Microflora of Respiratory Tract
When breathing, humans inspire a large number of aerosol dust particles contaminated with microorganisms. It has been found that the number of microbial cell within inspired air exceeds 200-500 times the amount of microbes in expired air. The penetrated bacteria are easily trapped or expelled out by ciliated epithelium of the nasal cavity, larynx or large bronchi. Therefore, only a lesser amount of the microbials enters the bronchial tree. As the result of successful clearance, the terminal bronchioles and alveoli are not available for microorganisms.
In general, the nasal cavity confines only moderate or small amounts of microorganisms. It depends in part on bactericidal activities of mucosal mucins, lysozyme and secretory IgA. Various staphylococci, diphtheroids, hemophiles, viridans streptococci are capable of growing there. In addition, many viruses maintain their viability in these conditions for a long time.
The upper respiratory tract (nasopharynx and larynx) harbors relatively stable composition of a limited number of microbial species. Among them are S. epidermidis and S. saprophyticus, various streptococci, diphtheroids and some others.
The lowest parts of respiratory tract that include bronchioles and alveoli are normally sterile.
When the body protection dampens from some internal or external challenge (like cooling, starvation, or secondary immune suppression) the facultative pathogens – normal inhabitants of the respiratory tract – can be re-activated and cause certain respiratory infections such as sinusitis (the common agents are haemophilic bacteria and pneumococci), tonsillitis (induced by streptococci), bronchitis, or pneumonia (caused by pneumococci or staphylococci).