STEPTRACKERMICROBIOLOGY + PULMONOLOGY | INFECTIONS BY SYSTEM | HIGH-YIELD REVIEW

Pneumonia by Setting & Age

The likeliest organism depends on where the patient caught it, how old they are, and what host factor they carry — then the X-ray pattern and one clue confirm it.

FOCUS ON

Community-acquired: S pneumoniae most common at every adult age (lobar, rusty sputum); atypicals (Mycoplasma, Chlamydophila, Legionella) in young adults or with GI/neuro features; H influenzae and Moraxella in COPD; S aureus after influenza; Klebsiella in alcoholics. Hospital/ventilator: Pseudomonas, MRSA, Enterobacterales. Aspiration: oral anaerobes → abscess. By age: neonates — GBS, E coli; infants — RSV, C trachomatis (afebrile staccato cough); children — viruses, Mycoplasma, pneumococcus. Patterns: lobar (pneumococcus, Klebsiella), bronchopneumonia (S aureus, H flu, Pseudomonas), interstitial (viruses, Mycoplasma, Pneumocystis), cavity (S aureus, Klebsiella, anaerobes, TB).

SETTING · HOST

LIKELY ORGANISMS

CLUE

EMPIRIC APPROACH

Healthy adult outpatient

S pneumoniae, Mycoplasma, Chlamydophila, H influenzae, respiratory viruses

Lobar consolidation (pneumococcus) vs patchy, mild "walking" (atypical)

Amoxicillin or doxycycline (macrolide where resistance low)

Inpatient community-acquired

Same + Legionella, S aureus, gram-negatives

Urine antigens for pneumococcus and Legionella

Ceftriaxone + azithromycin, or respiratory fluoroquinolone

After influenza

S aureus (MRSA), S pneumoniae, H influenzae

Second fever, necrotizing or cavitating pneumonia

Add vancomycin or linezolid if MRSA suspected

Alcohol use disorder

S pneumoniae, Klebsiella, oral anaerobes

Currant-jelly sputum, upper-lobe cavity · foul sputum

Cover anaerobes if aspiration

COPD, smoker

H influenzae, Moraxella, S pneumoniae, Legionella

Exacerbation with purulent sputum

—

Cystic fibrosis

S aureus (childhood), Pseudomonas (later), Burkholderia

Mucoid colonies

Antipseudomonal combinations

Hospital / ventilator-associated

Pseudomonas, MRSA, Klebsiella, Acinetobacter, E coli

Onset ≥ 48 h after admission

Antipseudomonal β-lactam + MRSA coverage per risk

Aspiration

Oral anaerobes (Bacteroides, Prevotella, Fusobacterium, Peptostreptococcus)

Seizure, stroke, alcohol, dental disease; RLL superior or RUL posterior segment

Ampicillin-sulbactam or clindamycin

Neonate (< 4 weeks)

Group B strep, E coli, Listeria

Sepsis picture

Ampicillin + gentamicin

Infant 1–3 months, afebrile

C trachomatis

Staccato cough, conjunctivitis history, eosinophilia

Azithromycin (pyloric stenosis risk)

Children & teens

RSV and viruses (young), Mycoplasma (school age), S pneumoniae

Wheezing infant · teen with headache, cold agglutinins

Amoxicillin; macrolide for Mycoplasma

Exposure clues

Legionella (AC/water), C psittaci (birds), Coxiella (livestock), Francisella (rabbits), hantavirus (rodents), fungi (region)

Ask about travel, animals, work

See 4.07 and 8.01

Immunocompromised

Pneumocystis, CMV, Aspergillus, Nocardia, TB, Cryptococcus

CD4 count, neutropenia, transplant timing

See 10.07

Mechanism — how organisms reach the alveoli

Microaspiration of colonizersPneumococcus, H flu

Large-volume aspirationAnaerobes

Inhaled aerosolLegionella, TB, fungi

Blood-borneS aureus (IV drugs, septic emboli)

Defenses breached: cough reflex, mucociliary clearance (smoking, CF, influenza), alveolar macrophages and complement.

X-ray pattern → organism

PATTERN

DESCRIPTION

ORGANISMS

Lobar

One lobe, air bronchograms

S pneumoniae, Klebsiella, Legionella

Bronchopneumonia

Patchy, multifocal around airways

S aureus, H influenzae, Pseudomonas

Interstitial

Diffuse reticular / ground glass

Viruses, Mycoplasma, Pneumocystis, Chlamydophila

Cavitary

Necrosis with air-fluid level

S aureus, Klebsiella, anaerobes, TB, fungi

How it's tested

COMMON CONFUSIONTypical (abrupt, high fever, productive, lobar) vs atypical (gradual, dry cough, extrapulmonary symptoms, patchy) — atypicals don't respond to β-lactams.

THE TRAPPneumococcus remains the most common cause of community pneumonia even in young adults — "atypical" is the classic, not the most frequent.

YOUR APPROACHSetting (community, hospital, aspiration) → age → host factor → X-ray pattern → one exposure clue → coverage.

If you see… think…

Abrupt fever, rigors, rusty sputum, lobar consolidation → S pneumoniae

College student, dry cough, headache, cold agglutinins → Mycoplasma

Pneumonia + diarrhea + hyponatremia → Legionella

Cavitating pneumonia a week after the flu → S aureus

Alcoholic with currant-jelly sputum → Klebsiella

Foul sputum, RLL superior segment abscess after a seizure → anaerobic aspiration

Ventilated patient on day 6 with new infiltrate → Pseudomonas, MRSA

Afebrile 6-week-old with staccato cough → C trachomatis

Newborn with pneumonia and sepsis → group B strep

Bird owner with pneumonia and splenomegaly → C psittaci