Respiratory Bacterial Pathogens Identification
Gram stain and culture appearances for some of the more common upper and lower respiratory tract bacterial agents.
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Large Gram Negative Rods Blood Agar Beta Hemolysis, Metallic Sheen Oxidase Positive Lactose Negative, Greenish Pyocyanin Pigment Mueller-Hinton Agar Blue-green Pigment
Large Gram Negative Rods MacConkey Agar - Positive Lactose Indole Negative - - Urea Positive
Gram Negative Coccobacili Growth on Chocolate Agar Hemin (X factor) and NAD (V factor) Required for Nutrient Agar Growth
Fastidious Aerobic Gram Negative Slender Rods Growth on BYCE Media - Gray-White Colonies Confirm By Direct Fluorescent Antigen Staining BYCE Positive - - DFA Positive
Gram Negative Cocci - Oxidase Positive Growth on Blood and Chocolate Agar: Gray-White Hemispheric 1 mm Colonies With a Waxy Surface
Gram Negative Coccobacilli Growth on Charcoal-Cephalexin Blood Agar
Small Gram Positive Rods Catalase Negative Gray-Black Colonies on Tellurite Blood Agar
Large Spore Forming Rods Anthrax Forms Long Chains - Catalase Positive
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.
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:
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).
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.