Gram-positive flora and specific infections
Staphylococcal pneumonia caused by Staphylococcus aureus typically develops via aspiration of upper respiratory tract secretions. Risk factors include COPD, carcinoma, renal failure, and diabetes mellitus, as well as recent viral infections. Of particular concern is severe community-acquired pneumonia caused by methicillin-resistant strains (MRSA) carrying Panton-Valentine leukocidin genes. This form affects even young immunocompetent individuals.
On computed tomography, S. aureus manifests as bronchopneumonia, which is bilateral in 40% of patients. Typical features include necrosis (predominantly in the upper lobes), centrilobular nodules, and branching opacities (tree-in-bud sign / tree-in-bud). Cavitation, pneumatocele formation, and spontaneous pneumothorax are frequently observed, while abscesses form in 15–30% of cases.
Similar changes can be seen in pleuropulmonary actinomycosis, which affects debilitated patients (e.g., in the setting of alcoholism). In this case, lower lobe consolidation with areas of low attenuation and air bubbles, as well as pleural thickening, is identified on CT. Nocardiosis pneumonia (Nocardia) forms a specific crazy-paving pattern (crazy-paving) on CT, consisting of extensive bilateral ground-glass opacity areas with superimposed thickened septal and intralobular lines.
Gram-negative pneumonias
Infections caused by Gram-negative bacteria generally follow an extremely severe course, forming a multilobar bronchopneumonia pattern with lower lobe predominance.
- Pseudomonas aeruginosa (Pseudomonas aeruginosa). This is the leading cause of nosocomial infections. It induces extensive confluent bronchopneumonia with a tendency toward cavitation. CT visualizes multifocal consolidation zones (often in the upper lobes), randomly distributed large nodules, ground-glass opacities, and the tree-in-bud sign. Contrast administration may reveal vascular structures within the necrotic zones.
- Escherichia coli (Escherichia coli). Causes 5–20% of hospital-acquired pneumonias in debilitated patients with chronic pathologies. The disease presents acutely with fever, chills, dyspnea, and productive cough. CT typically demonstrates multiple bilateral peripheral consolidation areas.
- Haemophilus influenzae (Haemophilus influenzae). This pleomorphic coccobacillus frequently infects individuals with COPD, malignancies, and HIV infection. Radiography reveals segmental or lobar consolidation with indistinct margins, often accompanied by pleural effusion.
Atypical, zoonotic, and rickettsial pneumonias
The most common pathogen of community-acquired pneumonia is Mycoplasma pneumoniae. The infection is prevalent among military recruits and young adults, and runs a severe course in patients with COPD. The most specific radiographic pattern of mycoplasmosis is focal reticulonodular opacification within a single lobe. CT reveals ground-glass opacity areas, centrilobular nodules, and thickening of bronchovascular bundles.
Zoonotic infections such as tularemia (Francisella tularensis) are transmitted via inhalation from farm animals and are endemic to rural areas of North America, Europe, and Asia. In the primary pulmonary form, radiography shows scattered multifocal consolidation areas, pleural effusion, and bilateral hilar lymphadenopathy.
Rickettsial infections (e.g., Rocky Mountain spotted fever) transmitted via tick bites can also involve the lungs. Radiographically, they manifest as diffuse heterogeneous or homogeneous opacities, most commonly as a secondary consequence of severe vasculitis or cardiogenic edema.
Association with viral lesions
It is important to remember that any lower respiratory tract viral infection (whether influenza A, respiratory syncytial virus, or adenovirus) severely compromises local immunity. Not only can they trigger a non-specific reaction resembling organizing pneumonia (Organising pneumonia), but they are also a major risk factor for secondary bacterial superinfections. Therefore, if signs of destruction and cavitation appear in a patient with a viral illness, a bacterial component must be immediately ruled out.
Briefly: answers to topical questions
Which bacterial pathogens most commonly cause typical lobar (croupous) pneumonia?
Typical lobar (croupous) pneumonia is most frequently caused by pneumococcus and several other bacteria.
- Pneumococcus (Streptococcus pneumoniae) is the classic and primary pathogen; it is the most common cause of complete lobar consolidation.
- Klebsiella (Klebsiella pneumoniae) is Friedländer's bacillus, capable of causing a croup-like (lobar) pattern.
- Staphylococcus (Staphylococcus aureus) and other pyogenic bacteria can also cause lobar involvement.
- Other causative agents of lobar pneumonia include Legionella pneumophila and Haemophilus influenzae; rarely, Mycoplasma pneumoniae.
Which microorganisms can cause the tree-in-bud sign on chest CT?
The tree-in-bud sign on chest CT can be caused by various bacterial and mycobacterial pathogens.
- Staphylococcus aureus (Staphylococcus aureus) causes bronchopneumonia with centrilobular nodules and branching opacities.
- Pseudomonas aeruginosa (Pseudomonas aeruginosa) causes multifocal involvement with characteristic opacities.
- Mycobacterium tuberculosis (Mycobacterium tuberculosis) — this sign occurs with endobronchial spread of post-primary tuberculosis.
- Nontuberculous mycobacteria (Mycobacterium avium-intracellulare complex) form multiple centrilobular opacities 1–3 mm in diameter.
This pattern also reflects infectious bronchiolitis in bronchiectasis and ciliary dyskinesia syndrome.
What specific radiographic features characterize pneumonia caused by Klebsiella pneumoniae?
Pneumonia caused by Klebsiella pneumoniae (Friedländer's bacillus) presents with a lobar pattern and a destructive clinical course.
- Lobar consolidation — a croup-like (lobar) radiographic pattern is formed.
- Abscess formation — frequent abscess formation is characteristic due to alveolar septal necrosis.
- Interstitial fibrosis — areas of carnification and pronounced fibrosis may be identified during the healing phase.
While no specific radiographic pattern exists exclusively for this microorganism, the combination of lobar consolidation and purulent-destructive complications is typical of Klebsiella infection.
What CT features are characteristic of Pneumocystis pneumonia in immunocompromised patients?
Pneumocystis pneumonia (Pneumocystis jirovecii) is characterized on CT by bilateral interstitial lung disease.
- Ground-glass opacity — perihilar or extensive bilateral opacities are typical, sometimes combined with focal consolidation.
- Distribution — changes are patchy or geographic in nature.
- Subpleural sparing (subpleural sparing) — may be observed in the acute phase of the disease.
- Cystic changes — air cysts may form in the subacute phase during ongoing treatment.
What else to study on this topic
- Pathogenesis and CT patterns of viral pneumonias (Influenza A, RSV, Adenovirus)
- Bronchiolitis obliterans as a post-infectious complication
- Radiological signs of Swyer-James-MacLeod syndrome
- Features of Epstein-Barr virus (EBV) pulmonary involvement
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