This section provides an overview of the important microorganisms involved in ophthalmology. Bacteria and viruses are not discussed here, instead, they are outlined in-depth in the microbial sections of the cornea and conjunctiva chapters.
Culture Media
Culturing organisms is a difficult process. Different organisms require specific media to grow. This section summarises the high-yield culture media.
Media |
Organisms |
|---|---|
Löwenstein–Jensen |
Mycobacteria and Nocardia |
Non-nutrient agar with E.col |
Acanthamoeba |
Sabouraud dextrose agar |
Fungi |
Blood agar |
Broad-spectrum other than Neisseria, Haemophilus and Moraxella |
Chocolate agar |
Neisseria, Haemophilus and Moraxella |
Fungi
Broadly classified as yeast (unicellular), filamentous (grow in branches) or dimorphic (features of both yeast and filamentous types).
The yeast Candida albicans infects the retina of immunocompromised people and it is also the commonest endogenous cause of endophthalmitis. It typically presents with fluffy white retinal lesions.
- Aspergillus and Fusarium are filamentous fungi associated with keratitis following trauma involving soil or bark.
- Histoplasma is a dimorphic soil fungus, endemic to the Mississippi and Ohio river valleys and causes presumed ocular histoplasmosis syndrome (POHS).
Protozoa
Protozoa are single-celled eukaryotic parasites.
Acanthamoeba is found in water and soil. It causes acanthamoeba keratitis (an orphan disease), typically in contact lens wearers who do not remove lenses whilst swimming or are storing lenses incorrectly.
- Toxoplasma gondii is transmitted to humans via cats (definitive host). It can also be transmitted vertically by humans and cause congenital toxoplasmosis
Helminth
Helminths are infection-causing parasitic worms.
Onchocerca volvulus causes onchocerciasis (African river blindness) and is the commonest helminth related ocular infection. It is transmitted by Simulium blackflies and is endemic to Africa
- Toxocara canii causes endophthalmitis in children. The definitive host is dogs.
