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 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.
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.