TABLE ONE. Gram-Positive Bacteria of Ocular Importance

VIRULENCE FACTORS AND IDENTIFICATION

B. anthracis is a large organism that is arranged as single or paired bacilli in clinical specimens and as long serpentine chains and clumps in cultures. In addition to its capsular antigen, B. anthracis has a polysaccharide cell-wall somatic antigen and a toxin associated with it. This toxin consists of three antigenically distinct, heat-labile components: protective antigen, lethal factor, and edema factor. Although none of the components are active alone, the combination of protective antigen with either of the other components has toxic properties. The two major factors responsible for B. anthracis virulence are the presence of the capsule and toxin production.^1,2

B. anthracis can be readily detected by the microscopic examination and culture of corneal ulcers. Large gram-positive bacilli are seen in tissue. It will grow on nonselective laboratory media with the appearance of nonhemolytic, readily growing, adherent colonies. Definitive identification requires the use of selected chemical tests and demonstration that B. anthracis is nonmotile. B. cereus also grows rapidly and is readily detected in specimens collected from infected eyes. It can resemble B. anthracis but generally is motile and produces markedly hemolytic colonies.

OCULAR INFECTION

Bacillus organisms are a rare but serious cause of endophthalmitis and suppurative keratitis.^3,4 Several species recovered from eye infections have been identified. Failure of contact lens disinfection is an important predisposing factor to corneal infection by these organisms.^5–7 In one study, 3% of contact lens-associated corneal ulcers were caused by Bacillus organisms.⁸ Bacillus organisms have been found in the contact lens care systems of 3% to 7% of asymptomatic patients.^9,10 These organisms form spores that are resistant to heat and many types of chemical disinfectants. Once a care system is contaminated, it may be difficult to eradicate the organism.

The clinical onset of B. anthracis keratitis can begin with multifocal epithelial opacities that lead to a focal stromal infiltrate. B. cereus keratitis also can be extremely severe and can rapidly progress to perforation. It may be characterized by a ring infiltrate in the stroma that is remote from the site of the injury (Fig. 1).¹¹ A history of a corneal foreign body or other corneal trauma preceding the infection generally can be obtained (Fig. 2). B. cereus is one of the most destructive organisms affecting the inner eye that often follows trauma. The source of the organisms can be either soil contamination of the object penetrating the eye or direct inoculation of organisms colonizing the eye surface. B. cereus panophthalmitis almost always ends in complete loss of light perception within 48 hours of the injury with massive vitreoretinal destruction. The pathogenesis of B. cereus panophthalmitis is incompletely defined. At least three toxins have been implicated: necrotic toxin (a heat-labile enterotoxin), cereolysin (a potent hemolysin named after the species), and phospholipase C (a potent lecithinase). It is likely that the rapid destruction of the eye, which is characteristic of B. cereus infection, is the result of interactions of these toxins and unidentified factors.

SUSCEPTIBILITY

The choice of treatment should be confirmed by in vitro testing. In general, B. cereus is sensitive to clindamycin, vancomycin, and the aminoglycosides, whereas B. anthracis is sensitive to penicillin, tetracycline, and chloramphenicol.

The genus Corynebacterium is restricted to bacteria with cell walls containing mesodiaminopimelic acid, arabino-galactose polymers, and short-chain mycolic acids. They are aerobic, nonspore forming, nonacid-fast, nonmotile, and catalase-positive and can ferment carbohydrates to produce lactic acid. A spectrum of corynebacterial isolates from clinical specimens has been examined systemically.¹²

VIRULENCE FACTORS AND IDENTIFICATION

C. diphtheriae, the major pathogen in this group, produces a necrotizing toxin that enables it to penetrate an intact corneal epithelium. This protein is a 63-kd protein that consists of two fragments. One of them binds the cell surface, and the other enters the cell and catalyzes the irreversible inactivation of elongation factor-2 (EF-2), which is required for movement of nascent peptidic chains on ribosomes. It has been estimated that one exotoxin molecule can inactivate the entire content of EF-2 in a cell. The concentration of iron also is critical. Iron is an essential element for bacteria growth; however, high concentrations of iron suppress toxin production. EF-2 toxin also has been found in strains of C. ulcerans and C. pseudotuberculosis.

Corynebacterial species often stain irregularly because of the presence of metachromatic granules. Most of the species are slow growing. Colonies may not become visible on culture plates for up to 1 week or more.^13–16 Although most species of corynebacteria grow well on ordinary media, some species (C. ovis and C. ulcerans) require Loeffler's or tellurite media for optimal growth.

Differentiation among corynebacterial species usually is difficult. Characteristics such as colony size, pigmentation, odor, and presence of hemolysis are helpful for identification. Key reactions include catalase production, fermentative or oxidative metabolism, motility, nitrate reduction, urea production, esculin hydrolysis, acid production, and the cyclic AMP reaction.¹⁷

OCULAR INFECTION

Corynebacterial species commonly are encountered in the normal flora of the skin and mucous membranes. They were formerly regarded as nonpathogenic contaminants of the human external eye, but there have been many eye infections reported to be caused by corynebacteria.¹⁸ Some species have been reported only as ocular pathogens (e.g., coryneform group A-4 and A-5 bacteria).¹⁹

Membranous or pseudomembranous conjunctivitis caused by C. diphtheriae is a classically described entity usually seen in children between 2 and 8 years of age.²⁰ Corneal ulcers caused by corynebacteria usually have an indolent onset and a clinical course with an epithelial defect, an underlying stromal infiltrate, minimal or absent purulent discharge, and a variable amount of anterior chamber reaction and pain (Fig. 3). Old age, ocular surface disease, exposed corneal sutures, eyelid abnormalities, extended-wear contact lenses, diabetes mellitus, and viral keratitis²¹ are frequent risk factors.

SUSCEPTIBILITY

Antibiotic sensitivities among corynebacteria vary greatly. Erythromycin, vancomycin, penicillins, tetracycline, and aminoglycosides often are effective. However, individual susceptibilities should be determined by the microbiology laboratory.

L. monocytogenes was first reported in 1926,²³ and most early reports recognized its role as an animal pathogen. Ocular infection in humans was first noted in 1934, when purulent conjunctivitis developed in a laboratory assistant after accidental inoculation of the organism.²⁴

Although L. monocytogenes has a widespread distribution in nature, human disease is uncommon and restricted to several well-defined populations: neonates, the elderly, pregnant women, and immunocompromised patients, particularly those with defective cell-mediated immunity. Any organ can be infected by the pathogen, but there is a marked tropism for the central nervous system. Meningitis is the most common disease associated with L. monocytogenes.^25,26 Bacteremia, endocarditis, pneumonia, serositis, osteomyelitis, septic arthritis, and dermatitis also have been described. Although epidemic food-borne transmission and vertical transmission from mother to offspring are established routes of infection, most cases are sporadic with the source of the organism and the mode of transmission remaining unknown.

VIRULENCE FACTORS AND IDENTIFICATION

L. monocytogenes is an intracellular pathogen, capable of growth in macrophages, epithelial cells, and cultured fibroblasts. All virulent strains produce a beta-hemolysin, listeriolysin O. This hemolysin is required for release of the bacterium after phagocytosis and intracellular growth. Intracellular survival and spread of the organisms are critically important factors because no other virulence factors have been identified.

Listeria can grow on most conventional media. Small, round colonies appear on agar media after incubation for 1 to 2 days. Because of its morphology and hemolysis pattern, L. monocytogenes occasionally is mistaken for Corynebacterium or streptococci. Laboratory identification is based on the presence of a narrow band of beta-hemolysis on blood agar and on its unique tumbling motility when grown at 20°C to 25°C.²⁷ Polymerase chain reaction can identify L. monocytogenes within hours.²⁸

OCULAR INFECTION

Ocular involvement with L. monocytogenes is very rare. Conjunctivitis is the most common presentation. Keratitis^29–32 and oculoglandular syndrome^33,34 also have been reported.³⁵

To date, 15 cases of L. monocytogenes endophthalmitis have been reported. All are presumed to be endogenous,^36–49 although L. monocytogenes has the ability to penetrate intact corneal epithelium. Approximately 60% of these patients were apparently healthy individuals, and the mean age of these cases was 62 years. The clinical appearance of L. monocytogenes endophthalmitis is remarkably consistent. Large keratic precipitates, fibrinous anterior chamber reaction, elevated intraocular pressure, dark hypopyon, and pigment dispersion often are noted. Usually, the outcome is poor with a visual acuity of less than 20/200.

SUSCEPTIBILITY

The choice of treatment for L. monocytogenes infection is controversial. A combination of ampicillin and gentamicin usually is recommended. The aminoglycosides are bactericidal, and there is synergism with the penicillins. There are other antibiotics with activity in vitro against L. monocytogenes, such as tetracycline, erythromycin, trimethoprim/sulfamethoxazole, chloramphenicol, ciprofloxacin, ofloxacin, and rifampim. Fluoroquinolones, aminoglycosides, and trimethoprim/sulfamethoxazole are bactericidal, whereas the other agents are only bacteriostatic.^50,51

Similarities exist between mycobacterial and nocardial species with respect to antigens of the cell wall and bacteriophage susceptibility.⁵³ Nocardial organisms, however, are differentiated from mycobacteria because they are only partially acid-fast and form fragmenting mycelia with true branching. Furthermore, the lipid composition of the cell walls between the two groups differs.⁵⁴

Nocardia species are soil organisms, often found in decaying organic matter such as wet hay or straw. Although distributed worldwide, nocardial ocular infections are rarely encountered. The commonly identified species are N. asteroides, N. brasiliensis, N. otitidis-caviarum, N. farcinica, and N. transvaliensis. N. asteroides, the most common species, was named after Nocard, a French bacteriologist and veterinary pathologist and for the starlike appearance of the colonies on the agar plate (Fig. 4).⁵⁵

VIRULENCE FACTORS AND IDENTIFICATION

N. asteroides is an intracellular organism that can persist and grow within macrophages. No exotoxins have been described. Nocardial organisms usually do not multiply quickly enough within host tissues to produce a fulminant infection. Iron seems to be an important factor for the intracellular growth of most organisms.

Nocardial organisms are gram-positive, branching pleomorphic rods with intermittent or beaded staining patterns, especially when invading tissues (Fig. 5). The organisms stain black with the methenamine-silver stain. They are partially acid-fast by the Ziehl-Nielsen technique with acid decolorization. In culture, Nocardia are not fastidious but do tend to grow slowly. Colonies will grow on most bacterial, fungal, or mycobacterial media that lack antibiotics. Blood and Sabouraud's agars are good substrates for pathogenic organisms that usually grow satisfactorily at temperatures between 35°C and 37°C. Growth of N. asteroides is facilitated by 10% carbon dioxide.

OCULAR INFECTION

The two most common modes of nocardial infection are inhalation of organisms with suppurative or cavitary pulmonary infection, often simulating tuberculosis, and contamination of skin wounds with soil, causing a localized mycetoma. Pulmonary or generalized nocardial infections have become increasingly prevalent in immunocompromised patients. Nocardia occasionally may cause infections in other tissue such as brain, kidney, and bone. Keratitis,^56–68 conjunctivitis,⁶⁹ dacryoadenitis,⁷⁰ uveitis,⁷¹ endophthalmitis,^72,73 and contaminated scleral buckles have been reported.

Nocardia species are a somewhat rare cause of primary conjunctivitis that may be either mucopurulent or granulomatous, sometimes associated with the development of subconjunctival scar tissue and an interstitial or superficial punctate keratitis. Corneal ulceration may occur from the direct extension of conjunctivitis (Fig. 6). Nocardial keratitis usually is preceded by minor trauma. Corneal infection occurs more frequently in young adult males and in rural areas. Such preponderances are likely related to the predisposing roles of minor corneal trauma and soil contact. Nocardial keratitis is characterized by a relatively slow and recalcitrant keratitis. Delay in identification of the causative organism sometimes causes the mistaken topical administration of corticosteroids as adjunctive therapy before a specific diagnosis. The characteristic morphology of nocardial keratitis is its resemblance to filamentous fungal ulcers. Nocardia species should be kept in mind as possible causative organisms in patients who do not respond to antifungal therapy. Minimal involvement of the epithelium with nonspecific punctate epitheliopathy often is encountered. The infiltrate usually is located in the midperiphery of the cornea, adjacent to sites of minor corneal trauma or abrasion, with superficial or midstromal involvement. The typical ulcer has a gray, sloughing base with overhanging necrotic edges (Fig. 7). The stromal involvement tends to be grossly granular or nodular in appearance. A “brush-fire” border of the infiltrates can be observed (Fig. 8). Multiple foci of infection or satellite lesions can sometimes be encountered.

Nocardial keratitis usually is refractory to conventional topical antibiotics, resulting in a protracted clinical course and progressive extension of keratitis. Corneal thinning and a moderate anterior chamber reaction or hypopyon can be observed. Eventual resolution leaves superficial stromal scarring and surface irregularities. Successful medical management is feasible. Corneal scarring often is minimal. Nocardial keratitis tends to heal with peripheral corneal vascularization or with vascularized scars. The visual prognosis usually is optimistic.

Most intraocular nocardial infections are caused by metastatic spread from pulmonary or infections,^74–78 although some follow surgical trauma. Endogenous nocardiosis occurs principally in patients with systemic immunosuppression related to antineoplastic therapy or systemic corticosteroids.^79,80

SUSCEPTIBILITY

Different nocardial strains vary in antibiotic sensitivity.^81–85 Most isolates are sensitive to sulfonamides, doxycycline, and amikacin. Many are resistant to trimethoprim, penicillin, and other beta-lactam agents, vancomycin, and ciprofloxacin. Sulfonamides, both systemic and topical, remain a drug of choice for ocular nocardial infections. If the refractory or severe cases occur, topical amikacin can be considered with oral doxycycline.

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