Klebsiella pneumoniae is a bacterium found in the environment and as a normal inhabitant of human intestines. It can act as an opportunistic pathogen, causing infections in healthcare settings where individuals may have weakened immune systems. The physical traits of this bacterium, its morphology, are fundamental to its laboratory identification and to understanding how it causes disease.
Microscopic Characteristics
Under a microscope, individual Klebsiella pneumoniae cells are straight rods, a shape known as a bacillus. These bacteria are observed on their own, in pairs, or in short chains, but do not form large clusters. A single cell measures between 0.3 to 1.0 micrometers (µm) in width and 0.6 to 6.0 µm in length.
A defining feature is its reaction to the Gram stain. K. pneumoniae is Gram-negative, meaning it does not retain the primary crystal violet stain and instead takes up the red or pink counterstain. This result indicates a cell wall with a thin peptidoglycan layer between inner and outer cellular membranes, a structure characteristic of Gram-negative organisms.
Unlike many bacteria that can move, K. pneumoniae is characteristically non-motile. This is because the bacterium does not produce flagella, the whip-like appendages used for propulsion. This lack of independent movement is a consistent feature used in its identification.
The Prominent Polysaccharide Capsule
One of the most notable features of Klebsiella pneumoniae is its prominent capsule, a thick, gelatinous layer that encases the cell wall. This structure is composed of complex sugars called polysaccharides, linked together to form a dense matrix. The composition of these sugars can vary between strains, leading to different capsular types, known as K-antigens.
When viewed with specialized negative staining techniques, like India ink, the capsule becomes visible. These stains color the background but not the capsule, resulting in an image of the bacterial cell surrounded by a clear halo. This halo vividly outlines the capsule’s size and shape.
The capsule is a primary virulence factor that contributes to the bacterium’s ability to cause disease. Its main function is to protect the bacterium from the host’s immune defenses, particularly phagocytosis—the process where immune cells engulf and destroy invaders. The thick capsule makes it difficult for phagocytes to ingest the bacterial cell, allowing the pathogen to survive and establish an infection.
Colony Appearance on Culture Media
When grown in a laboratory on a nutrient-rich surface called an agar plate, Klebsiella pneumoniae multiplies to form visible colonies. These colonies have a distinct macroscopic appearance, and are typically large, raised in a dome shape, and have a glistening, wet look. This appearance is a direct result of the abundant polysaccharide capsule produced by the bacteria.
The capsule production gives the colonies a “mucoid” or slimy texture. This quality is pronounced enough for an informal identification method called the “string test.” When a sterile loop touches a mucoid colony and is lifted, the viscous material can be pulled into a sticky, string-like thread. This mucoid quality is a visual hallmark of many K. pneumoniae strains.
On MacConkey agar, a medium that differentiates bacteria, K. pneumoniae colonies appear a prominent pink to red color. This color change is caused by the bacterium fermenting lactose in the agar, which produces acid and lowers the pH. This change in pH triggers a color shift in the medium’s indicator, helping microbiologists distinguish it from non-lactose fermenting bacteria.
Significance of Morphological Variations
Some Klebsiella pneumoniae strains exhibit an exaggerated version of the mucoid capsule, which has clinical consequences. These hypervirulent strains are distinguished by producing an even thicker, more viscous capsule. This trait leads to a “hypermucoviscous” phenotype, an extreme version of the typical mucoid appearance.
This hypermucoviscous morphology is linked to an increase in the bacterium’s disease-causing potential. The thicker capsule provides enhanced protection against the immune system, allowing these strains to cause more aggressive and invasive infections. Unlike classic strains that affect immunocompromised individuals, hypervirulent strains can cause severe, life-threatening infections in healthy people.
These infections often include pyogenic liver abscesses, a condition where pus-filled pockets form in the liver, which can then spread to other parts of the body like the lungs, central nervous system, and eyes. The observation of a hypermucoviscous colony serves as a warning sign in clinical laboratories, suggesting a highly aggressive pathogen that can influence treatment decisions.