Pseudomonas aeruginosa comprises slender gram-negative rods (Fig 10-4) 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.
The family Enterobacteriaceae includes multiple genera of enteric non–spore-forming gram-negative rods, including Escherichia, Klebsiella, Enterobacter, Citrobacter, Serratia, Salmonella, Shigella, and Proteus. In particular, the genera Klebsiella, Enterobacter, Citrobacter, Serratia, and Proteus include species that are important causes of keratitis. Pathogenetic factors include pili, adhesins, cytolysins, and toxins. Enteropathogenic Escherichia coli expresses a protein similar to cholera toxin.
Figure 10-4 Gram-negative rods (Pseudomonas aeruginosa). (Gram stain, original magnification ×1000.)
Haemophilus species vary in morphology from oval (coccobacilli) to short rods. Culture isolation requires enriched media such as chocolate agar. Along with streptococci, Haemophilus species are important etiologic agents of bleb infections following glaucoma filtering surgery. Haemophilus influenzae can be divided into biotypes based on biochemical reactions; capsulated 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, is a gram-negative aerobic rod best seen with Warthin-Starry staining of a biopsy specimen. B henselae infection can be confirmed by culture, polymerase chain reaction (PCR), immunocytologic staining (of histologic specimens), and serologic testing. 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.)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.