Respiratory Bacterial Pathogens Identification

Gram stain and culture appearances for some of the more common upper and lower respiratory tract bacterial agents.



Bacterial infections of the lower respiratory tract are characterized by pneumonia. In general, just like acute infections elsewhere, the time course is short (days) with clinical features of fever, cough, and dyspnea. Since acute inflammatory reactions to most bacteria are neutrophilic, an exudate is produced, which fills alveolar spaces, resulting in radiographic infiltrates (bright areas with x-rays), pathologic consolidation (firm tissue, not spongy normal lung), and a productive cough as the exudate is brought up.

Listed below are the most common organisms causing bacterial pneumonia, but the potential list is much longer. The rank order of this list can change by individual patients circumstance. Thus, a 'community acquired' pneumonia has historically often been due to Streptococcus pneumoniae while a 'hospital acquired' pneumonia is more likely due to Staphylococcus aureus or Pseudomonas aeruginosa.

  • Streptococcus pneumoniae

  • Staphylococcus aureus

  • Klebsiella pneumoniae

  • Pseudomonas aeruginosa

  • Hemophilus influenzae

  • Legionella pneumophila

Upper respiratory tract infections are characterized by airway obstruction, and cough.

One of the most common conditions is sinusitis. The common "cold" is characterizedd by involvement of upper airways, with nasal stuffiness and coryza. Common causes include rhinoviruses and coronaviruses. However, anatomic abnormalities of the ostiomeatal complex, or any abnormality that affects air passages and sinus drainage contribute to development of chronic sinusitis.

Chronic sinusitis is most often associated with three bacterial organisms:

  • Streptococcus pneumoniae

  • Hemophilus influenzae

  • Moraxella catarrhalis

The stagnation of mucus leads to bacterial growth with inflammation that causes further obstruction and a continuing cycle of infection-inflammation.

Two of the most severe examples of upper respiratory infection are listed below, but are now rare childhood diseases due to widespread immunization (the "D" and the "P" in the DPT vaccine).

  • Bordetella pertussis - whooping cough produced by bacterial exotoxin.

  • Corynebacterium diptheriae - diptheria characterized by an obstructive pseudomembrane of ulcerated necrotic epithelium along with production of an exotoxin that damages heart.

The organism Bacillus anthracis is a large spore-forming gram positive rod. It most often causes disease in herbivores (cattle, sheep, goats, horses) but can infect humans as cutaneous anthrax from contact with infected animals or animal products, intestinal anthrax from ingestion of spores, or inhalation anthrax. Any of these forms can lead to septicemia and to meningitis B. anthracis has marked virulence due to elaboration of exotoxins such as the aptly named "lethal" toxin and "edema" toxin which combine to inhibit neutrophil function and phagocytosis.