Bacillus species

Bacillus species are ubiquitous gram-positive or gram-variable rods commonly found in soil and characterized by the production of spores, a form of the bacteria that allows survival for extended periods under extremely harsh conditions. Bacillus species are typically motile, and this feature may play a role in the explosive character of Bacillus cereus–induced posttraumatic endophthalmitis. B cereus produces a number of toxins that may rapidly damage ocular tissues. The closely related genus Clostridium is anaerobic; Bacillus species are aerobes or facultative anaerobes.

Gram-negative Rods

Pseudomonas aeruginosa comprises slender gram-negative rods (Fig 5-4) commonly found as contaminants of water. P aeruginosa ocular infections are among the most fulminant. Permanent tissue damage and scarring are the rule following corneal infection. Structural virulence factors of P aeruginosa include polar flagella, adhesins, and surface pili. P aeruginosa organisms secrete a number of toxins that disrupt protein synthesis and damage cell membranes of ocular cells, as well as proteases that degrade the corneal stromal extracellular matrix.

Figure 5-4 Gram-negative rods (Pseudomonas aeruginosa). (Gram ×1000.)

Enterobacteriaceae

The Enterobacteriaceae family includes multiple genera of enteric non–spore-forming gram-negative rods, including Escherichia coli, Klebsiella, Enterobacter, Citrobacter, Serratia, Salmonella, Shigella, and Proteus. In particular, Klebsiella, Enterobacter, Citrobacter, Serratia, and Proteus are important causes of keratitis. Pathogenetic factors include pili, adhesins, cytolysins, and toxins. Enteropathogenic E coli express a protein similar to cholera toxin.

Haemophilus species

Haemophilus species vary in morphology from coccobacilli to short rods. Culture isolation requires enriched media such as chocolate agar. These species are obligate parasites of mammalian mucous membranes and commonly inhabit the human upper respiratory tract and mouth. Along with streptococci, they are important agents of bleb infections following glaucoma filtering surgery. Haemophilus influenzae can be divided into biotypes based on biochemical reactions; encapsulated strains are further divided into serotypes based on their capsular polysaccharides. H influenzae type B

(Hib) is the primary human pathogen, and its capsule is a major virulence factor.

Bartonella henselae

The etiologic agent of cat-scratch disease, B henselae appear as gram-negative aerobic rods, best seen by Warthin-Starry staining of tissue biopsies. B henselae infection can be confirmed by culture, polymerase chain reaction (PCR), immunocytologic staining (of histologic specimens), and serology. Cats, especially young cats, are a natural reservoir of B henselae, and despite the disease’s association with a history of cat scratch or contact with fleas, infection may be transmitted by any contact with an infected cat. (See Parinaud Oculoglandular Syndrome later in the chapter.)

Gram-positive Filaments

Mycobacterium species

Mycobacteria are nonmotile, aerobic, weakly gram-positive, but acid-fast; they appear on smears as straight or slightly curved rods. Löwenstein-Jensen medium is most commonly used for culture isolation. Mycobacteria are obligate intracellular pathogens and fall into 2 main groups based on growth rate. Mycobacterium tuberculosis and Mycobacterium leprae are slow growers. Ocular infection by M tuberculosis is uncommon, but it can manifest as a posterior uveitis or scleritis. The fast-growing atypical mycobacteria, including Mycobacterium fortuitum and Mycobacterium chelonei, are a more common cause of ulcerative keratitis in the setting of an immunocompromised ocular surface. Although their importance as a cause of keratitis following refractive surgery remains, atypical mycobacteria have recently been supplanted by MRSA as the predominant causative agent in this setting.

Nocardia species

Nocardia asteroides and related filamentous bacilli are gram-variable or gram-positive and weakly acid-fast. They may cause keratitis clinically similar to that caused by the atypical mycobacteria.

Actinomyces species

Actinomycetes are gram-positive, non–acid-fast anaerobic bacteria that colonize the mouth, intestines, and genital tract. They are an important cause of canaliculitis.

Chlamydia Species

Chlamydiae are spherical or ovoid obligate intracellular parasites of mucosal epithelium with a dimorphic life cycle. The infectious form is the elementary body (EB), which develops within an infected host eukaryotic cell into the intracellular replicating form, the reticulate body (RB). Only the EB survives outside the host, and only the EB is infectious. Reticulate bodies divide by binary fission to produce 1 or more EBs within a cytoplasmic vacuole, seen on light microscopy as a cellular inclusion.

Spirochetes

Spirochetes are characterized by the periplasmic location of their flagella (endoflagella). They are too narrow to be seen by light microscopy. Visualization in fresh clinical specimens requires darkfield illumination. Silver staining or immunocytology can aid identification in histologic specimens.