Causative Agents

In all three conditions the most common bacterial pathogens are Haemophilus influenzae, a tiny Gram-negative rod, and Streptococcus pneumoniae, the Gram-positive encapsulated diplo-coccus known as the pneumococcus. Strains that infect the conjunctiva have adhesins that allow firm attachment to the epithelium. ■ adhesins, p. 466 ■ Streptococcus pneumoniae, p. 576 ■ Haemophilus influenzae, p. 666

Less commonly, Moraxella lacunata, enterobacteria, Neisseria gonorrhoeae, and many others infect the conjunctiva. Among these are agents that can contaminate eye medications and contact lens solutions such as Bacillus sp. (free-living aerobic Gram-positive spore-forming rods), Pseudomonas sp., (free-living Gram-negative aerobic rods), and Acanthamoeba sp. (free-living protozoa that live in soil, and fresh and salt water that have a cyst form carried by dust). Infections caused by these organisms are very uncommon, but when they occur they are usually serious and can lead to destruction of the eye. Contact lens wearers need to be very careful to follow exactly the instructions on use of cleansing solutions, and cloudy or outdated solutions and eye medications should be discarded.

Besides Streptococcus pneumoniae and Haemophilus influenzae, Mycoplasma pneumoniae, Streptococcus pyogenes, and Staphylococcus aureus can cause otitis media. About one-third of the cases are caused by respiratory viruses. This helps explain why some infections fail to respond to antimicrobial medications, which have no effect on viruses. The same infecting agents are often involved in sinusitis.

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