Normal Ocular Flora

Bacterial colonization of the eyelid margin and conjunctiva is normal and can be beneficial as long as normal flora competitively inhibit pathogenic strains. The spectrum of normal ocular flora varies with the age and geographic locale of the host. In the eye of an infant delivered vaginally, multiple bacterial species predominate, including Staphylococcus aureus, Staphylococcus epidermidis, streptococci, and Escherichia coli; streptococci and pneumococci then predominate during the first 2 decades of life. Although gram-negative bacteria are more commonly isolated with age, S epidermidis and other coagulase-negative staphylococci, S aureus, and diphtheroids remain some of the most common species identified from the external eye (Table 9-1). Nonpathogenic colonization of the eyelid margin with Demodex folliculorum and Demodex brevis also becomes more common with age, and these parasites become almost ubiquitous over time. Any use of topical antibiotics or corticosteroids can alter the character and spectrum of eyelid and conjunctival flora.

Table 9-1 Relative Prevalence of the Normal Flora of the External Eye

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Graham JE, Moore JE, Jiru X, et al. Ocular pathogen or commensal: a PCR-based study of surface bacterial flora in normal and dry eyes. Invest Ophthalmol Vis Sci. 2007;48(12): 5616–5623.

Kemal M, Sümer Z, Toker MI, Erdoğan H, Topalkara A, Akbulut M. The prevalence of Demodex folliculorum in blepharitis patients and the normal population. Ophthalmic Epidemiol. 2005;12(4):287–290.

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.