STEPTRACKERRESPIRATORY | PATHOLOGY + MICROBIOLOGY | HIGH-YIELD REVIEW

Pneumonia Patterns & Organisms

Lobar, bronchopneumonia, interstitial and aspiration patterns — and the host, exposure or geography that names the organism.

FOCUS ON

Pattern → organism: lobar (pneumococcus, Klebsiella), patchy broncho (S aureus, H influenzae, Pseudomonas), interstitial (Mycoplasma, viruses, Legionella, Pneumocystis), aspiration (anaerobes, right lower lobe). Learn the lobar stages by day and the endemic fungi by map.

PATTERN / SETTING

ORGANISMS

PATHOLOGY · IMAGING

CLUE

Lobar pneumonia

S pneumoniae (most common CAP), Klebsiella

Consolidation of an entire lobe; intra-alveolar exudate

Rusty sputum (pneumococcus) · currant-jelly sputum in alcoholics (Klebsiella)

Bronchopneumonia

S aureus, H influenzae, Pseudomonas, Moraxella, E coli

Patchy, multifocal consolidation centered on bronchioles; often bilateral

Post-influenza → S aureus (cavities) · COPD → H influenzae, Moraxella

Interstitial (atypical) pneumonia

Mycoplasma, Chlamydophila, Legionella, viruses (influenza, RSV, adenovirus, CMV, SARS-CoV-2)

Inflammation in alveolar septa; diffuse reticular infiltrates; little sputum

Mycoplasma: young adults, cold agglutinins · Legionella: diarrhea, hyponatremia, urine antigen

Aspiration pneumonia

Oral anaerobes (Bacteroides, Fusobacterium, Peptostreptococcus)

Right lower lobe (upright) or posterior RUL/superior RLL (supine)

Alcohol, seizures, stroke, dysphagia · foul sputum

Lung abscess

Anaerobes, S aureus, Klebsiella

Necrotic cavity with air-fluid level

Clindamycin (aspiration) · also post-obstructive (tumor)

Hospital / ventilator

Pseudomonas, MRSA, gram-negative rods

Often multilobar

Pseudomonas also in cystic fibrosis

Immunocompromised

Pneumocystis (CD4 < 200), CMV, Aspergillus, Nocardia, MAC

PCP: bilateral ground-glass; Aspergillus: nodule with halo

PCP: ↑ LDH, silver-stained cysts · TMP-SMX treatment and prophylaxis

Endemic fungi

Histoplasma (Ohio/Mississippi valleys, caves, bird/bat droppings), Blastomyces (Great Lakes, broad-based budding; skin, bone), Coccidioides (Southwest desert, spherules)

Granulomas; hilar adenopathy; can mimic TB

Coccidioides: erythema nodosum ("desert bumps") · itraconazole; amphotericin if severe

Complications

Any organism

Parapneumonic effusion, empyema, abscess, sepsis, ARDS

Empyema → chest tube drainage

Mechanism — when defenses fail

EntryAspirated oral flora (most common), inhalation, blood

→

Defenses failCough, cilia, macrophages, IgA

→

Alveolar inflammationExudate

→

PatternLobar, broncho, interstitial

Risks: smoking (cilia), alcohol (aspiration, weak neutrophils), influenza (S aureus superinfection), CF, immunosuppression.

Stages of lobar pneumonia

STAGE

TIMING

FINDINGS

Congestion

Days 1–2

Dilated capillaries, fluid, many bacteria

Red hepatization

Days 2–4

Neutrophils, RBCs and fibrin fill alveoli — firm, red, liver-like

Gray hepatization

Days 4–8

RBCs break down; fibrinopurulent exudate remains

Resolution

~Day 8 onward

Enzymes digest exudate; macrophages clear it; architecture restored

How it's tested

COMMON CONFUSION"Atypical" pneumonia means an interstitial pattern with a mild course and little sputum. Legionella breaks the "mild" rule — it can be severe.

THE TRAPAn alcoholic found down after a binge, with fever and foul sputum, has anaerobic aspiration. The answer is clindamycin (or ampicillin-sulbactam), not a pneumococcal regimen alone.

YOUR APPROACHHost (age, alcohol, HIV, ventilator, CF) + exposure (travel, water, animals) + pattern → organism → drug.

If you see… think…

Rusty sputum and lobar consolidation in an older adult → S pneumoniae

Alcoholic with currant-jelly sputum, upper-lobe cavity → Klebsiella

Cavitating pneumonia a week after influenza → S aureus

College student, dry cough, mild illness, cold agglutinins → Mycoplasma

High fever, diarrhea, Na⁺ 128, cooling-tower exposure → Legionella

Foul sputum + RLL abscess after a binge → anaerobic aspiration

HIV with CD4 90, bilateral ground-glass, ↑ LDH → Pneumocystis

Neutropenic patient with a nodule and halo sign → invasive Aspergillus

Cave explorer from the Ohio valley with hilar nodes → Histoplasma

Arizona resident after a dust storm, erythema nodosum → Coccidioides