Ocular bacteriology

Abstract

This review comprises two sections. The first describes the physiological external ocular microbiota which are present in 50% of individuals and consist of Gram-positive organisms, including coagulase-negative staphylococci, Corynebacterium spp and Propionibacterium spp. As with other mucosae, the resident microbiota may prevent colonization by pathogens. Common causative agents of ocular disease include Gram-positive species such as Staphylococcus aureus, Streptococcus spp, and coagulase-negative staphylococci. Corneal infection in contact lens wear, however, is more frequently associated with Gram-negative organisms, particularly Pseudomonas aeruginosa. In inflammatory disease in contact lens wear, high numbers of organisms frequently colonize the lens itself. Mechanisms of bacterial persistence and pathogenic traits in the ocular environment are discussed next. Mechanisms of adhesion to epithelial cells are described for the common disease-causing bacteria, including adhesion via protein-protein interactions, e.g. fibronectin binding by staphylococci, or via protein-carbohydrate interactions, e.g. Ps. aeruginosa binding to asialo-GM1-glycolipid. Bacteria also interact with tear proteins and cellular defence mechanisms. Evasion of host defence systems, including immunoglobulin destruction and evasion of phagocytosis, may be critical for bacterial persistence. Bacterial products which may produce disease in the contact lens or non-lens-wearing eye include endotoxin and exoproteins, e.g. α-toxin of Staph. aureus or exotoxin A of Ps. aeruginosa.