Listeria monocytogenes is a bacterium that causes listeriosis, a serious foodborne illness. This microorganism has a distinctive method of movement once it enters host cells. This internal motility is a central characteristic that contributes to its ability to establish and spread disease. Understanding this movement is important for comprehending how Listeria causes illness.
Understanding Listeria’s Movement
Bacterial motility refers to a microorganism’s ability to move independently. Many bacteria use external structures like flagella, whip-like appendages that propel them through liquids. Listeria monocytogenes uses flagella to move outside host cells, navigating various environments. Once inside a host cell, however, Listeria employs a completely different and unique mechanism for movement, abandoning its flagella for intracellular propulsion. This internal, actin-based movement is a defining feature distinguishing its pathogenesis.
The Mechanism of Actin-Based Motility
Inside host cells, Listeria moves by recruiting and manipulating the host cell’s actin cytoskeleton. This process begins with ActA, a bacterial surface protein located at one pole of the bacterium. ActA acts as a molecular mimic, manipulating the host cell’s machinery for building actin filaments. This protein directly activates the Arp2/3 complex, a host protein complex that initiates the branching and nucleation of new actin filaments.
The activated Arp2/3 complex promotes the rapid assembly of actin monomers into long, branched filaments directly behind the bacterium. As new actin filaments are continuously added, they push against the bacterium, generating propulsive force. This continuous polymerization of actin creates a “comet tail” structure that elongates behind the bacterium, propelling it forward through the cell’s cytoplasm. This directed assembly of actin provides the force for Listeria to move within the host cell.
How Motility Drives Infection
The actin-based motility of Listeria is important to its infectious cycle and ability to spread throughout a host. Using this propulsion system, the bacteria move directly from one host cell into an adjacent one. As Listeria moves within an infected cell, it pushes against the cell membrane, forming finger-like protrusions. These protrusions extend into neighboring cells, which then engulf them in a process similar to phagocytosis.
Once engulfed, the bacteria are enclosed within a double-membraned vacuole, from which they escape to begin a new cycle of intracellular replication and movement. This cell-to-cell spread allows Listeria to bypass the extracellular environment, shielding itself from the host’s immune system. This direct transfer helps the infection disseminate widely within tissues and organs without being readily detected or neutralized by immune cells.
Motility’s Role in Food Safety
The motility of Listeria monocytogenes contributes to its persistence in food processing environments and its capacity to cause severe foodborne illness. Its ability to move within host cells and spread directly makes it a challenging pathogen to combat. This intracellular movement helps Listeria evade immune responses, allowing it to establish systemic infections.
Once inside the body, this cell-to-cell spread can lead to widespread infection, impacting various organs. This can cause serious conditions like meningitis or septicemia, especially in vulnerable populations such as pregnant women, newborns, older adults, and individuals with weakened immune systems. Understanding and mitigating Listeria’s unique motile characteristics are important for developing effective food safety measures and preventing listeriosis outbreaks.