Master Listeria Monocytogenes Characteristics with Picmonic for Medicine

This organism stains positive on Gram stain due to thick peptidoglycan layer which absorbs crystal violet.

This organism has a bacillus (rod) shape. After 26 hours it can often be visualized as an intermediate shape as cocci (sphere).

Listeria is a facultative intracellular microbe that grows and reproduces inside host's cells.

Listeria is a tough organism that can survive in 4 degrees Celsius environment, which is why it is not hindered by refrigeration. This helps distinguish this organism from other gram-positive organisms.

Listeria monocytogenes is a facultative intracellular microbe that grows and reproduces inside host's cells, and moves within eukaryotic cells by explosive polymerization of actin filaments called actin rockets.

This organism exhibits a characteristic motility described as tumbling motility when viewed with light microscopy, which can help with identification.

Characteristically, Listeria is catalase-positive, meaning it produces the enzyme catalase. This enzyme allows the bacterium to convert hydrogen peroxide to water and oxygen.

Listeria expresses a beta hemolysin, which causes destruction of red blood cells forming a clear area of hemolysis on blood agar.

Listeria was initially thought to be unique among gram-positive bacteria because its cell wall appeared to be composed of lipopolysaccharides, which was believed to serve as an endotoxin. Later, it was classified as not a true endotoxin. In fact, the cell walls contain lipoteichoic acids, and not lipopolysaccharides, which they closely resemble. Thus, listeria are unique because of the endotoxin-like structure they possess, which is composed of lipoteichoic acids that resemble the lipopolysaccharide cell walls of gram negative bacteria.