dissolve into a greenish-yellow mucous discharge that fluoresces under ultraviolet (but not under cobalt blue) light. The suppurative ulcer frequently thins to a descemetocele that perforates. The ulcer often is associated with a marked anterior chamber reaction and hypopyon formation. Extensive keratitis can extend to the limbus and produce an infectious scleritis.
Less virulent strains follow a more indolent course. Multifocal epithelial infection also can be seen, with multiple intraepithelial gray-white nodules accompanied by a granular-appearing stromal infiltrate and anterior chamber inflammation ( Fig. 4.12.8 ). Diffuse epithelial disease is most commonly seen in association with hydrophilic contact lens wear.
Serratia
These Gram-negative rods are found in soil, water, food, and the gastrointestinal tract. Keratitis often occurs in association with hydrophilic contact lens wear. The infection may begin as a superficial central or paracentral ulcer that invades the deeper layers of the cornea, producing a deep, ring-shaped keratitis. Exotoxins and protease can produce aggressive ulceration and perforation. Contact lens–associated disease also can present with multiple gray intraepithelial nodules that assume a branching linear pattern, accompanied by a granular-appearing stromal infiltrate and anterior chamber inflammation.
Escherichia , Klebsiella , and Proteus
Infection by this group of Gram-negative rods is associated with contact lens wear and diseased eyes. The features of corneal infection can be similar to those seen in a virulent pseudomonal infection, with aggressive necrosis, ring ulcer formation, and perforation. Alternatively, the keratitis might be less aggressive with indolent ulceration and a moderate anterior chamber reaction. Suppurative keratitis caused by Escherichia coli is typically more indolent but is usually accompanied by severe iridocyclitis and hypopyon formation.
Moraxella
Moraxella species are large, Gram-negative (or Gram-variable) bacilli that are described as having a square “boxcar” shape. They are found in pairs and chains. Moraxella keratitis occurs most frequently in patients with alcoholism and debilitated patients. Marginal ulcers occur in association with angular blepharoconjunctivitis characterized by a gray, ulcerated infiltrate separated from the limbus by a clear crescent. Central indolent ulcers also occur. These most often develop in the lower half of the cornea as a gray infiltrate that eventually forms an oval-shaped ulcer. The infection tends to extend deeply rather than peripherally and does so slowly. The anterior chamber reaction can be vigorous. Uncommonly, perforation can occur.
Haemophilus
Haemophilus influenzae is a Gram-negative bacillus or coccobacillus that can cause conjunctivitis that leads to keratitis. Infrequently, keratitis has been associated with contact lens wear and chronic corneal disease. The infection usually is superficial but extensive; it can be suppurative and might be associated with a hypopyon.
Gram-Negative Cocci
Neisseria
Neisseria gonorrhoeae and N. meningitidis are Gram-negative, intracellular diplococci. In corneal and conjunctival scrapings they are found within epithelial cells. In a newborn with ophthalmia neonatorum, gonorrheal conjunctivitis is a significant concern because the organism can invade through an intact epithelium. Corneal infection often is peripheral and can progress to perforation and endophthalmitis. In adults, ocular gonorrhea is accompanied by a copious weeping, hyperpurulent discharge. Keratitis most commonly occurs after prolonged conjunctivitis. The transient, usually peripheral subepithelial infiltrates that might be seen likely represent a type III hypersensitivity reaction. Keratitis is characterized by diffuse edema or a ring ulcer with hypopyon. A significant risk of corneal necrosis and perforation exists.
Meningococcal conjunctivitis also can be complicated by keratitis, although this is less common than with gonococcal conjunctivitis. Typically, the keratitis is multifocal, and a peripheral infiltrate progresses to ulceration.
Moraxella (Branhamella) catarrhalis
Moraxella catarrhalis is a Gram-negative diplococcus that resembles N. gonorrhoeae. However, on smears of the conjunctiva, it is not found within epithelial cells. It can be a constituent of the normal flora of the conjunctiva and is an opportunistic pathogen. Although it can be associated with both neonatal and adult conjunctivitis, it is an infrequent cause of keratitis.
Mycobacteria
Nontuberculous Mycobacteria
Of this group of organisms, the Mycobacterium abscessus/chelonae complex and M. fortuitum are most commonly associated with ocular disease, although M. avium-intracellulare and M. gordonae also have been reported to cause infectious keratitis. These long rods are acid-fast; that is, they retain red basic fuchsin dye with Ziehl–Neelsen staining. Nontuberculous mycobacteria can grow in disinfectant and are found free in the environment, including soil. Keratitis most commonly follows trauma or surgery and has been associated with penetrating keratoplasty and refractive surgery. These corneal ulcers tend to be indolent and have been confused with mycotic keratitis. The infiltrated base of this typically nonsuppurative ulcer characteristically assumes a “cracked windshield” appearance, with multiple radiating lines. Satellite lesions, immune ring, and endothelial plaque formation might develop as the infection progresses.
Gram-Positive Cocci
Staphylococcus
Staphylococci are Gram-positive cocci, which, on stained smears, tend to appear singly or in pairs, although clusters of organisms can be seen. The two most common species that cause keratitis are Staphylococcus aureus and Staphylococcus epidermidis , both of which are commonly found on skin, eyelids, and the conjunctiva. Although non- aureus strains are usually less virulent, antibiotic resistance tends to be more common, and aggressive keratitis occasionally occurs.
Infection tends to occur in compromised corneas, such as those with bullous keratopathy, herpetic disease, and persistent epithelial defect. Usually a well-defined, cream-colored or gray-white stromal infiltrate with an overlying epithelial defect is seen. Sometimes multiple foci of abscesses can develop that resemble fungal satellite lesions. S. aureus tends to cause more severe infiltration and necrosis than S. epidermidis. Over time, the former can extend deep into the stroma, and necrosis of this abscess can lead to perforation. A hypopyon and endothelial plaque can be seen.
Streptococcus
Streptococci are Gram-positive cocci. On stained smears, most species tend to appear in chains, but they also can be arranged singly, in pairs, or in loose clusters. The most common species causing keratitis is Streptococcus pneumoniae ( Pneumococcus ), which appears as lancet-shaped diplococci arranged with the flattened ends together. Streptococcal species are distinguished by their ability to hemolyze red blood cells. Streptococcus viridans and S. pneumoniae do so partially (alpha-hemolysis), S. pyogenes completely (beta hemolysis), and gamma-hemolytic species do not hemolyze red blood cells at all.
Pneumococcal infections can readily spread, producing a deep stromal abscess, fibrin deposition, plaque formation, severe anterior chamber reaction, hypopyon, and iris synechiae ( Fig. 4.12.4 ). The advancing necrosis often produces an undermined leading edge with overhanging tissue. Infection by Streptococcus pyogenes occurs less frequently but has a similar, severe presentation and course. Group viridans Streptococcus spp . tend to cause less aggressive disease and are associated with a more indolent course, as is seen in infectious crystalline keratopathy.
Infectious crystalline keratopathy describes a particular pattern of corneal infiltration characterized by needle-like opacities that can be found at all levels of the corneal stroma ( Fig. 4.12.5 ). The crystalline opacities range from fine, feathery, and white to thick, brown, arborizing aggregations, without apparent cellular infiltrate or ocular inflammation. The entity most frequently occurs in corneal grafts but has also been associated with other conditions, such as incisional keratotomy, epikeratophakia, contact lens wear, chemical burns, and topical anesthetic abuse. Long-term corticosteroid therapy is thought to play a role in the pathogenesis. Although the viridans group of streptococci accounts for most cases, other organisms associated with infectious crystalline keratopathy include S. pneumoniae , Haemophilus aphrophilus , Peptostreptococcus , Pseudomonas aeruginosa , and a number of other bacteria, as well as Candida and Alternaria fungal species.
Staphylococcus
Staphylococci are Gram-positive cocci, which, on stained smears, tend to appear singly or in pairs, although clusters of organisms can be seen. The two most common species that cause keratitis are Staphylococcus aureus and Staphylococcus epidermidis , both of which are commonly found on skin, eyelids, and the conjunctiva. Although non- aureus strains are usually less virulent, antibiotic resistance tends to be more common, and aggressive keratitis occasionally occurs.
Infection tends to occur in compromised corneas, such as those with bullous keratopathy, herpetic disease, and persistent epithelial defect. Usually a well-defined, cream-colored or gray-white stromal infiltrate with an overlying epithelial defect is seen. Sometimes multiple foci of abscesses can develop that resemble fungal satellite lesions. S. aureus tends to cause more severe infiltration and necrosis than S. epidermidis. Over time, the former can extend deep into the stroma, and necrosis of this abscess can lead to perforation. A hypopyon and endothelial plaque can be seen.
Streptococcus
Streptococci are Gram-positive cocci. On stained smears, most species tend to appear in chains, but they also can be arranged singly, in pairs, or in loose clusters. The most common species causing keratitis is Streptococcus pneumoniae ( Pneumococcus ), which appears as lancet-shaped diplococci arranged with the flattened ends together. Streptococcal species are distinguished by their ability to hemolyze red blood cells. Streptococcus viridans and S. pneumoniae do so partially (alpha-hemolysis), S. pyogenes completely (beta hemolysis), and gamma-hemolytic species do not hemolyze red blood cells at all.
Pneumococcal infections can readily spread, producing a deep stromal abscess, fibrin deposition, plaque formation, severe anterior chamber reaction, hypopyon, and iris synechiae ( Fig. 4.12.4 ). The advancing necrosis often produces an undermined leading edge with overhanging tissue. Infection by Streptococcus pyogenes occurs less frequently but has a similar, severe presentation and course. Group viridans Streptococcus spp . tend to cause less aggressive disease and are associated with a more indolent course, as is seen in infectious crystalline keratopathy.
Infectious crystalline keratopathy describes a particular pattern of corneal infiltration characterized by needle-like opacities that can be found at all levels of the corneal stroma ( Fig. 4.12.5 ). The crystalline opacities range from fine, feathery, and white to thick, brown, arborizing aggregations, without apparent cellular infiltrate or ocular inflammation. The entity most frequently occurs in corneal grafts but has also been associated with other conditions, such as incisional keratotomy, epikeratophakia, contact lens wear, chemical burns, and topical anesthetic abuse. Long-term corticosteroid therapy is thought to play a role in the pathogenesis. Although the viridans group of streptococci accounts for most cases, other organisms associated with infectious crystalline keratopathy include S. pneumoniae , Haemophilus aphrophilus , Peptostreptococcus , Pseudomonas aeruginosa , and a number of other bacteria, as well as Candida and Alternaria fungal species.
Gram-Positive Bacilli
Bacillus
Bacillus cereus is an aerobic, spore-forming, typically Gram-positive rod, although considerable variability in staining characteristics has been noted. They are ubiquitous and are found in water, in soil, and on vegetation. Corneal infection, therefore, can be seen after penetrating injury, especially when soil contamination occurs. Bacillus infection also has been reported in contact lens–related keratitis. Posttraumatic infection characteristically develops within 24 hours of injury and is associated with chemosis, profound lid edema, and proptosis. A diffuse or a peripheral ring of microcystic edema often occurs, followed by a circumferential corneal abscess. This is an extremely virulent organism, and perforation of the cornea can develop within hours.
Corynebacterium
The corynebacteria, which include C. diphtheriae , are Gram-positive, club-shaped or pleomorphic rods arranged in the so-called Chinese-letter formation, Y’s, or palisades. An infrequent cause of keratitis, the clinical picture characteristically begins with diffuse epithelial haze, followed by stromal necrosis and melting.
Listeria
Listeria monocytogenes is a Gram-positive, short, rod-shaped facultative anaerobe. Infection usually occurs in animal handlers. It can colonize persistent epithelial defects and lead to a necrotizing keratitis. Typically, a ring ulcer and an exuberant anterior chamber reaction with fibrinous exudate and a hypopyon occur.
Clostridium
Clostridia are anaerobic, spore-forming, Gram-positive bacilli. Infrequently, clostridial conjunctivitis can be associated with the development of a marginal keratitis. Direct corneal infection is associated with marked edema and a frothy, bullous keratitis caused by trapped intraepithelial, subepithelial, and intrastromal gas produced by the organism. Gas might also be seen in the anterior chamber.
Propionibacterium acnes
Propionibacterium acnes is an anaerobic, nonspore-forming, Gram-positive rod. It forms part of the normal flora of the eyelid and conjunctiva. Keratitis can be established in the setting of corneal disease, trauma, surgery, contact lens wear, or chronic topical corticosteroid use. Although keratitis caused by P. acnes can assume the appearance of typical infectious keratitis, the infection can be indolent, with a stromal abscess covered by an intact epithelium.
Bacillus
Bacillus cereus is an aerobic, spore-forming, typically Gram-positive rod, although considerable variability in staining characteristics has been noted. They are ubiquitous and are found in water, in soil, and on vegetation. Corneal infection, therefore, can be seen after penetrating injury, especially when soil contamination occurs. Bacillus infection also has been reported in contact lens–related keratitis. Posttraumatic infection characteristically develops within 24 hours of injury and is associated with chemosis, profound lid edema, and proptosis. A diffuse or a peripheral ring of microcystic edema often occurs, followed by a circumferential corneal abscess. This is an extremely virulent organism, and perforation of the cornea can develop within hours.
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