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PNEUMONIA - Comprehensive 2nd Year MBBS Microbiology Notes

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PART 1: LOBAR PNEUMONIA (Pneumococcal Pneumonia)

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DEFINITION

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ETIOLOGY

• Primary causative organism: Streptococcus pneumoniae (accounts for most lobar pneumonias)
• Common serotypes in adults: types 1-8 cause ~75% of cases; types 1, 3, 7, and 8 are most frequent
• In children: types 6, 14, 19, and 23 predominate
• Other organisms (less common): Klebsiella pneumoniae, Legionella pneumophila

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PATHOGENESIS

Source and transmission

1. Microaspiration of oropharyngeal secretions (most common route)
2. Hematogenous spread from a distant infected site
3. Macroaspiration (in obtunded patients)

Virulence factors

• Polysaccharide capsule - the primary virulence factor; prevents opsonization and phagocytosis by masking complement and antibody binding sites
• Pneumolysin - a cholesterol-binding cytotoxin that damages respiratory epithelium, inhibits ciliary function, and directly injures myocardium
• Secretory IgA protease - cleaves secretory IgA, overcoming a key mucosal defense
• Surface adhesins - mediate binding to nasopharyngeal epithelial cells
• Autolysin - releases cell wall components (teichoic acid, peptidoglycan) that trigger intense inflammatory response

Predisposing factors (loss of natural resistance)

1. Viral respiratory infections (damage surface cells, impair mucociliary clearance)
2. Alcohol/drug intoxication (depresses phagocytic activity, cough reflex, facilitates aspiration)
3. Abnormal mucus accumulation (allergy, COPD) - protects pneumococci from phagocytosis
4. Pulmonary congestion and heart failure
5. Malnutrition, sickle cell anemia, hyposplenism, complement deficiency, agammaglobulinemia
6. Crowding (facilitates droplet transmission, especially in winter/early spring)

Disease mechanism

1. Innate host defenses are bypassed (mucociliary escalator, secretory IgA, alveolar macrophages)
2. Massive outpouring of fibrinous edema fluid into alveoli
3. Neutrophil recruitment - alveolar exudate forms
4. Bacteremia possible via lymphatic drainage of the lung
5. The alveolar walls remain intact (unlike destructive organisms like S. aureus)

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PATHOLOGY - THE FOUR CLASSIC STAGES OF LOBAR PNEUMONIA

Key point: In Stage 4, the exudate is resorbed, ingested by macrophages, expectorated, or organized by fibroblasts. Alveolar walls remain intact - unlike in abscess-forming organisms.

Pleural fibrinous reaction (pleuritis) may accompany the early stages if consolidation reaches the lung surface; it may resolve or leave fibrous thickening/adhesions.

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CLINICAL MANIFESTATIONS

• High fever (39-40°C) with rigors/chills
• Sharp, stabbing pleuritic chest pain (due to pleural involvement)
• Cough - initially dry, then productive
• Rusty/blood-tinged sputum - classic; due to red cells and hemoglobin in the alveolar exudate (rusty = degraded hemoglobin in stage 2-3)
• Tachypnea and breathlessness
• Tachycardia

• Inspection: Tachypnea, reduced chest movement on affected side
• Palpation: Increased tactile vocal fremitus over consolidation
• Percussion: Dull note over consolidated lobe
• Auscultation: Bronchial breath sounds, crepitations (crackling rales) - localized to involved lobe
• Pleural rub if pleuritis present

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SUPPURATIVE (DIRECT/LOCAL) COMPLICATIONS

1. Empyema thoracis - pus in the pleural cavity; most common suppurative complication; results from spread of infection to the pleural space. Forms a fibrinopurulent intrapleural reaction
2. Lung abscess - less common with pneumococcus (more typical with S. aureus, Klebsiella, anaerobes); occurs when tissue necrosis exceeds resolution
3. Necrotizing pneumonia - extensive parenchymal destruction with multiple micro-abscesses
4. Parapneumonic effusion - sterile pleural effusion adjacent to pneumonia; if untreated may progress to empyema
5. Pneumothorax - rare, but can complicate empyema
6. Bacteremia - spreads hematogenously to distant sites, leading to metastatic infections (see non-suppurative complications below)
7. Pericarditis - direct spread or hematogenous

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NON-SUPPURATIVE (METASTATIC/SYSTEMIC) COMPLICATIONS

1. Meningitis - pneumococcal meningitis; most serious complication; bacteremia from pneumonia leads to seeding of the meninges. Confusion, obtundation, and stiff neck should prompt immediate consideration
2. Endocarditis - seeding of cardiac valves; acute pneumococcal endocarditis (now rare with early antibiotic treatment)
3. Septic arthritis - hematogenous seeding of joints
4. Peritonitis - especially in patients with ascites
5. Otitis media / Sinusitis - contiguous or hematogenous spread
6. Cardiovascular events - up to one-third of hospitalized patients experience major cardiovascular events (MI, heart failure, arrhythmias); pneumolysin may directly injure myocardium
7. Overwhelming pneumococcal sepsis (OPSI) - risk is greatly increased in splenectomized patients because the spleen is the principal site for clearance of encapsulated organisms
8. ARDS (Acute Respiratory Distress Syndrome) - in severe/overwhelming disease

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LABORATORY DIAGNOSIS

1. Specimen Collection

• Sputum - should be actual sputum, NOT saliva; specimen must show >25 PMNs and <10 epithelial cells per low-power field to be valid ("acceptable sputum")
• Blood cultures - 2 sets before antibiotics; yield only 5-15% in CAP but S. pneumoniae accounts for ~2/3 of positives
• CSF if meningitis suspected
• Pleural fluid if effusion present

2. Direct Gram Stain of Sputum

• Finding: Gram-positive, lancet-shaped (flame/candle-shaped) diplococci, often in pairs or short chains, surrounded by a clear halo (the capsule)
• Highly specific and rapid; can detect pneumococci in 60-80% of confirmed cases
• Quellung (capsular swelling) reaction - mixing sputum with type-specific antiserum causes the capsule to swell (positive Quellung), confirming both presence and serotype; uses polyvalent "omniserum" for rapid screening; rarely used now due to high cost and expertise required

3. Culture

• Media: Blood agar (5% sheep blood), chocolate agar; incubate at 37°C with 5-10% CO2
• Colonies: Alpha-hemolytic (greenish halo) - 1-2 mm; draughtsman/checker colonies (central depression due to autolysis)
• Identification tests:
  • Optochin (P disk) sensitivity - zone of inhibition >14 mm (positive = pneumococcus; viridans streptococci are resistant)
  • Bile solubility - colonies dissolve in 10% sodium deoxycholate (positive = pneumococcus)
  • Alpha-hemolysis (vs beta-hemolysis of Group A Strep)

4. Urinary Antigen Test

• Detects pneumococcal C-polysaccharide (common to all serotypes) in urine
• Sensitivity: 50-80%; Specificity: >90%
• Particularly useful when: culture cannot be obtained, antibiotics already started, or rapid diagnosis needed
• C-polysaccharide can also be detected in CSF

5. Molecular Methods (PCR/NAAT)

• Nucleic acid amplification techniques detect multiple respiratory pathogens from a single respiratory sample
• Increasingly standard of care for community-acquired pneumonia
• High sensitivity, unaffected by prior antibiotic use

6. Chest X-Ray

• Homogeneous lobar or segmental alveolar infiltrate (white-out of a lobe)
• Air bronchograms - outline of airways made visible because surrounding alveoli are fluid-filled; characteristic of lobar pneumonia
• Small pleural effusion often present
• CT chest: may demonstrate abscess, empyema, adenopathy more clearly

7. Blood Tests

• CBC: Leukocytosis with left shift (predominance of band forms/neutrophilia) - typical
• Leukopenia in a minority - associated with poor prognosis
• Elevated CRP and ESR
• Elevated procalcitonin supports bacterial etiology

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TREATMENT

Antibiotic Therapy

Supportive Treatment

• Oxygen supplementation for hypoxia
• Antipyretics, analgesics (for pleuritic pain)
• IV fluids if dehydrated
• Management of complications (chest drain for empyema, etc.)

Prevention

• Pneumococcal vaccines: PCV13 (conjugate), PPSV23 (polysaccharide) - recommended for elderly, immunocompromised, asplenic, and children
• Stabilization of predisposing conditions (diabetes, heart failure)

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PART 2: HAEMOPHILUS INFLUENZAE PNEUMONIA

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DEFINITION

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MICROBIOLOGY OF H. INFLUENZAE

• Morphology: Small (0.3-0.5 x 1.5 µm), pleomorphic Gram-negative rods (coccobacilli to long filaments)
• Capsule: Type b (Hib) has polyribosylribitol phosphate (PRP) capsule - main virulence factor; types a-f; non-typeable strains lack a capsule
• Growth requirements: X factor (hemin) + V factor (NAD) - "XV" requiring; grows on chocolate agar (heating lyses RBCs, releasing X factor and inactivating V-factor inhibitors)
• Satellite phenomenon: Grows as satellite colonies around S. aureus on blood agar (staphylococci lyse RBCs releasing X factor and excrete NAD)
• Facultative anaerobe

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PATHOGENESIS

Who gets H. influenzae pneumonia?

• Nontypeable H. influenzae: Causes pneumonia in adults with:
  • Chronic obstructive pulmonary disease (COPD) - most important risk factor
  • Smoking history (damages mucociliary clearance)
  • Diabetes mellitus
  • Congestive heart failure
  • Alcoholism
  • Elderly patients
• Type b (Hib): Now uncommon due to vaccine; caused serious invasive disease (meningitis, epiglottitis, pneumonia) primarily in children <5 years before vaccination

Mechanism

• Organisms colonize the oropharynx, then descend to the lower respiratory tract
• Non-encapsulated strains use adhesins (Hap protein, pili, outer membrane proteins) to attach to respiratory epithelium
• Produce IgA protease which cleaves secretory IgA
• Endotoxin (lipooligosaccharide) triggers inflammatory response
• Dense, fibrin-rich exudates containing neutrophils are produced, plugging smaller bronchi (similar to pneumococcal pneumonia but lobular/patchy distribution)
• Consolidation is usually lobular and patchy but may become confluent involving an entire lobe

Classic clinical case pattern (Jawetz)

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CLINICAL MANIFESTATIONS

• Onset can be subacute (days) or acute
• Fever, productive cough with purulent sputum
• Pleuritic chest pain (less prominent than pneumococcal)
• Tachypnea, breathlessness
• Signs of consolidation on examination
• Following viral respiratory infection (influenza, RSV) - secondary bacterial pneumonia
• In children (Hib): high mortality; descending laryngotracheobronchitis may cause airway obstruction
• Complications: pleural effusion, empyema (associated with Hib); rarely lung abscess

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SUPPURATIVE COMPLICATIONS

1. Pleural effusion and empyema - associated with Hib infections
2. Bacteremia - leading to metastatic disease
3. Lung abscess - less common
4. Other sites of hematogenous spread (before vaccine era in children):
   • Meningitis (most feared; H. influenzae was the commonest cause of bacterial meningitis in children <5 years pre-vaccine)
   • Epiglottitis (supraglottitis)
   • Septic arthritis
   • Pericarditis
   • Cellulitis (orbital, buccal)
   • Pyelonephritis, cholecystitis, endocarditis (in predisposed adults)

NON-SUPPURATIVE COMPLICATIONS

• Mainly arise from bacteremic spread (as above)
• Acute exacerbations of COPD - H. influenzae is the most common bacterial cause

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LABORATORY DIAGNOSIS

1. Specimen Collection

• Sputum from lower airways (not contaminated with oral secretions)
• Blood cultures - useful in pneumonia, especially if bacteremia suspected
• CSF + blood if meningitis suspected
• Direct needle aspiration for sinusitis/otitis

2. Gram Stain

• Gram-negative coccobacilli (small rods ranging from coccobacilli to long pleomorphic filaments)
• Detected in >80% of CSF specimens in untreated Haemophilus meningitis
• Also useful for rapid diagnosis in arthritis and lower respiratory tract disease

3. Culture

• Media: Chocolate agar (most reliable) - X and V factors released by heated RBCs
• Incubate at 37°C in 5-10% CO2
• Colonies: 1-2 mm, smooth, opaque after 24 hours incubation
• Satellite phenomenon: Small colonies growing around S. aureus on blood agar (S. aureus lyses RBCs to provide X factor and excretes NAD as V factor)
• Blood cultures: growth often delayed (media not always optimally supplemented)
• H. influenzae often grows better in anaerobically incubated blood cultures (does not require X factor under anaerobic conditions)

4. Identification

• X and V factor requirement: Growth around X+V factor disk but not X alone or V alone
• Confirmed as H. influenzae by requiring both X and V factors for growth

5. Antigen Detection (Hib only)

• PRP capsular antigen detection using latex particle agglutination
• Detects <1 ng/mL of PRP
• Samples: CSF, urine (antigen excreted intact)
• Limitation: Only detects type b strains; not useful for non-typeable or non-b capsular types
• Limited utility in vaccinated populations where Hib is now rare

6. Molecular Methods

• PCR for H. influenzae - useful when culture is negative due to prior antibiotic treatment

7. Chest X-Ray

• Patchy bilateral or unilateral bronchopneumonia
• May show segmental consolidation that can coalesce into lobar pattern
• Spherical pneumonia (rounded consolidation) possible, especially in children

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TREATMENT

• Beta-lactamase production is the main resistance mechanism (~30-40% of strains produce beta-lactamase)
• Prevention: Hib conjugate vaccine - dramatically reduced invasive Hib disease in children; not effective against non-typeable strains

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PART 3: ATYPICAL PNEUMONIA

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DEFINITION

• The causative organisms are not visible on Gram stain (intracellular, lack cell walls, or stain poorly)
• They do not grow on standard bacteriologic media
• They are resistant to beta-lactam antibiotics
• The clinical picture is milder - "walking pneumonia"
• Predominantly interstitial rather than alveolar infiltrate
• Dry, non-productive cough rather than productive sputum

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CAUSATIVE ORGANISMS

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MYCOPLASMA PNEUMONIAE - THE PROTOTYPE (Primary Atypical Pneumonia)

Microbiology

• Smallest free-living organism capable of self-replication
• No cell wall - hence beta-lactams and other cell wall active antibiotics are ineffective
• Very slow growing in artificial media (Eaton agent); cannot be detected on Gram stain
• Pleomorphic (no fixed shape due to absence of cell wall)

Pathogenesis

• Transmitted by respiratory droplets (requires close prolonged contact)
• Long incubation period: 2-3 weeks
• Organism attaches to respiratory epithelium via P1 adhesin tip structure
• Damages ciliated cells, causing ciliastasis (paralysis of cilia)
• Produces hydrogen peroxide and superoxide - oxidative injury to epithelium
• Triggers intense immune/inflammatory response - interstitial pneumonitis
• Small outbreaks in households, schools, and military camps; may not be recognized due to long incubation period
• Community-wide epidemics every 4-7 years

Clinical Manifestations

• Affects all ages but most common in young adults, school-age children, military recruits
• Insidious onset (gradual, over days)
• Malaise, low-grade fever
• Prominent headache (more prominent than in pneumococcal pneumonia)
• Sore throat (common); coryza minimal or absent
• Dry, non-productive cough - the hallmark; develops over a few days
• Myalgias
• The patient is often sick enough to feel unwell but "well enough to walk" - hence "walking pneumonia"
• Extrapulmonary manifestations: myalgias, arthralgias, skin rashes (erythema multiforme), ear pain (bullous myringitis - pathognomonic if present), hemolytic anemia (cold agglutinins)

Suppurative Complications

1. Secondary bacterial superinfection
2. Pleural effusion (small; seen in some cases)
3. Lung abscess (very rare)

Non-Suppurative (Extrapulmonary) Complications

1. Hemolytic anemia (cold agglutinin-mediated, IgM anti-I antibodies) - most classic; may cause Raynaud's phenomenon
2. Stevens-Johnson syndrome / Erythema multiforme - skin manifestation
3. Bullous myringitis (vesicular tympanic membrane) - highly characteristic
4. Neurological complications: Meningoencephalitis, Guillain-Barre syndrome, transverse myelitis, cerebellar ataxia
5. Cardiac: Myocarditis, pericarditis (rare)
6. Gastrointestinal: Nausea, vomiting, diarrhea, hepatitis
7. Arthritis (reactive)
8. Glomerulonephritis (rare)

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LABORATORY DIAGNOSIS OF ATYPICAL PNEUMONIA

1. Chest X-Ray

• Interstitial infiltrates - bilateral, patchy, peribronchial thickening ("shaggy heart" border); often worse than clinical findings (X-ray-clinical dissociation)
• Consolidation is usually unilateral (lower lobe), but may be bilateral
• Small pleural effusion in some cases

2. Cold Agglutinin Test

• Nonspecific but classically associated with Mycoplasma pneumonia
• Cold agglutinins = IgM antibodies that agglutinate human group O red blood cells at 4°C but not 37°C
• Positive in ~50-70% of Mycoplasma pneumonia cases
• Bedside test: Collect blood in a tube, refrigerate at 4°C for a few minutes - agglutination of red cells is positive; re-warming dissolves the agglutinate
• A titer of ≥1:32 is significant; ≥1:64 is highly suggestive
• Not specific - can be positive in EBV, CMV, adenovirus, influenza

3. Serology (Complement Fixation / ELISA)

• Complement fixation (CF) test: Fourfold rise in CF antibody titer between acute and convalescent serum (2-3 weeks apart) is diagnostic
• ELISA: Detection of IgM (acute infection) and IgG (past infection / convalescence)
• IgM anti-Mycoplasma antibodies appear at 7-10 days and peak at 3-4 weeks

4. Culture

• Very difficult and impractical - organisms grow slowly (weeks) on special media (SP4 broth/agar, Hayflick medium)
• Colonies show "fried-egg" appearance under microscope
• Not routinely done clinically

5. PCR / Molecular Methods

• Most sensitive and specific; detects Mycoplasma DNA from respiratory specimens
• Becoming the standard of care; rapid results
• Can also diagnose Chlamydophila, Legionella, viruses from the same specimen

6. Legionella-specific Tests

• Urinary antigen test for Legionella serogroup 1 (sensitivity 70-90%, specificity >99%) - rapid, highly reliable
• Culture: BCYE (buffered charcoal yeast extract) agar - specific enriched medium
• Gram stain: poorly staining Gram-negative rods (not visible on standard stain)
• Direct fluorescent antibody (DFA) test on sputum

7. Chlamydophila

• Serology (micro-immunofluorescence - MIF test) or PCR
• Culture possible in cell cultures but impractical

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TREATMENT OF ATYPICAL PNEUMONIA

• Macrolides (azithromycin, clarithromycin) and tetracyclines (doxycycline) are the cornerstone
• Fluoroquinolones (levofloxacin, moxifloxacin) cover both typical and atypical organisms

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SUMMARY COMPARISON TABLE

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PNEUMONIA - Complete 2nd Year MBBS Microbiology Notes

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PART 1: LOBAR PNEUMONIA (Pneumococcal Pneumonia)

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DEFINITION

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ETIOLOGY

• Primary causative organism: Streptococcus pneumoniae - Gram-positive, lancet-shaped diplococci; 91 serotypes based on capsular polysaccharide
• Common serotypes in adults: Types 1-8 cause ~75% of pneumococcal pneumonia and >50% of bacteremic deaths
• Common serotypes in children: Types 6, 14, 19, 23
• Other causes of lobar pattern: Klebsiella pneumoniae (alcoholics; bulging fissure); Legionella pneumophila

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PATHOGENESIS

Transmission and colonization

• S. pneumoniae colonizes the nasopharynx in 5-50% of healthy people (carriage is highest in winter/early spring)
• Transmitted by respiratory droplet nuclei
• Carriage is most common in school-age children
• From the nasopharynx, the organism reaches the lungs by microaspiration of oropharyngeal secretions (main route), hematogenous spread, or direct spread from contiguous sites

Virulence factors

1. Polysaccharide capsule - the principal virulence factor; anti-phagocytic (prevents complement deposition and opsonin-mediated phagocytosis); immunity is type-specific
2. Pneumolysin - cholesterol-binding cytotoxin; damages respiratory epithelium, inhibits ciliary function, directly injures myocardium; releases bacterial components that trigger inflammation
3. Secretory IgA protease - cleaves secretory IgA, overcoming mucosal defense
4. Surface adhesins - bind pneumococci to nasopharyngeal epithelial cells (strains with smaller capsules have enhanced adhesin exposure and colonize better)
5. Autolysin - releases peptidoglycan and teichoic acid (C-polysaccharide) from cell wall, triggering intense host inflammatory response
6. Endopeptidase O - degrades complement proteins

Predisposing factors (factors that lower natural resistance)

1. Viral respiratory infections - damage surface cells; impair mucociliary clearance
2. Abnormal mucus accumulation (allergy, COPD) - protects pneumococci from phagocytosis
3. Bronchial obstruction (atelectasis)
4. Alcohol/drug intoxication - depresses phagocytic activity, depresses cough reflex, facilitates aspiration
5. Pulmonary congestion and heart failure
6. Malnutrition, general debility
7. Sickle cell anemia, hyposplenism
8. Complement deficiency, agammaglobulinemia, nephrosis
9. Crowding (winter/early spring epidemics)

Disease mechanism

• The disease is caused primarily by the host inflammatory response to infection, not by bacterial toxins
• Organisms overwhelm host defenses (innate: mucociliary, secretory IgA, alveolar macrophages)
• Massive outpouring of fibrinous edema fluid into alveoli occurs
• Neutrophil recruitment and exudate formation follow
• Bacteremia occurs via lymphatic drainage in 10-20% of cases
• Alveolar walls remain intact (unlike S. aureus or Klebsiella which cause necrosis)

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PATHOLOGY - FOUR CLASSIC STAGES

Pleural fibrinous reaction (pleuritis) is present early if consolidation reaches the lung surface; may resolve or leave fibrous thickening/adhesions. Alveolar walls remain intact throughout.

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CLINICAL MANIFESTATIONS

• High fever (39-40°C) with rigors and chills
• Sharp, stabbing pleuritic chest pain - accentuated by inspiration/cough (due to pleural involvement)
• Cough - initially dry, then productive
• Rusty/blood-tinged sputum - pathognomonic; due to red cells and hemoglobin in alveolar exudate (degraded hemoglobin = rust color in stage 2-3)
• Breathlessness (tachypnea, dyspnea)
• Tachycardia
• Myalgias, arthralgias, vomiting, diarrhea may accompany
• Older patients may present with delirium as the first sign

• Reduced chest expansion on affected side
• Increased tactile vocal fremitus (TVF)
• Dull percussion note
• Bronchial breath sounds
• Crackling (crepitant) rales - localized to the involved lobe
• Pleural friction rub if pleuritis present

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SUPPURATIVE COMPLICATIONS

1. Empyema thoracis - pus in the pleural cavity; the most common suppurative complication; results from spread of infection to the pleural space forming a fibrinopurulent intrapleural reaction; requires drainage
2. Parapneumonic effusion - initially sterile pleural effusion adjacent to pneumonia; may progress to empyema if untreated
3. Lung abscess - less common with pneumococcus (more typical with S. aureus, Klebsiella, anaerobes); occurs when tissue necrosis exceeds resolution capacity
4. Necrotizing pneumonia - extensive parenchymal destruction with multiple microabscesses
5. Pericarditis - direct extension or hematogenous; purulent pericarditis
6. Bacteremia / Pneumococcal septicemia - occurs in 10-20%; leading to systemic metastatic complications

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NON-SUPPURATIVE (METASTATIC/SYSTEMIC) COMPLICATIONS

1. Meningitis - most feared complication; confusion, obtundation, and stiff neck should prompt urgent evaluation; pneumococcal meningitis has high mortality
2. Endocarditis - seeding of heart valves; acute pneumococcal endocarditis
3. Septic arthritis - joint infection via hematogenous spread
4. Peritonitis - especially in patients with ascites
5. Overwhelming pneumococcal sepsis (OPSI) - greatest risk in asplenic patients (spleen is the principal site for clearance of encapsulated bacteria)
6. Major cardiovascular events - up to 1/3 of hospitalized patients develop MI, arrhythmias, or heart failure; pneumolysin directly injures myocardium
7. Otitis media / Sinusitis - by contiguous or hematogenous spread
8. ARDS - in overwhelming/severe disease

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LABORATORY DIAGNOSIS

1. Specimen Collection

• Sputum: Actual sputum, not saliva; valid if >25 PMNs and <10 squamous epithelial cells per low-power field
• Blood cultures: 2 sets before antibiotics; yield 5-15%; S. pneumoniae accounts for ~2/3 of positives
• CSF if meningitis suspected
• Pleural fluid if effusion present
• Urine for antigen detection

2. Direct Gram Stain of Sputum

• Gram-positive, lancet-shaped diplococci (candle-flame shape), often surrounded by a clear halo (capsule)
• Rapid, highly specific
• Can detect pneumococci in 60-80% of confirmed cases

3. Culture

• Media: Blood agar (5% sheep blood) or chocolate agar; CO2 incubation at 37°C
• Colonies: Alpha-hemolytic (green halo), 1-2 mm; "draughtsman" or "checker" colonies (central depression due to autolytic collapse)
• Identification:
  • Optochin (P disk) sensitivity test - zone of inhibition >14 mm distinguishes S. pneumoniae from viridans streptococci (which are optochin resistant)
  • Bile solubility test - colonies dissolve in sodium deoxycholate (positive = S. pneumoniae; viridans streptococci are bile-resistant)

4. Quellung (Capsular Swelling) Reaction

• Mixing pneumococci with type-specific antiserum or polyvalent "omniserum" causes the capsule to swell visibly and organisms agglutinate
• Useful for rapid identification and serotyping directly from sputum
• Rarely used now due to high reagent cost and expertise required

5. Urinary Antigen Detection

• Detects pneumococcal C-polysaccharide (common to all 91 serotypes) in urine by immunochromatography
• Sensitivity: 50-80%; Specificity: >90%
• Useful when cultures cannot be obtained in time or antibiotics already started
• C-polysaccharide also detectable in CSF

6. Molecular Methods (PCR/NAAT)

• Highly sensitive and specific; unaffected by prior antibiotic use
• Detects multiple pathogens (bacteria + viruses) from a single respiratory sample
• Increasingly standard of care

7. Imaging

• Chest X-ray: Homogeneous lobar or segmental alveolar infiltrate; air bronchograms (airways visible against background of fluid-filled alveoli) - characteristic; small pleural effusion common
• CT chest: Better defines complications (abscess, empyema, adenopathy)
• Lung ultrasound: Shows subpleural consolidation, hepatization pattern

8. Blood Tests

• CBC: Leukocytosis with left shift (neutrophilia + band forms); leukopenia = poor prognosis
• Elevated CRP, ESR, procalcitonin

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TREATMENT

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PART 2: HAEMOPHILUS INFLUENZAE PNEUMONIA

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DEFINITION

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MICROBIOLOGY

• Morphology: Small (0.3-0.5 x 1.5 µm), pleomorphic Gram-negative rods (coccobacilli to long filaments)
• Capsule: Type b capsule contains polyribosylribitol phosphate (PRP) - main virulence factor; 6 capsular types (a-f); non-typeable strains lack a capsule
• Growth requirements: Requires both X factor (hemin, heat-stable) and V factor (NAD, heat-labile) - hence grows on chocolate agar (but not plain blood agar where V-factor inhibitors persist)
• Satellite phenomenon: Grows as tiny satellite colonies around S. aureus on plain blood agar (S. aureus lyses RBCs to release X factor and excretes NAD as V factor)
• Facultative anaerobe (under anaerobic conditions does not require X factor)

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PATHOGENESIS

Risk groups

• Adults with COPD (most important) - H. influenzae is the most common bacterial cause of acute COPD exacerbations
• Smokers (damaged mucociliary escalator)
• Diabetics, patients with congestive heart failure
• Elderly patients
• Children (pre-vaccine era): Hib caused meningitis, epiglottitis, pneumonia in children <5 years

Mechanism

• Organisms colonize oropharynx, then descend to lower respiratory tract
• Non-typeable strains use adhesins (Hap, pili, OMP P2, P5) to attach to damaged respiratory epithelium
• IgA protease cleaves secretory IgA
• Lipooligosaccharide (LOS/endotoxin) triggers inflammatory cascade
• Dense fibrin-rich exudates containing neutrophils plug the smaller bronchi
• Consolidation is usually lobular and patchy (bronchopneumonia) but may become confluent and involve an entire lobe

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CLINICAL MANIFESTATIONS

• Onset: subacute to acute
• Fever, productive cough with purulent sputum
• Pleuritic chest pain (less prominent than pneumococcal)
• Tachypnea, breathlessness
• Signs of consolidation on examination
• Often follows a viral upper respiratory tract infection (secondary bacterial pneumonia)
• In children (Hib): high-mortality pneumonia; descending laryngotracheobronchitis with airway obstruction from fibrin-rich exudate plugging small bronchi

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SUPPURATIVE COMPLICATIONS

1. Pleural effusion and empyema - especially with Hib in children
2. Bacteremia - leads to metastatic infections
3. Lung abscess - less common
4. In children (Hib, pre-vaccine era) - hematogenous spread to:
   • Meningitis (commonest cause of bacterial meningitis in children <5 years, pre-vaccine)
   • Epiglottitis (supraglottitis) - life-threatening airway emergency
   • Septic arthritis
   • Pericarditis, cellulitis (orbital, buccal), osteomyelitis

NON-SUPPURATIVE COMPLICATIONS

• COPD exacerbations (chronic recurrent inflammation)
• Pyelonephritis, cholecystitis, endocarditis (predisposed adults, rare)

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LABORATORY DIAGNOSIS

1. Specimen Collection

• Sputum (from lower airways, minimizing oral contamination)
• Blood cultures (2 sets before antibiotics)
• CSF + blood if meningitis suspected
• Direct needle aspiration for sinusitis/otitis

2. Gram Stain

• Gram-negative coccobacilli (small rods ranging from coccobacilli to long pleomorphic filaments)
• Detected in >80% of CSF in untreated Hib meningitis
• Useful for rapid diagnosis in pneumonia and arthritis

3. Culture

• Media: Chocolate agar - essential; X and V factors are released by heated RBCs and V-factor inhibitors destroyed; incubate at 37°C in 5-10% CO2
• Colonies: 1-2 mm, smooth, opaque after 24 hours
• Satellite phenomenon on plain blood agar around S. aureus colonies (diagnostic clue)
• Blood cultures: growth often delayed (commercial media may not have optimal X+V factor concentrations)

4. Identification

• X and V factor requirement: Spot tests with X, V, and X+V disks on factor-free media; H. influenzae grows only around X+V disk
• Biotype determination by biochemical tests

5. Antigen Detection (for Hib only)

• Latex particle agglutination detects PRP capsular antigen in CSF and urine
• Detects <1 ng/mL of PRP
• Limitation: Only detects type b; not useful for non-typeable strains (now more common)

6. Molecular Methods

• PCR detects H. influenzae from respiratory specimens; useful after antibiotic treatment

7. Imaging

• Patchy bilateral or unilateral bronchopneumonia
• May show segmental/lobar consolidation
• Spherical pneumonia possible in children (rounded edges; due to exudate spread through pores of Kohn)

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TREATMENT

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PART 3: ATYPICAL PNEUMONIA

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DEFINITION

• Cannot be seen on Gram stain (intracellular, lack cell walls, or stain poorly)
• Do not grow on standard bacteriologic media
• Are resistant to beta-lactam antibiotics
• Produce a milder, more indolent clinical picture ("walking pneumonia")
• Cause predominantly interstitial (not alveolar) infiltrates on chest X-ray

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CAUSATIVE ORGANISMS

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MYCOPLASMA PNEUMONIAE (Primary Atypical Pneumonia) - THE PROTOTYPE

Microbiology

• Smallest free-living self-replicating organism
• NO CELL WALL - hence beta-lactams, vancomycin, and all cell-wall-active antibiotics are completely ineffective
• Pleomorphic (no fixed shape)
• Very slow growing on artificial media; cannot be detected on Gram stain
• Eaton agent (originally thought to be a virus)

Pathogenesis

• Transmitted by respiratory droplets (requires close, prolonged contact)
• Incubation period: 2-3 weeks (long - explains why household/school outbreaks may go unrecognized)
• Community-wide epidemics every 4-7 years
• Organism attaches to respiratory epithelium via P1 adhesin (tip structure)
• Damages ciliated cells - ciliastasis (paralysis of cilia)
• Produces hydrogen peroxide and superoxide radicals - oxidative injury to epithelium
• Triggers intense lymphocytic and monocytic interstitial inflammation (interstitial pneumonitis)

Clinical Manifestations

• Most common in young adults, school-age children, college students, military recruits
• Insidious onset (gradual, over days to a week)
• Malaise, low-grade fever
• Prominent headache - more prominent than in typical pneumonia
• Sore throat (common), minimal coryza
• Dry, non-productive cough - the hallmark; develops progressively over days
• Myalgias, arthralgias
• Patients feel sick but often continue daily activities - hence "walking pneumonia"

• Bullous myringitis (vesicular hemorrhagic blebs on tympanic membrane) - highly characteristic when present
• Skin rashes (maculopapular, erythema multiforme)
• Ear pain, otitis media

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SUPPURATIVE COMPLICATIONS

1. Small pleural effusion (uncommon)
2. Secondary bacterial superinfection
3. Lung abscess (very rare)

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NON-SUPPURATIVE (EXTRAPULMONARY) COMPLICATIONS

1. Cold agglutinin hemolytic anemia - most classic complication; IgM anti-I antibodies agglutinate RBCs at 4°C; may cause intravascular hemolysis and jaundice; can precipitate Raynaud's phenomenon
2. Stevens-Johnson syndrome / Erythema multiforme - mucocutaneous immune reaction
3. Bullous myringitis - hemorrhagic blisters on tympanic membrane
4. Neurological complications: Meningoencephalitis, cerebellar ataxia, Guillain-Barre syndrome, transverse myelitis, peripheral neuropathy
5. Cardiac: Myocarditis, pericarditis, complete heart block (rare)
6. Gastrointestinal: Hepatitis, nausea/vomiting/diarrhea, pancreatitis (rare)
7. Reactive arthritis
8. Glomerulonephritis (rare, immune-complex mediated)

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LABORATORY DIAGNOSIS OF ATYPICAL PNEUMONIA

1. Chest X-Ray

• Interstitial infiltrates - bilateral, peribronchial, reticular pattern; "shaggy heart" border
• X-ray-clinical dissociation - X-ray looks much worse than the patient appears clinically (classic teaching point)
• Usually unilateral lower lobe but may be bilateral
• Small pleural effusion in some cases

2. Cold Agglutinin Test (Mycoplasma-specific pointer)

• Cold agglutinins = IgM antibodies against the I antigen on human group O RBCs; agglutinate at 4°C, dissolve at 37°C
• Positive in ~50-70% of Mycoplasma pneumonia cases
• Bedside test: Blood in a glass tube, put in ice bucket for 2-3 minutes - visible agglutination = positive; re-warm in hand and agglutinate dissolves = cold agglutinin confirmed
• Significant titer: ≥1:32; highly suggestive: ≥1:64
• Not specific - can be positive in EBV, CMV, adenovirus, influenza

3. Serology

• Complement Fixation (CF) test: Fourfold or greater rise in CF antibody titer between acute serum (day 1) and convalescent serum (2-3 weeks later) is diagnostic
• ELISA: Detection of specific IgM (acute infection; appears at 7-10 days, peaks at 3-4 weeks) and IgG (past infection or convalescence)
• Paired serology is confirmatory but retrospective

4. Culture

• Impractical clinically - organisms grow on SP4 medium (special broth/agar) or Hayflick medium over weeks
• Colonies show "fried-egg" appearance under microscope
• Rarely done in routine diagnosis

5. PCR / Molecular Methods (Preferred)

• Most sensitive and specific
• Detects Mycoplasma DNA from nasopharyngeal swab, sputum, or BAL
• Can simultaneously detect Chlamydophila, Legionella, and respiratory viruses
• Rapid (hours); unaffected by prior antibiotic therapy
• Increasingly the standard of care for atypical pneumonia

6. Legionella-Specific Tests

• Urinary antigen test - detects L. pneumophila serogroup 1 (sensitivity 70-90%, specificity >99%); rapid, reliable; test of choice for Legionella
• Culture: BCYE (buffered charcoal yeast extract) agar - specific for Legionella; takes 3-5 days
• DFA (Direct Fluorescent Antibody) test on sputum/BAL

7. Chlamydophila

• Micro-immunofluorescence (MIF) test - reference standard for serology
• PCR from respiratory specimens

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TREATMENT OF ATYPICAL PNEUMONIA

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COMPREHENSIVE COMPARISON TABLE

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• Jawetz Melnick & Adelberg's Medical Microbiology 28e
• Robbins & Cotran Pathologic Basis of Disease (Robbins, Kumar, Abbas)
• Goldman-Cecil Medicine International Edition
• Harrison's Principles of Internal Medicine 22e (2025)
• Fishman's Pulmonary Diseases and Disorders
• Medical Microbiology 9e (Murray, Rosenthal, Pfaller)