24/07/2026
Pneumonia occurring before or within 48 hours of hospital admission.
Common Organisms
• Streptococcus pneumoniae (most common)
• Haemophilus influenzae
• Staphylococcus aureus
• Moraxella catarrhalis
• Mycoplasma pneumoniae
• Respiratory viruses (Influenza virus, RSV)
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# # Hospital-Acquired Pneumonia (HAP)
Definition
Pneumonia developing 48 hours or more after hospital admission and was not incubating at the time of admission.
Common Organisms
• Staphylococcus aureus (including MRSA)
• Pseudomonas aeruginosa
• Klebsiella pneumoniae
• Escherichia coli
• Acinetobacter species
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# # Streptococcus pneumoniae
• Most common cause of community-acquired pneumonia.
• Gram-positive lancet-shaped diplococci.
• Produces rust-colored sputum.
• Common in elderly, alcoholics, post-splenectomy patients, immunocompromised patients, and those with chronic heart or lung disease.
• May cause bacteremia, meningitis, empyema, and pleural effusion.
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# # Staphylococcus aureus
• Gram-positive cocci in clusters.
• Most common cause of secondary bacterial pneumonia following influenza.
• Seen in intravenous drug users, diabetics, cystic fibrosis patients, and immunocompromised individuals.
• Causes lung abscess, cavitary pneumonia, empyema, and pneumatoceles.
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# # Klebsiella pneumoniae
• Gram-negative encapsulated bacillus.
• Common in alcoholics, diabetics, elderly, and debilitated patients.
• Produces thick red currant-jelly sputum.
• Causes severe necrotizing pneumonia with upper lobe cavitation.
• Frequently multidrug resistant.
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# # Pseudomonas aeruginosa
• Gram-negative rod.
• Produces green sputum due to pyocyanin pigment.
• Important cause of hospital-acquired pneumonia.
• Most common pathogen in cystic fibrosis.
• Common in bronchiectasis, ventilated patients, and immunocompromised individuals.
• Causes severe necrotizing pneumonia with high mortality.
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# # Mycoplasma pneumoniae
• Smallest free-living bacterium.
• Has no cell wall; therefore penicillins and cephalosporins are ineffective.
• Causes atypical pneumonia.
• Presents with fever, headache, malaise, myalgia, sore throat, and dry cough.
• Diagnosis is by PCR or serology.
• Cold agglutinins may cause autoimmune hemolytic anemia.
• Associated with erythema multiforme, Stevens–Johnson syndrome, Guillain–Barré syndrome, encephalitis, and myocarditis.
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# # Legionella pneumophila
• Found in contaminated water systems, cooling towers, air-conditioning systems, and hot water tanks.
• Causes Legionnaires' disease.
• Presents with high fever, dry cough, dyspnea, myalgia, and headache.
• Extrapulmonary manifestations include diarrhea, abdominal pain, confusion, and neurological symptoms.
• Laboratory findings include hyponatremia and elevated liver enzymes.
• Diagnosis is by urinary antigen test, culture on BCYE agar, or PCR.
• Treatment is azithromycin or levofloxacin.
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# # Haemophilus influenzae
• Gram-negative coccobacillus.
• Common in smokers and patients with COPD.
• Important bacterial cause of acute exacerbation of COPD.
• Most common organism isolated in non-cystic fibrosis bronchiectasis.
• May cause otitis media, sinusitis, epiglottitis, and pneumonia.
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# # High-Yield IMM/FCPS Pearls
• Most common cause of community-acquired pneumonia: Streptococcus pneumoniae.
• Most common bacterial pneumonia after influenza: Staphylococcus aureus.
• Rust-colored sputum: Streptococcus pneumoniae.
• Currant-jelly sputum: Klebsiella pneumoniae.
• Green sputum: Pseudomonas aeruginosa.
• No cell wall: Mycoplasma pneumoniae.
• Air conditioner-associated pneumonia: Legionella pneumophila.
• Pneumonia with hyponatremia: Legionella pneumophila.
• Common bacterial cause of acute exacerbation of COPD: Haemophilus influenzae.
• Most common pathogen in cystic fibrosis: Pseudomonas aeruginosa.
• Most common organism isolated in non-cystic fibrosis bronchiectasis: Haemophilus influenzae.
• Pneumonia with upper lobe cavitation in alcoholics: Klebsiella pneumoniae.
