Bacterial Pneumonia

Pneumonia can be generally defined as inflammation of the lung parenchyma, in which consolidation of the affected part and a filling of the alveolar air spaces with exudate, inflammatory cells, and fibrin is characteristic.^[1] Infection by bacteria or viruses is the most common cause, although inhalation of chemicals, trauma to the chest wall, or infection by other infectious agents such as rickettsiae, fungi, and yeasts may occur.^[1] (See images below.)
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. For example, historical review of the 1918-19 influenza pandemic suggests that the majority of deaths were not a direct effect of the influenza virus, but they were from bacterial coinfection.^[2]
Discussion of bacterial pneumonia involves classification and categorization schemes based on various characteristics of the illness, such as anatomic or radiologic distribution, the setting, or mechanism of acquisition, and the pathogen responsible. A major part of what distinguishes these various categories from each other is the varying risk of exposure to multidrug-resistant (MDR) organisms.^{[3, 4, 5]}
Anatomic or radiologic distribution of pneumonia includes the following (see Chest Radiography for details):

• Lobar - Known as focal or nonsegmental pneumonia (see the images below)
• Multifocal/lobular (bronchopneumonia)
• Interstitial (focal diffuse)Bacterial pneumonia. Radiographic images in a patient with right upper lobe pneumonia. Note the increased anteroposterior chest diameter, which is suggestive of chronic obstructive pulmonary disease (COPD). Bacterial pneumonia. Radiographic images in a patient with bilateral lower lobe pneumonia. Note the spine sign, or loss of progression of radiolucency of the vertebral bodies Bacterial pneumonia. Radiographic images in a patient with early right middle lobe pneumonia.

The setting of pneumonia includes the community, Institutional (healthcare/nursing home setting), and nosocomial (hospital).

Community-acquired pneumonia

Community-acquired pneumonia (CAP) is defined as pneumonia that develops in the outpatient setting or within 48 hours of admission to a hospital. CAP should not meet the criteria for healthcare-associated pneumonia (HCAP), as defined below.
Go to Community-Acquired Pneumonia for complete information on this topic.

Institutional-acquired pneumonia

Institutional-acquired pneumonia (IAP) includes HCAP and nursing home–associated pneumonia (NHAP).
HCAP is defined as pneumonia that develops in the outpatient setting or within 48 hours of admission to a hospital in patients with increased risk of exposure to MDR bacteria as a cause of infection. Risk factors for exposure to MDR bacteria in HCAP include the following:

• Hospitalization for 2 or more days in an acute care facility within 90 days of current illness
• Exposure to antibiotics, chemotherapy, or wound care within 30 days of current illness
• Residence in a nursing home or long-term care facility
• Hemodialysis at a hospital or clinic
• Home nursing care (infusion therapy, wound care)
• Contact with a family member or other close person with infection due to MDR bacteria

NHAP is generally included in the category of HCAP because of the high incidence of infection with gram-negative bacilli and Staphylococcus aureus. However, some authors accept NHAP as a separate entity because of distinct epidemiologic associations with infection in nonhospital healthcare settings.^[1] Pneumonia in patients in nursing homes and long-term care facilities has been associated with greater mortality than in patients with CAP. These differences may be due to factors such as disparities in functional status, likelihood of exposure to infectious agents, and variations in pathogen virulence, among others.
It is important to note that nursing home patients with pneumonia are less likely to present with classic signs and symptoms of the typical pneumonia presentation, such as fever, chills, chest pain, and productive cough, but instead these individuals often have delirium and altered mental status. Thus, the degree to which the HCAP definition applies to such settings is not yet clear.^{[3, 4]}
Go to Nursing Home Acquired Pneumonia for complete information on this topic.

Nosocomial pneumonia

Nosocomial infections are generally described as those acquired in the hospital setting. The term nosocomial pneumonia has evolved into the more succinct clinical entities of hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP); however, the term nosocomial pneumonia still has an appropriate place in the descriptive language of pneumonia. Nosocomial infections have been viewed as a "tribute to pay to the more aggressive management of the population, characterized by the use of sophisticated technologies and invasive devices," an important consideration in the pulmonary care of critically ill patients.^[6]
Go to Ventilator-Associated Pneumonia and Nosocomial Pneumonia for complete information on these topics.

Hospital-acquired pneumonia

HAP is defined as pneumonia that develops at least 48 hours after admission to a hospital and, as in HCAP, is characterized by increased risk of exposure to MDR organisms,^[3] as well as gram-negative organisms.^[7] Risk factors for exposure to such organisms in HAP include the following^[3] :

• Antibiotic therapy within 90 days of the hospital-acquired infection
• Current length of hospitalization of 5 days or more
• High frequency of antibiotic resistance in the local community or within the specific hospital unit
• Immunosuppressive disease or therapy
• Presence of HCAP risk factors for exposure to MDR bacteria

Common mechanisms for the acquisition of pneumonia include ventilator use and aspiration.

Ventilator-associated pneumonia

VAP is defined as pneumonia that develops more than 48 hours after endotracheal intubation or within 48 hours of extubation. Risk factors for exposure to MDR bacteria that cause VAP are the same as those for HCAP and/or HAP.^{[3, 5]} VAP may occur in as many as 10-20% of patients who are on ventilators for more than 48 hours.^[8]
Go to Ventilator-Associated Pneumonia for complete information on this topic.

Aspiration pneumonia

Aspiration pneumonia develops after the inhalation of oropharyngeal secretions and colonized organisms. Although organisms frequently implicated in CAP, such as Haemophilus influenzae and Streptococcus pneumoniae, can colonize the nasopharynx and oropharynx and their aspiration can contribute to the development of CAP, the term aspiration pneumonia refers specifically to the development of an infectious infiltrate in patients who are at increased risk of oropharyngeal aspiration.
Patients may be at increased risk of aspiration and/or the development of aspiration pneumonia for a number of reasons, as follows:

• Decreased ability to clear oropharyngeal secretions - Poor cough or gag reflex, impaired swallowing mechanism (eg, dysphagia in stroke patients), impaired ciliary transport (eg, from smoking)
• Increased volume of secretions
• Increased bacterial burden of secretions
• Presence of other comorbidities - Anatomic abnormalities, gastroesophageal reflux disease (GERD), achalasia.

Critically ill patients are at notably increased risk of aspiration due to the following:

• The challenge of appropriate, risk-minimizing positioning
• Gastroparesis/dysmotility
• Impaired cough/gag/swallow reflexes (illness- or drug-induced)
• Impaired immune response
• Intubation/extubation

Historically, the bacteria implicated in aspiration pneumonia have been the anaerobic oropharyngeal colonizers such as Peptostreptococcus, Bacteroides, Fusobacterium, and Prevotella species. However, now evident is that the vast majority of cases of aspiration pneumonia result from the same pathogens implicated in CAP and HAP, depending on the setting in which the aspiration event occurred. The clinical course of aspiration pneumonia is, thus, similar to that of CAP or HAP.^[9] However, recurrence of aspiration pneumonia is common unless the risk factors for underlying aspiration are treated or minimized.
Go to Aspiration Pneumonia for complete information on this topic.

Although pneumonia may be caused by myriad pathogens, a limited number of agents are responsible for most cases,^{[15, 16, 17, 18]} 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.

Risk factors

Coinfection with H1N1 influenza increases the risk of secondary bacterial pneumonia, with S pneumoniae the most likely coinfection.^[19] However, pregnant patients with H1N1 influenza in the 2009 pandemic were at increased risk of developing secondary Klebsiella pneumonia with poor clinical outcome.^[20]
Other risk factors include local lung pathologies (eg, tumors, chronic obstructive pulmonary disease [COPD], bronchiectasis), chronic gingivitis and periodontitis, and smoking, which impair resistance to infection. Furthermore, any individual with an altered sensorium (eg, seizures, alcohol or drug intoxication) or central nevous system (CNS) impairment (eg, stroke) may have a reduced gag reflex, which allows aspiration of stomach or oropharyngeal contents and which enables aspiration pneumonias.

Typical organisms

Although several of the organisms discussed in this section may be implicated in pneumonia, only a few of them are responsible for the vast majority of cases.
Gram-positive bacteria that can cause pneumonia include the following:

• Streptococcus pneumoniae: This organism is a facultative anaerobe identified by its chainlike staining pattern. Pneumococcosis is by far the most common cause of typical bacterial pneumonia.
• Staphylococcus aureus: S aureus is a facultative anaerobe identified by its clusterlike staining pattern. S aureus pneumonia is observed in intravenous drug abusers (IVDAs) and other individuals with debilitations. In patients who abuse intravenous drugs, the infection probably is spread hematogenously to the lungs from contaminated injection sites. Methicillin-resistant S aureus (MRSA) has had a large impact on empiric antibiotic choices at many institutions.
• Enterococcus (E faecalis, E faecium): These organisms are group D streptococci that are well-known normal gut florae that can be identified by their pair-and-chain staining pattern. The emergence of vancomycin-resistant Enterococcus (VRE) is indicative of the importance of appropriate antibiotic use.
• Actinomyces israelii: This is a beaded, filamentous anaerobic organism that grows as normal flora in the gastrointestinal (GI) tract. A israelii is known to form abscesses and sulfur granules.
• Nocardia asteroides: N asteroides is a weakly gram-positive, partially acid-fast bacillus (AFB) that forms beaded, branching, thin filaments. It is known to cause lung abscesses and cavitations. Erosion into the pleura can also occur, resulting in hematologic spread of the organism.

Gram-negative pneumonias occur most often in individuals who are debilitated, immunocompromised, or recently hospitalized. Individuals living in long-term care facilities where other residents are intubated are also at risk for these infections. Gram-negative bacteria include the following:

• Pseudomonas aeruginosa: P aeruginosa is an aerobic, motile bacillus often characterized by its distinct (grapelike) odor.
• Klebsiella pneumoniae: K pneumoniae is a facultatively anaerobic, encapsulated bacillus that can lead to an aggressive, necrotizing, lobar pneumonia. Patients with chronic alcoholism, diabetes, or COPD are at increased risk.
• Haemophilus influenzae: H influenzae is an aerobic bacillus that comes in both encapsulated and nonencapsulated forms. Several major subtypes have been identified, which have varying levels of pathogenicity; encapsulated type B (HiB) is known to be particularly virulent, although routine vaccination against this subtype has decreased the prevalence of severe disease caused by H influenzae.
• Escherichia coli: E coli is a facultatively anaerobic, motile bacillus; it is well known to colonize the lower GI tract and produce the essential vitamin K.
• Moraxella catarrhalis: M catarrhalis is an aerobic diplococcus known as a common colonizer of the respiratory tract.
• Acinetobacter baumannii: A baumannii is a pathogen that has been well described in the context of ventilator-associated pneumonia (VAP).
• Francisella tularensis: F tularensis is the causative agent of tularemia, or rabbit fever. F tularensis is a facultative intracellular bacterium that multiplies within macrophages and that is typically transmitted to humans via a tick bite; its reservoir animals include rodents, rabbits, and hares. F tularensis can also be transmitted in an airborne manner or contracted from handling dead, infected animals. It is commonly spoken of in terms of its potential use as a biologic weapon.^[21]
• Bacillus anthracis: B anthracis is the agent responsible for inhalational anthrax.
• Yersinia pestis: Y pestis infection is better known as the black plague, but other members of the Yersinia family are responsible for a wide variety of infectious presentations.

Atypical organisms

Atypical organisms are generally associated with a milder form of pneumonia, the so-called "walking pneumonia." A feature that makes these organisms atypical is the inability to detect them on Gram stain or to cultivate them in standard bacteriologic media.^{[15, 16]} Atypical organisms include the following:

• Mycoplasma species: The mycoplasmas are the smallest known free-living organisms in existence; they lack cell walls (and therefore are not apparent after Gram stain) but have protective 3-layered cell membranes.
• Chlamydophila species (C psittaci, C pneumoniae): Psittacosis, also known as parrot disease or parrot fever, is caused by C psittaci and is associated with the handling of various types of birds.
• Legionella species: Legionella species are gram-negative bacteria found in freshwater and are known to grow in complex water distribution systems. Institutional water contamination is frequently noted in endemic outbreaks. Legionella species are the causative agents of Legionnaires disease.
• Coxiella burnetii:C burnetii is the causative agent of Q fever. It is spread from animals to humans; person-to-person transmission is unusual. Animal reservoirs typically include cats, sheep, and cattle.
• Bordetella pertussis:B pertussis is the agent responsible for pertussis or whooping cough.

Anaerobic organisms

Pneumonia due to anaerobes typically results from aspiration of oropharyngeal contents, as previously mentioned. These infections tend to be polymicrobial and may consist of the following anaerobic species, some of which have already been discussed above: Klebsiella, Peptostreptococcus, Bacteroides, Fusobacterium, and Prevotella.