Which gram-negative bacteria can cause bacterial pneumonia?

Updated: Sep 30, 2020
  • Author: Justina Gamache, MD; Chief Editor: Guy W Soo Hoo, MD, MPH  more...
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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 for infection with this organism.

  • 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. Infection with this bacteria is more common in patients with COPD.

  • 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. [28]

  • Bacillus anthracis: B anthracis is the agent responsible for inhalational anthrax.

  • Yersinia pestis: Y pestis infection is better known as the black plague. It is the most commonly accepted cause of the pandemic known as the bubonic plague. This organism can also cause the pneumonic plague. The pneumonic plague causes a lung infection by the direct inhalation of aerosolised plague bacteria or, secondary, when the organism spreads to the lungs from the bloodstream. Pneumonic plague is, therefore, not exclusively vector-borne like bubonic plague. Instead, it can be spread from person to person. Other members of the Yersinia family are responsible for a wide variety of infectious presentations.

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