Normal Flora

The mastoid air cells, middle ear, sinuses, trachea, bronchi, bronchioles, and alveoli are normally sterile. The nasal cavity, nasopharynx, and pharynx are colonized by numerous bacterial species. Aerobes, facultative anaerobes, aerotolerant organisms, and anaerobes are all represented. Although generally harmless, members of the normal bacterial flora are opportunists and can cause disease when host defenses are impaired. ■ oxygen requirements, p. 88 ■ opportunists, p. 462

Surprisingly, even though the eyes are constantly exposed to a multitude of microorganisms, normal healthy people commonly have no bacteria on their conjunctivae, the moist membrane covering the eye and inner surfaces of the eyelids. Presumably, this results from the frequent automatic washing of the eye with lysozyme-rich tears and from the eyelid's blinking reflex, which cleans the eye surface like a windshield wiper sweeps a windshield. Unless they are able to attach to the epithe lium, the viruses and microorganisms that impinge on the conjunctiva are swept into the nasolacrimal duct and into the nasopharynx. Organisms recovered from the normal conjunctiva are usually few in number and originate from the skin flora. They are adapted to live in a different environment and are generally unable to colonize the respiratory system. ■ lysozyme, p. 60

Some of the bacterial genera that inhabit the upper respiratory system are shown in table 23.1. The secretions of the nasal entrance usually contain diphtheroids and staphylococci. About 20% of healthy people constantly carry Staphylococcus aureus in their noses, and an even higher percentage of hospital personnel are likely to be carriers of these important opportunistic pathogens. Farther inside the nasal passages, the micro-bial population increasingly resembles that of the nasopharynx. The nasopharynx contains mostly a-hemolytic streptococci of the viridans group, non-hemolytic streptococci, Moraxella catarrhalis, and diphtheroids. Anaerobic Gram-negative bacteria, including species of Bacteroides, are also present in large numbers in the nasopharynx. In addition, commonly pathogenic bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and the Gram-negative diplococcus Neisseria meningitidis are often found, especially during the cooler seasons of the year.

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