Normal Flora at Respiratory Tract

Respiratory Tract

Respiratory tract is subdivided into (a) the upper airways, which include the anterior and posterior nares and the nasopharynx; (b) the middle airways comprising the oropharynx and tonsils; and (c) the lower airways with larynx, trachea, bronchi, and lungs. The structural and physiological differences at each site provide an environment compatible for some organisms and hostile for others.

◗ Nares and nasopharynx

Nares are inhabited by a small number of organisms, which include Staphylococcus, Corynebacterium, Peptostreptococcus, and Fusobacterium species. The microbial population in the nasopharynx is more complex with the predominance of streptococci and Neisseria species. Streptococcus salivarius, Streptococcus parasanguis, and Streptococcus pneumoniae can bereadily recovered from the nasopharynx. Twelve species of Neisseria have been recovered from nasopharyngeal carriers.Colonization with Neisseria meningitidis varies from 10% to 95% with the highest incidence in young adults. Other Neisseria species that colonize the nasopharynx are Neisseria subflava, Neisseria sicca, Neisseria mucosa, and Neisseria lactamica. Gram-negative coccobacilli that colonize the nasopharynx include Moraxella catarrhalis and Kingella species. Noncapsulatedstrains of Haemophilus influenzae are also commonly found in the nasopharynx.

◗ Oropharynx and tonsils

The oropharynx is a complex mixture of ecosystems. Gram-positive and Gram-negative cocci predominate in the oropharynx. Overall, anaerobes outnumber aerobes with a ratio of 100:1. The most common anaerobic bacteria present in the oropharynx are Peptostreptococcus, Veillonella, Actinomyces, and Fusobacterium, whereas the most common aerobic bacteria are Streptococcus and Neisseria. Nearly 20% of individuals are colonized with Staphylococcus aureus. Streptococcus species that colonize the oropharynx are S. salivarius (saliva), Streptococcussanguis and S. mutans (tooth surface), Streptococcus vestibularis and S. sanguis (oral mucosa), S. pneumoniae, and beta-hemolytic streptococci. Other Gram-positive cocci that are present in the oropharynx include Stomatococcus mucilaginosus, Gemella species, etc.

Gram-negative cocci and coccobacilli also colonize the oropharynx. Veillonella, anaerobic Gram-negative cocci, are the most numerous Gram-negative cocci found in the oropharynx. Other Gram-negative cocci include Neisseria, Moraxella, Kingella, Cardiobacterium, and Eikenella species.

Haemophilus spp. are present in almost all individuals in lessthan 5% of the microbial population. Fifty percent of them are noncapsulated. H. parainfluenzae makes 10% of the bacterial flora in saliva. Members of the Enterobacteriaceae and non-fermentative Gram-negative bacilli are also present in the oro-pharynx in small numbers or transiently.

Gram-positive bacteria are also predominant microbes that constitute normal flora of the oropharynx. Actinomyces are present in large numbers, comprising 20% of bacterial flora in saliva and on the tongue, 35% in the gingival crev-ices, and 40% in the dental plaque. The ability of Actinomyces spp. to colonize various surfaces is mediated by the fimbriae and other extracellular polysaccharides present in the bac-teria. Other Gram-positive bacilli present in the oropharynx are Actinobacillus, Corynebacterium, Eubacterium, Lactobacillus, Propionibacterium, and Rothia species.

The predominant anaerobes that are found in the oropharynx are Fusobacterium, Bacteriodes, Porphyromonas, Prevotella, and Selenomonas species. Fusobacterium nucleatum is the most common Fusobacterium found in the mouth.

Fungal colonization of the oropharynx is restricted to yeasts only; Candida albicans is present in almost all individuals.

Entamoeba gingivalis and Trichomonas tenax are the only protozoapresent in the oropharynx.

◗ Trachea, larynx, bronchi, and lungs

Colonization of the lower airways is transient with few organisms present at any one time. Long-term colonization may occur when the ciliated epithelial cells are damaged or altered in a disease process.