Bacterial Pneumonia
Acute bacterial pneumonia is the most common life-threatening infectious disease in adults and the 6th leading cause of death in the US. 30% of children < 5 years old worldwide die of pneumonia annually.
Pathophysiology
Infection of the lower respiratory tract (pneumonia) may occur by (1) inhalation/aspiration of bacteria, (2) hematogenous or lymphatic spread, or (3) direct extension or penetrating injury.
The nature and extent of the inflammatory response is determined by the host’s immune system and the bug’s virulence.
New organisms are combated by non-specific mechanisms while B and T cell responses are developing. Macrophages produce chemo-attractants which bring in PMNs. Opsonization by complement, and later by IgG, greatly aids in phagocytosis of the invading bacteria.
The presence of a capsule may prevent exposure of the cell wall and subsequent activation of complement unless a capsule specific Ab is present. The capsule may also prevent phagocytosis directly to a minor extent.
Once phagocytized the bacterium is subjected to antibacterial substances in primary and secondary granules that fuse with the phagosome. Intracellular killing in PMNs occurs by both oxygen-dependent and non-oxidative mechanisms.
Three Common Pneumonias
(1) Pneumococcal Pneumonia
Streptococcus pneumoniae is a significant cause of death in the elderly and very young. Case fatality ~5%
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Immuno-compromised patients and asplenic patients at high risk
Gram + , lancet shaped, diplococci. A facultative anaerobe with type specific complex capsular polysaccharide.
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smooth capsules are more virulent
Pneumococcal pneumonia arises from aspiration of the organism from the pharynx (which it transiently colonizes in 70% of the adult population).
Symptoms: classically sudden onset of fever, cough, Rigor, and rusty sputum. Consolidation signs of PE.
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also hypoxemia due to V/Q mismatch and shunting, pleural effusions (4% Þ empyema), meningitis, and ARDS
Results in Lobar pneumonia. Copious intra-alveolar fluid and scanty cellular inflammation. Usually a single lobe.
Pneumococcal pneumonia has 4 pathologically recognized stages
- (1) congestion
- (2) red hepatization
- (3) grey hepatization
- (4) resolution
* there is no scarring or fibrosis after resolution (this is in contrast to Staphylococcal pneumonia)
Diagnosis: presumptive if Gram stain of sputum reveals PMNs, squamous cells, and Gram + diplococci, definitive if pneumococcus is cultured from sputum.
Treatment: penicillin G, vancomycin
Prevention: Vaccine available for high risk individuals (elderly, immuno-compromised, asplenic, etc)
(2) Staphylococcus aureus Pneumonia
Staphylococcus aureus affects children > adults. Adults with S. aureus pneumonia have co-morbid conditions.
Develops from hematogenous spread or by oropharyngeal aspiration. Often occurs 3-4 days after the onset of the flu.
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S. aureus pneumonia associated with the flu has a high mortality (30-50%) in the elderly and pregnant
Gram + cocci in clusters
Symptoms: pleuritic chest pain, cough and fever, rigor uncommon.
Rapid cavitation and empyema formation are commonly seen and are suggestive of the disease.
Lungs are diffusely hemorrhagic with multiple small abscesses that may coalesce.
Treatment: parenteral b -lactamase resistant penicillins or vancomycin for MRSA (methicillin resistant S. aureus)
(3) Hemophilus influenza Pneumonia
Gram -, aerobic, fastidious, coccobacillus that frequently colonizes the oropharynx in adults and children.
Pathogenesis is by aspiration. Smoking, COPD and EtOHism are important predisposing conditions.
Fastidious, requires Factors V and X (hemin and NADH) for growth. Difficult to culture. Gram stain extremely important
Symptoms: fever, chills, cough, purulent sputum and clinical toxicity are usual. Empyema and abscesses are unusual.
Treatment: because of resistance, 2nd generation macrolides are used until b -lactamase production can be assessed.
Prevention: Vaccine, a conjugate of the H. flu capsular polysaccharide polyribosylribitol phosphate (PRP) with a protein (from N. meningitides or mutant diphtheria toxoid) is used to provide protection to infants > 18 months old.
Emperic treatment
Pts are grouped into categories according to age and symptoms. Each class receives a treatment until the organism can be identified and targeted specifically. because of the time involved in typing the causative agent, emperic treatment has been shown to save lives.
Poor response to Treatment – might be:
- (1) the wrong drugs
- (2) an unusual organism
- (3) a non-infectious illness.