The word "pneumonia" originates from the ancient Greek word "pneumon" which means "lung," so the word "penumonia" becomes "lung disease." Medically it is an inflammation of one or both lung's parenchyma that is more often but not always caused by infections. The many causes of pneumonia include bacteria, viruses, fungi, and parasites. This article is about bacterial causes of pneumonia as it is the major cause of mortality and morbidity by pneumonia. According to the new classification of pneumonia, there are four categories: community-acquired (CAP), hospital-acquired (HAP), healthcare-associated (HCAP) and ventilator-associated pneumonia (VAP). 
Types of Bacterial Pneumonia
Some articles include both HAP and VAP under the category of HCAP, so defining HCAP is problematic and controversial.
Etiology of community-acquired pneumonia is an extensive list of agents that include bacteria, viruses, fungi, and parasites, but this article is about bacterial pneumonia and its causes. Bacteria have classically been categorized into two divisions on the basis of etiology, "typical" and "atypical" organisms. Typical organisms can be cultured on standard media or seen on Gram stain, but "atypical" organisms do not have such properties. 
The most common causes of community-acquired pneumonia (CAP) is S. pneumoniae followed by Klebsiella pneumoniae, Haemophilus influenzae, and Pseudomonas aeruginosa. The most common causes of HCAP and HAP are MRSA (methicillin-resistant Staphylococcus aureus) and Pseudomonas aeruginosa respectively. The causative agents of VAP include both multi-drug resistant (MDR) agents (e.g., S. pneumoniae, other Strep spp, H. influenzae and MSSA) and non-MDR (e.g., P. aeruginosa, methicillin-resistant Staphylococcus aureus, Acinetobacter spp. and antibiotic-resistant Enterobacteriaceae) bacterial pathogens.
The incidence of CAP in the United States is more than 5 million per year; 80% of these new cases are treated as outpatients with the mortality rate of less than 1%, and 20% are treated as inpatients with the mortality rate of 12% to 40%. The incidence of CAP varies among different genders; for example, it is more common in males and African Americans than females and other Americans. The incidence rates are higher at extremes of age distribution range; the adult rate is usually 5.15 to 7.06 cases per 1000 persons per year, but in the population of age less than 4 years and greater than 60 years, the rate is more than 12 cases per 1000 persons. In 2005, influenza and pneumonia combined was the eighth most common cause of death in the United States and the seventh Most common cause of death in Canada. The mortality rate also is variable among different regions at 7.3% for the United States and Canada, 9.1% for Europe, and 13.3% for Latin America.
The lower respiratory tract is not sterile, it always is exposed to environmental pathogens. Invasion and propagation of the above-mentioned bacteria into lung parenchyma at alveolar level causes bacterial pneumonia, and the body's inflammatory response against it causes the clinical syndrome of pneumonia. To prevent this proliferation of microorganisms there are a number of host defenses working together in lungs such as mechanical (e.g., hair in nostrils and mucus on nasopharynx and oropharynx) and chemical (e.g., proteins produced by alveolar epithelial cells like surfactant protein A and D, which have the intrinsic property of opsonizing bacteria). Another component of the pulmonary defense system is made up of immune cells such as alveolar macrophages, which work to engulf and kill proliferating bacteria, but once bacteria overcome the capacity of host defenses, they start proliferation. In this setting, the alveolar macrophages kickoff the inflammatory response to strengthen the lower respiratory tract defenses. This inflammatory response is the main culprit of clinical manifestation of bacterial pneumonia. Cytokines are released in response to the inflammatory reaction and cause the constitutional symptoms, for example, IL-1 (interleukin-1) and TNF (tumor necrosis factor) causes fever. Chemokine-like IL-8 (interleukin-8) and colony-stimulating factors like G-CSF (granulocyte colony-stimulating factor) promote chemotaxis and neutrophils maturation respectively, resulting in leukocytosis on serological lab and purulent secretions. These cytokines are responsible for the leakage of the alveolar-capillary membrane at the site of inflammation, causing a decrease in compliance and shortness of breath. Sometimes even erythrocytes cross this barrier and result in hemoptysis.
Pathologically, lobar pneumonia is the acute exudative inflammation of a lung lobe. It has the following four advanced stages if left untreated:
The history findings of bacterial pneumonia may vary from indolent to fulminant. Clinical manifestation includes both constitutional findings and findings due to damage to the lung and related tissue. The following are major history findings:
Physical findings also vary from patient to patient and mainly depend on the severity of lung consolidation and existence or nonexistence of pleural effusion. The following are major clinical findings:
Confusion manifests earlier in older patients. A critically ill patient may present with sepsis or multi-organ failure.
The approach to evaluate and diagnose pneumonia depends on different modalities but primarily it is like a tripod stand which has 3 legs which are summed up as:
In all patients with bacterial pneumonia, empirical therapy should be started as soon as possible. The first step in treatment is a risk assessment to know whether the patient should be treated in an outpatient or inpatient setting. Cardiopulmonary conditions, age, and severity of symptoms affect risk for bacterial pneumonia, especially CAP.
An expanded CURB-65 or CURB-65 pneumonia severity score can be used for risk quantification. It includes C = Confusion, U = Uremia (BUN greater than 20 mg/dL), R = Respiratory rate (greater than 30 per min), B = B.P (BP less than 90/60 mmHg) and age greater than 65 years. One point is scored for each previously mentioned risk factor. If the total of the score is 2 or more than 2, it indicates hospital admission. If the total is 4 or more than 4, it indicates ICU admission. Recommended therapy for different settings are as follows:
After getting a culture-positive lab result, therapies should be directed to the culture-specific pathogen.
The patient also can benefit from smoking cessation counseling and influenza and pneumococcal vaccination.
All patients treated at home should be scheduled for a follow-up visit within 2 days to assess any complication of pneumonia.
Differential Diagnosis in Children
Differential Diagnosis in Adults
Prognosis of pneumonia depends on many factors including age, comorbidities, and hospital setting (inpatient or outpatient). Patients older than 60 years or younger than 4 years of age have a relatively poorer prognosis than young adults. Antibiotic resistance, very concerning due to the enhancement of antibiotic regimens, and infectious diseases, especially those like bacterial pneumonia, can be easily cured.
The most common complications of bacterial pneumonia are respiratory failure, sepsis, multiorgan failure, coagulopathy, and exacerbation of preexisting comorbidities. Three distinct complications are metastatic infections, lung abscess, and complicated pleural effusion.
The management of a pneumonia is multidisciplinary. Besides the administration of antibiotics, these patients often require chest physical therapy, a dietary consult, physical therapy to help regain muscle mass and a dental consult. The key is to educate the patient on discontinuation of smoking and abstaining from alcohol. Further, patients should be encouraged to get the appropriate influenza and pneumococcal vaccines. Finally, it is important to educate the patient on compliance with antibiotics if they want a complete resolution of the infectious process. (Level V)
In healthy people, the outcome after a bacterial pneumonia is excellent. However, in people with advanced age, lung disease, immunosuppression, infection with aggressive gram-negative organisms (Klebsiella) and other comorbidities, the outcomes are usually poor. When a pneumonia is left untreated, it carries a mortality in excess of 25%. Pneumonia can also lead to extensive lung damage and lead to residual impairment in lung function. Other reported complications of pneumonia that occur in 1-5% of patients include lung abscess, empyema, and bronchiectasis. (Level V)
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