2020–2021 BCSC Basic and Clinical Science Course™
8 External Disease and Cornea
Chapter 10: Infectious Diseases of the External Eye: Microbial and Parasitic Infections
Corynebacterium species are pleomorphic bacilli that display palisading or cuneiform patterns in smears. Corynebacterium diphtheriae is an exotoxin-producing agent and cause of acute membranous conjunctivitis. Other Corynebacterium species, referred to as diphtheroids, are routinely isolated from the external eye in the absence of clinical infection. Corynebacterium xerosis is commonly seen on histologic sections of vitamin A deficiency–associated conjunctival Bitôt spots, but its significance in conjunctival xerosis is unknown.
Figure 10-2 Gram-negative cocci (Neisseria gonorrhoeae). (Gram stain, original magnification ×1000.)
Propionibacterium acnes and related species are normal inhabitants of human skin. They are aerotolerant but prefer an anaerobic environment. These slender, slightly curved gram-positive rods sometimes have a beaded appearance (Fig 10-3). P acnes is a major cause of chronic postoperative endophthalmitis and can cause microbial keratitis.
Figure 10-3 Gram-positive rods (Propionibacterium acnes). (Gram stain, original magnification ×1000.)
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.
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.