Is Streptococcus pneumoniae part of normal flora in the upper respiratory tract?

Streptococcus pneumoniae as Part of Normal Flora in the Upper Respiratory Tract

Yes, Streptococcus pneumoniae is part of the normal bacterial flora of the upper respiratory tract, particularly the nasopharynx, despite its potential to cause invasive disease. 1

Colonization and Carriage

Streptococcus pneumoniae (pneumococcus) naturally colonizes the human nasopharynx as part of the normal bacterial flora. This colonization typically occurs in early childhood:

• Approximately 21% of nasopharyngeal cultures from healthy children contain S. pneumoniae 2
• Colonization rates increase to 32% during periods of viral upper respiratory infections 2
• Colonization is often asymptomatic but serves as a reservoir for potential infection

The pneumococcus establishes itself in the nasopharynx through a highly efficient transmission process that usually occurs via respiratory droplets from asymptomatic carriers 1. Following acquisition, the organism may either:

1. Remain confined to the nasopharynx as part of normal flora
2. Progress to other sites such as the lungs, sinuses, middle ear, or bloodstream to cause disease

Pathogenic Potential

Despite being part of normal flora, S. pneumoniae is also recognized as:

• The most common bacterial cause of community-acquired pneumonia 3
• A leading cause of meningitis, bacteremia, sinusitis, and otitis media 2
• A significant cause of morbidity and mortality worldwide, especially in children under 5 years and adults over 65 years 2

This dual nature as both commensal and pathogen makes S. pneumoniae somewhat unique. The transition from colonization to disease depends on multiple factors:

• Host immune status
• Presence of co-infections (particularly viral respiratory infections)
• Specific pneumococcal serotype
• Environmental factors

Risk Factors for Invasive Disease

While S. pneumoniae is part of normal flora, certain conditions increase the risk of invasive pneumococcal disease:

• Age extremes (very young and elderly)
• Immunocompromised states (HIV infection, splenic dysfunction)
• Chronic medical conditions (diabetes, alcoholism, chronic lung disease)
• Recent viral respiratory infections 4, 5

Clinical Implications

Understanding that S. pneumoniae is part of normal flora has important clinical implications:

• Positive cultures from the upper respiratory tract must be interpreted carefully, as they may represent colonization rather than infection
• The presence of pneumococci in sputum cultures from patients with chronic bronchitis may signify colonization and not acute infection 2
• Pneumococcal vaccination strategies target prevention of invasive disease while recognizing that complete elimination of colonization is not the goal

Antimicrobial Resistance Concerns

The commensal nature of S. pneumoniae contributes to antimicrobial resistance development:

• Exposure to antibiotics during upper respiratory infections can select for resistant strains
• Resistant pneumococci can be transmitted between individuals
• Serotypes that frequently colonize children (6A, 6B, 9,14, 19F, and 23F) are most likely to develop resistance 2

In conclusion, while S. pneumoniae is a significant pathogen capable of causing serious invasive disease, it is indeed part of the normal bacterial flora of the upper respiratory tract in many healthy individuals.