Introduction
Background
Bacterial pneumonia is caused by a pathogenic infection of the lungs and may present as a primary disease process or as the final coup de grace in the individual who is already debilitated. Pneumonia may be further categorized into community-acquired pneumonia (CAP), or hospital-acquired pneumonia (HAP) or institutional-acquired pneumonia (IAP).
William Henry Harrison, the ninth president of the United States, contracted pneumonia during his inauguration in 1841 and died after being in office for only 31 days. Ronald Reagan, 40th US president, died of pneumonia after years of debilitation from Alzheimer disease. Other notable persons to succumb to pneumonia include Sir Francis Bacon (1626), who died after stuffing chickens with snow while conducting freezing experiments, and Thomas Stonewall Jackson (1863), whose arm required amputation after he was shot by one of his own sentries.
Pathophysiology
Bacteria from the upper airways or, less commonly, from hematogenous spread, find their way to the lung parenchyma. Once there, a combination of factors (including virulence of the infecting organism, status of the local defenses, and overall health of the patient) may lead to bacterial pneumonia. The patient may be made more susceptible to infection because of an overall impairment of the immune response (eg, HIV infection, chronic disease, advanced age) and/or dysfunction of defense mechanisms (eg, smoking, chronic obstructive pulmonary disease [COPD], tumors, inhaled toxins, aspiration). Poor dentition or chronic periodontitis is another predisposing factor.
Frequency
United States
More than 3 million cases occur annually in the United States. Pneumonia is more prevalent during the winter months and in colder climates. This condition is most likely from viral upper and lower respiratory infections, which increase in winter and result in impaired host defenses to bacterial superinfection.
Mortality/Morbidity
Left untreated, pneumonia may have an overall mortality rate of more than 30%. The individual's risk of mortality depends on the particular infectious agent and host response. Even with appropriate treatment, the risk of mortality may be high if the host is ill or infirm. The Pneumonia Severity Index (PSI) may be used as a guide to determine a patient's mortality risk, but it tends to overestimate the actual risk in many cases. Particularly virulent organisms, such as Klebsiella and Legionella species, may confer a higher mortality rate.
Morbidity may include destruction of lung tissue from infection, with subsequent scarring. Affected areas may be incapable of gas exchange, reducing respiratory reserve. In a patient with preexisting respiratory disease, onset of bacterial pneumonia may result in a downward spiral of infections, further impairment of respiratory status, and repeated infections owing to reduced local and systemic immune responses. Bronchiectasis may be a sequela of bacterial pneumonia. Infections with Staphylococcus and Klebsiella organisms result in subsequent bronchiectasis, especially if treatment is delayed.
Destroyed alveoli and small-to-medium airways may be replaced by dilated blind saccules filled with purulent material. Ongoing, chronic inflammation usually occurs in the surrounding area and may destroy local adjacent lung tissue over time. Empyema and lung abscess may occur as direct complications of bacterial pneumonia. Pneumonia has been associated with increased incidence of placental abruption in pregnant patients.
Sex
Incidence is greater in males than in females.
Age
In general, advanced age increases the risk of contracting the disease and mortality. In patients who are elderly, comorbidity and a diminished immune response and defense against aspiration increase the risk of bacterial pneumonia. In a 20-year US study, the average overall mortality rate in pneumococcal pneumonia with bacteremia was 20.3%; patients older than 80 years had the highest mortality rate, which was 37.7%.1
Clinical
History
The clinical presentation of bacterial pneumonia varies.
- Although not diagnostic of a particular causative agent, the characteristics of the sputum produced may suggest the presence of one bacterium instead of another.
- Pneumococci may produce bloody or rust-colored sputum.
- Pseudomonas, Haemophilus, and pneumococcal species may produce green sputum.
- Anaerobic infections may produce foul-smelling sputum.
- Klebsiella and type 3 pneumococci may produce sputum resembling currant jelly.
- Rigors or severe shaking chills may be observed in any infectious process. For unclear reasons, the presence of rigors may suggest pneumococcal pneumonia more often than pneumonia caused by other bacterial pathogens.2
- The patient also may have headache, malaise, nausea, vomiting, and diarrhea. Although these symptoms may be observed in any systemic illness, their presence with bacterial pneumonia is suggestive of infection with Legionella species.
- Other findings include the following:
- Malaise
- Myalgias
- Exertional dyspnea (dyspnea at rest with progressive disease)
- Pleuritic chest pain
- Abdominal pain
- Anorexia and weight loss
- Sudden onset of symptoms and rapid illness progression are associated with bacterial pneumonias.
Physical
- Findings at physical examination may include the following:
- Fever3
- Tachypnea
- Tachycardia or bradycardia
- Cyanosis
- Decreased breath sounds
- Wheezes, rhonchi, and rales
- Egophony on auscultation
- Pleural friction rub
- Dullness to percussion
- Altered mental status
Causes
Causes for the development of pneumonia are extrinsic or intrinsic, and various bacterial causes are noted.
- Extrinsic factors include exposure to a causative agent, exposure to pulmonary irritants, or direct pulmonary injury.
- Most authors categorize bacterial pneumonias by their infectious agents, which include pneumococcal agents; Haemophilus influenzae; Klebsiella, Staphylococcus, and Legionella species; gram-negative organisms; and aspirated materials.
- Inhalation of infectious aerosols is probably the most common mode of infection. Some agents, notably Staphylococcus species, may be spread hematogenously.
- Intrinsic factors are related to the host.
- Loss of protective upper airway reflexes allows aspiration of contents from the upper airways into the lung. Various causes for this loss include altered mental status due to intoxication and other metabolic states and neurologic causes, such as stroke and endotracheal intubation.
- Local lung pathologies (eg, tumors, chronic obstructive pulmonary disease [COPD], bronchiectasis) are predisposing factors. Smoking also impairs resistance to infection. Chronic gingivitis and periodontitis also are associated with increased risk of pneumonia.
- Bacterial pneumonia caused by Streptococcus pneumoniae remains the most common cause of all bacterial pneumonias.
- Pneumonia from H influenzae most commonly arises in the winter and early spring.
- This pneumonia is more often associated with hosts who are debilitated.
- Asthma, COPD, smoking, and a compromised immune system are risk factors for H influenzae infection.
- Klebsiella pneumonia results in an aggressive necrotizing lobar pneumonia. Patients with chronic alcoholism, diabetes, or COPD are at increased risk.
- Staphylococcus aureus pneumonia is observed in intravenous drug abusers and other individuals with debilitations. In patients who abuse intravenous drugs, the infection probably is spread hematogenously to the lungs from contaminated injection sites.
- Legionella pneumophila infections tend to occur sporadically and in local epidemic clusters. Legionella pneumonia was named after an outbreak in 1976 that affected more than 180 members of the American Legion who were staying at the same hotel for an annual convention. Twenty-nine legionnaires died, and the organism was not identified until 1977. These infections usually arise in the summer and fall and may be found in the water condensed from air conditioning systems. L pneumophila seems to have the following 2 forms:
- Pontiac fever has a viruslike presentation, with malaise, fever and/or chills, myalgias, and headache. This form of Legionella pneumonia usually subsides without sequelae.
- Frank Legionella pneumonia, the second form of the disease, is very aggressive, with a mortality rate as high as 75% unless treatment begins rapidly. It occurs in individuals who are elderly and debilitated, as well as in smokers and those with COPD, alcoholism, immunocompromise, or trauma.
- Unlike other pneumonias, Legionella pneumonia has GI symptoms associated with its infection more than 50% of the time. GI manifestations may include anorexia, nausea, vomiting, and diarrhea.
- Gram-negative pneumonias are observed in individuals who are debilitated, immunocompromised, or recently hospitalized.
- Causative organisms include Escherichia coli and Pseudomonas, Enterobacter, and Serratia species.
- Individuals living in long-term care facilities where other residents are intubated also are at risk for these infections.
- Any individual with an altered sensorium (eg, seizures, alcohol or drug intoxication) or CNS impairment (eg, stroke) may have a reduced gag reflex, which allows aspiration of stomach or oropharyngeal contents and which enables aspiration pneumonias.
- Causative organisms may include Moraxella catarrhalis and Bacteroides, Peptostreptococcus, and Fusobacterium species.
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Further Reading
Keywords
bacterial pneumonia, lung infection, bacterial lung infection, bronchopneumonia, lung parenchyma, smoking, chronic obstructive pulmonary disease, COPD, bronchiectasis, Staphylococcus aureus, pneumococcal pneumonia, treatment, diagnosis, symptoms
