Klebsiella pneumoniae

Klebsiella pneumoniae is a Gram-negative, non-motile, facultative anaerobe belonging to the Enterobacteriaceae family of the γ-Proteobacteria class in the phylum Proteobacteria. K. pneumoniae consists of straight rods 1 to 2 μm (micrometres) in length with a thick, surrounding capsule (Figure 1). When cultured, this species produces a distinctive yeasty odor and bacterial colonies have a viscous/mucoid appearance (Figure 2). K. pneumoniae is commonly found in the human digestive tract as part of the natural microflora, and is often the cause of hospital acquired, or nosocomial infections involving the urinary and pulmonary systems, especially since it is able to adapt to an existence in an oxygenated or deoxygenated environment. Immunocomprimised individuals (people with AIDS or cancer) infected with K. pneumoniae usually develop respiratory tract infections such as pneumonia, but blood infections (septecemia), wound or surgical site infections, and meningitis are also possible.

Figure 1. This scanning electron micrograph reveals some of the ultrastructural morphologic features of a Klebsiella pneumoniae bacterium. 


Figure 2. After 24 hours, this inoculated MacConkey agar culture plate cultivated colonial growth of Gram-negative, small rod-shaped and facultatively anaerobic Klebsiella pneumoniae bacteria.

This pathogen possesses many virulence factors that allow it to go undetected by the host's immune system and cause infection in a variety of ways. Firstly, this species uses ferric-siderophore receptors of the host to activate their enterobactin-mediated iron-sequestering system, allowing for bacterial growth. Their thick polysaccharide capsule prevents ingestion by phagocytes and their somatic antigens from being detected by the host’s antibodies. Also, serum complement activation is more difficult with the thick lipopolysaccharide capsule it possesses (Greenwood et al., 2002). In fact, K. pneumoniae avoids damage by complement proteins by the extreme length of the molecules comprising the capsule, essentially allowing the lytic C5b-9 (complement) complex to form too far away from the membrane. This prevents opsonization and membrane attack complex (MAC) insertion, which leads to lysis of the bacterium (Figure 3).

Figure 3. A schematic depicting a capsule from allowing C3b complement protein from initiating the complement cascade.

There are many metabolic features of K. pneumoniae that make it unique. To begin, this bacterium produces an enzyme called carbapenemase, which makes it resistant to the drug carbapenem. This species also produces bacteriocins, which are proteinaceous toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strains (Greenwood et al., 2002). Since K. pneumoniae is normally found in the microflora of the host, the production of bacteriocins could actually be harmful, as it may get rid of the essential or 'good' bacteria found in the intestines. K. pneumoniae also can utilize sodium citrate and can decarboxylate some amino acids to form amines (Forbes et al., 2002). Two other metabolic features
include the pathogen's ability to hydrolyze the glycoside esculin and its inability to split tryptophan to form indole (Greenwood et al., 2002).

The morphology, culture morphology, motility, and metabolic activities can all be used to identify K. pneumoniae. Once the bacteria are mounted and stained, straight rods 1 to 2 μm in length should be visible under a light microscope (Figure 4). Once the species is cultured, there should be a distinct yeasty odor, as well as a viscous/mucoid appearance (Figure 2). To ensure the absence of motile structures such as flagella, a motility test can be performed, involving inoculation and incubation a semi-solid agar deep test tube (Forbes et al., 2002). Test results should show that the bacteria remain at the site of inoculation. Also, a flagella stain can be performed, which should show the absence of the motile structure. When Simmons citrate agar is inoculated and incubated, growth should occur, changing the bromthymoll blue indicator from green to blue, indicating the metabolic feature of citrate utilization (Forbes et al., 2002).

Pneumoniae Bacteria Microscope

Figure 4. Photomicrograph of Klebsiella pneumoniae bacteria depicting rod-shape morphology.

Another metabolic feature of K. pneumoniae is its ability to decarboxylize amino acids; this can be tested by inoculating decarboxylase broths with arginine, lysine, and ornithine. After incubation, the results should show a colour change from yellowish-orange to purple, indicating an alkaline pH (>8.0) due to the decarboxylation of the amino acids. As mentioned earlier, K. pneumoniae is able to hydrolyze esculin, which can be tested by inoculating and incubating a medium with this bacterium. This medium should become blackened, and when examined under a Wood’s lamp, there should be a loss of fluorescence (Forbes et al., 2002). This bacteria's morphology, cultural morphology, motility, and metabolic features allow for its identification and possible treatment.


Forbes, B.A., Sahm, D.F., & Weissfeld, A.S. (2002). Bailey and Scott’s diagnostic microbiology (11th ed.). St. Louis, MO: Mosby.

Greenwood, D., Slack, R.C.B., & Peutherer, J.F. (2002). Medical microbiology: A guide to microbial infections: Pathogeneisis, immunity, laboratory diagnosis and control (16th ed.). Toronto, ON: Churchill Livingstone.