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
Bacteriology
Gram-positive Filaments
Mycobacterium species
Mycobacteria are nonmotile, aerobic, weakly gram-positive, but acid-fast; in smears, they appear as straight or slightly curved rods. Löwenstein-Jensen medium is most commonly used for culture isolation. Mycobacteria are obligate intracellular pathogens and are divided into 2 main groups based on their growth rate. Mycobacterium tuberculosis and Mycobacterium leprae are slow growers. Ocular infection by M tuberculosis is uncommon, but it can manifest as a scleritis or posterior uveitis. The fast-growing atypical mycobacteria, including Mycobacterium fortuitum and Mycobacterium cheloneae, are a more common cause of ulcerative keratitis in the setting of an immunocompromised ocular surface or refractive surgery. Although their importance as a cause of keratitis following refractive surgery remains, atypical mycobacteria have 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, weakly acid-fast bacteria. They may cause keratitis that is clinically similar to one 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.
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.