Streptococci, Pneumococci, and Enterococci

The mucous membranes of the upper respiratory tract that are exposed to air and food (nose, throat, mouth) normally display a variety of aerobic and anaerobic bacterial species: gram-positive cocci (Streptococcus, Staphylococcus, and Peptostreptococcus species); gram-negative cocci (Neisseria, Moraxella, and Veillonella species); gram-positive bacilli (Corynebacterium, Propionibacterium, and Lactobacillus species); gram-negative bacilli (Haemophilus, Prevotella, and Bacteroides species); and, sometimes, yeasts (Candida species).

This flora varies somewhat in various areas of the upper respiratory tract. The nasal membranes show a predominance of staphylococci. The throat (pharyngeal membranes) has the richest variety of microbial species, and the sinus membranes have few if any organisms. The deeper reaches of the respiratory tract (trachea, bronchi, alveoli) are not readily colonized by microorganisms, because the ciliated epithelium of the upper membranes together with mucous secretions trap and move them upward and outward.

Usually the normal flora present in the upper respiratory tract prevents entry and overgrowth of transient microorganisms at those sites. Some of these intruders might be pathogenic and capable of invading respiratory lining cells or deeper tissues. If the conditions maintained by commensal organisms are disturbed (by changes in the immune status of the host, by administration of antimicrobial agents to which the commensals are susceptible, or by unusual exposure to virulent, transient pathogens in large numbers), other microorganisms may then be able to colonize and invade the membranes.