Among bacteria, Listeria includes Listeria monocytogenes, the primary species causing listeriosis in humans. This bacterium is studied for its ability to survive in diverse environments and its unique behaviors within host cells. Its microscopic appearance reveals how it interacts with its surroundings and contributes to its survival and spread.
Physical Appearance of Listeria
Under a standard light microscope, Listeria monocytogenes appears as a small, rod-shaped bacterium. These bacilli are slender, measuring about 0.5 micrometers (µm) in width and 1 to 3 µm in length. They often appear individually, but can also be observed in short chains or V-shaped arrangements. A defining feature is its classification as a Gram-positive bacterium.
Microscopy and Staining Methods
To visualize Listeria, microbiologists employ the Gram stain. This method uses dyes that react differently with bacterial cell walls, causing Gram-positive bacteria like Listeria to appear purple or blue under the light microscope due to their thick peptidoglycan layer. For more intricate details beyond basic shape and staining, scientists utilize advanced electron microscopy. Scanning electron microscopy (SEM) provides high-resolution, three-dimensional images of the bacterium’s surface, allowing observation of surface features and biofilm formation. Transmission electron microscopy (TEM) offers greater magnification, enabling visualization of the bacterial cell’s internal structures.
The Motility of Listeria
Listeria monocytogenes exhibits two distinct forms of movement, depending on its environment. In a liquid culture at room temperature, the bacteria display “tumbling motility.” This movement is powered by peritrichous flagella, which are hair-like structures protruding from the bacterial cell surface.
At human body temperature (approximately 37°C), Listeria monocytogenes generally ceases to synthesize flagella and becomes non-motile externally. However, once inside a host cell, it employs an intracellular propulsion mechanism. The bacterium hijacks the host cell’s actin, a protein involved in cell shape and movement, to form “actin rockets” or “comet tails.” These tails of continuously polymerizing actin filaments push the bacterium through the host cell’s cytoplasm and can even propel it into neighboring cells, facilitating its spread without exiting the host.
Viewing Listeria Inside Host Cells
Studying Listeria’s behavior within living cells requires specialized microscopic techniques. Scientists often infect laboratory-grown host cells with Listeria to observe its intracellular journey. Fluorescence microscopy is a widely used tool for this purpose. Researchers can introduce fluorescent tags, which are molecules that emit light when illuminated, to either the Listeria bacteria themselves or to the host cell’s actin proteins. This labeling causes the bacteria or their associated actin tails to glow under the microscope, providing clear visual tracking of their movement and spread within and between cells.