Bacterial Pneumonia

Introduction

Streptococcus pneumoniae is also known as pneumococcus, a term coined soon after its discovery because of its association with pulmonary infections, and Diplococcus pneumoniae, because it has paired cocci in tissue and liquid culture. It is a leading cause of meningitis, otitis media, bacteremia, and pneumonia in the United States.

Clinical Features

Pneumonia due to S. pneumoniae is the most common cause of community-acquired pneumonia. It usually develops in infants or the elderly, in the setting of overcrowded environments, or in individuals with an increased risk of aspiration such as alcoholics. In addition to lung infections, S. pneumoniae is also the most common cause of otitis media and bacterial sinusitis with abscess formation. Suppressed immune responses including damage to the mucociliary tree (smokers), defective immunoglobulin response, neutropenia, or defects in the complement cascade are also risk factors for this type of pneumonia.

S. pneumoniae can be found in the respiratory tract of normal hosts. However, when a pathogen, the symptoms are characterized by sudden onset of chills, pleuritic chest pain, high fever, and cough productive of “rust-colored” sputum. Tachycardia, tachypnea, and dyspnea are frequently seen. Laboratory values are remarkable for a leukocytosis with a shift toward immature forms and, in severe cases, leukopenia.

Radiologic Features

Chest imaging usually shows patchy infiltrates that involve more than one lung segment and may progress to complete lobar consolidation, cavity formation, and pleural effusions.

However, from one third to one half of pneumonias due to this organism may present with atypical features—that is, not confined to a single lobe—and pneumococcal pneumonia is often bilateral.

Pathologic Features

Gross Findings

Pneumococcal pneumonia is the most common cause of lobar pneumonia that progresses from a red hepatization phase to a gray hepatization phase. The affected lobe may appear purple, and frothy fluid may exude from the surface and bronchi in the most exaggerated cases ( Figure 12-1 ). A bronchopneumonia (lobular) pattern is more commonly seen in the very young or the very old ( Figure 12-2 ). Fibrinous pleuritis and pleural effusions are present in about two thirds of the cases, and it is these pathologic features that are responsible for the pleuritic pain that is usually experienced by these patients. The consolidation is more common in the posterior and lower segments of the lobe and more commonly seen in the lower lobes. Necrosis and cavitation are rare, and empyemas occur in only about 2% of patients. Bacteremia occurs in approximately 25% to 35% of cases and is more commonly seen in the lobar pneumonia pattern of injury.

Microscopic Findings

In the early stages of the disease, alveoli show congestion with increased fluid. The alveolar space contains fibrin, neutrophils, mononuclear cells, and erythrocytes, and there are distended alveolar capillaries and hyaline membranes within alveolar ducts ( Figure 12-3 ). Gram-positive organisms are abundant within neutrophils and macrophages. The fibrinous exudate present in the alveoli may also be found within the large airways. As the disease progresses, the intra-alveolar exudates will begin to organize and the pleuritis may undergo fibrosis, resulting in a firm lobe of lung encased within a fibrous capsule.

Ancillary Studies

Differential Diagnosis

The differential diagnosis includes other predominantly non-necrotizing bacterial pneumonias due to Haemophilum influenzae and Klebsiella pneumoniae and some viral pneumonias, especially influenza.

Prognosis and Therapy

Penicillin is considered the drug of choice for S. pneumoniae pneumonia. However, because this infection has the potential to advance rapidly, usually initial treatment is started before microbiologic confirmation and consists of additional antibiotics including cephalosporins, doxycycline, macrolides, clindamycin, and fluoroquinolones. Antibiotic resistance, an increasing problem, is found in approximately 30% of isolates. With prompt treatment, most individuals begin to recover with 1 to 2 days. Radiographic resolution is usually complete by 2 to 3 weeks. A mortality rate of 15% to 20% can be seen in the setting of bacteremia.

Pneumococcal vaccine can prevent bacteremia in individuals at high risk for developing complications from this pneumonia. These include the young and the old and individuals with chronic obstructive pulmonary disease, diabetes, cardiovascular disease, alcoholism, asplenia, renal insufficiency, and immunosuppression of various types.

HAEMOPHILUM INFLUENZAE

Introduction

Haemophilum influenzae is colonizer of the upper airways in up to 50% of the population and may colonize the lower respiratory tract in patients with either chronic bronchitis or bronchiectasis. It causes pneumonia either through hematogenous spread or by aspiration into chronically inflamed lungs. Six serotypes (A-F) are defined based on their capsular antigens. Both children and adults are susceptible to these pulmonary infections, usually by type B, an encapsulated strain. These organisms are also responsible for a spectrum of other infections including meningitis, epiglottitis, otitis, and arthritis.

Clinical Features

H. influenzae pneumonia is found predominantly in children. In adults, it is found in the setting of underlying chronic lung disease such as chronic bronchitis, bronchiectasis, or cystic fibrosis, with HIV infection, or in alcoholics. Epiglottitis is a common manifestation of this infection in children. The pneumonia is usually preceded by a viral or mycoplasma infection that disrupts the airway mucosa, predisposing it to infections by these colonizing organisms. Symptoms for H. influenzae pneumonia are fever, a cough productive of purulent sputum, and myalgias. Most commonly found as a community-acquired pneumonia, its role as a HAP is increasing, especially in younger patients.

Radiologic Features

H. influenzae pneumonia is usually localized with a lobar or segmental consolidation. In patients with hematogenous spread, a diffuse miliary pattern may be present. Cavitation, pleural effusion, or empyemas can be seen but are not common.

Pathologic Features

Gross Findings

The pneumonia starts out as a lobular pneumonia that undergoes consolidation ( Figure 12-4 ). This progresses to a necrotizing pneumonia with a lobar pattern. Cavitation or pneumatoceles, when present, usually arise out of the lobar pattern and are more common in children.

Microscopic Findings

The infiltrate is rich in neutrophils with fibroblastic proliferation and organization in the peripheral areas ( Figure 12-5 ). A pleural effusion may be present with a fibrinous pleuritis.

Haemophilum Influenzae—Pathologic Features

Gross Findings

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  Lobar or segmental consolidation.

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  Fibrinous pleuritis.

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  Cavitation and empyemas are unusual but may develop in children.

Microscopic Findings

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  Bronchocentric, neutrophilic-rich alveolitis.

Microbiology

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  Gram-negative coccobacillus.

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  Most pneumonias caused by nonencapsulated, nontypable strain.

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  Difficult to culture; organisms are seen in gram-stained sputum.

Pathologic Differential Diagnosis

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  Streptococcal pneumonia.

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  Staphylococcal pneumonia.

Ancillary Studies

Differential Diagnosis

Streptococcal pneumonia or staphylococcal pneumonia and pneumonias caused by K. pneumoniae may mimic this predominantly non-necrotizing pneumonia.

Prognosis and Therapy

Patients usually survive this pneumonia when treated promptly with antimicrobials. Due to increasing bacterial resistance, the treatment of choice is third-generation cephalosporin antibiotics. However, amoxicillin/clavulanate (Augmentin) with trimethoprim/sulfamethoxazole (Bactrim) is effective in beta-lactamase positive strains. Mortality may be higher with bacteremia. A vaccine to H. influenzae type b (HIB vaccine) has been shown to be effective in preventing infections in children.

Introduction

Pneumonia due to Staphylococcus species accounts for approximately 10% of all of pneumonias. S. aureus, the most common organism isolated, is a gram-positive coccus that usually grows in clusters. S. epidermidis and S. saprophyticus may cause pneumonia but are not common pathogens in the lungs. Staphylococcus sp. give off multiple toxins that may cause both localized tissue injury in the form of abscesses and systemic tissue injury as is seen in toxic shock syndrome.

Clinical Features

Pneumonia due to Staphylococcus species usually develops from either aspiration of organisms from upper airway infections or oral secretions or from hematogenous seeding of the lung during bacteremia. It is twice as common as a hospital-acquired infection than as a community-acquired one. Staphylococci colonize the anterior nasal cavity or the skin in 30% to 50% of normal adults and in higher rates in diabetic patients, patients on hemodialysis, or intravenous drug abusers. Staphylococcal pneumonia is commonly a complication in patients with influenza, cystic fibrosis, in the immunocompromised, and in the very young and the very old. The clinical course in these patients is usually slow to progress but can quickly develop into high fevers, chills, cough productive of purulent bloody sputum, dyspnea, and, occasionally, pleuritic chest pain.

Radiologic Features

Staphylococcal pneumonia usually manifests as patchy, bilateral, lower-lobe infiltrates that may progress to complete consolidation. Early infection is based around the bronchioles as an early necrotizing bronchopneumonia and a “tree and bud” appearance. Large abscess cavities may occur in over half of patients who have pleural effusions, and the majority of these will progress to empyema ( Figure 12-6 ).

Pathologic Features

Gross Findings

During the acute infection, the lungs are congested with a purple-red color and a minimal pleural reaction. Pleural effusions are common and usually of a serosanguineous quality. Airways and cut surfaces may contain bloody fluid and a thick, exudative bronchitis with pseudomembrane formation. Progression of the disease usually results in a necrotizing, peribronchiolar pattern that develops into multiple, thin-walled cavities (pneumatoceles) from 1 to 5 cm in diameter with thick yellow-green pus ( Figure 12-7 ). These are especially common in children.