Virulence factors of Listeria monocytogenes

Listeria monocytogenes is a Gram-positive, rod-shaped bacterium responsible for listeriosis, a severe food-borne human infection with an overall mortality rate of $30\mathrm{\%}$‍. It has evolved efficient strategies to survive in the intestine and cross the intestinal, blood-brain and placental barriers leading to clinical features of the disease that include gastroenteritis, septicemia, central nervous system infections, and mother-to-child infections. The danger of food contamination as a result of the bacterium in the refrigeration process have drawn attention to the study of virulence factors associated with L. monocytogenes.

The spread of L. monocytogenes is accomplished through invasion from cell-to-cell. Aided by the listerial membrane surface protein ActA, from the gene actA, invasion and mobilization of the intracellular bacterium occurs when actin assembly is initiated inside a host cell. The bacterial membrane-bound ActA protein then recruits the Arp2/3 complex. Once recruited, Arp2/3 helps to polymerize actin filaments at the posterior end of the bacterium, creating an actin comet tail within the host cell and propelling the bacterium forward. This function allows the bacterium to move throughout the host cell cytoplasm as well as to invade neighboring eukaryotic cells.

CK-0944636, a molecular inhibitor can be used to bind Arp2/3 within its hydrophobic core and preventing its active conformation. This compound can be used to inhibit formation of actin filament comet tail by L. monocytogenes and is used as a way to study Arp2/3 complex in living cells.

Figure 1. ActA protein, tethered to the bacterial membrane, stimulates actin filament nucleation with the Arp2/3 complex. This generates branched arrays of filament that grow towards the membrane to which they are tethered, elongating the actin strand and moving the bacterium.

Attribution: “Benjamin A Smith, Shae B Padrick, Lynda K Doolittle, Karen Daugherty-Clarke, Ivan R Corrêa Jr, Ming-Qun Xu, Bruce L Goode,Michael K Rosen, Jeff Gelles et al, CC-BY 4.0.

Different colonies of L. monocytogenes were cultured inside media with freely soluble actin monomers in order to study the relative effects of actin polymerization. Strain A was exposed to CK-0944636 molecular inhibitor and had its actA gene disrupted. Strain B had only its actA gene disrupted while strain C was only incubated with CK-0944636 molecular inhibitor.

Figure 2. The rates of actin filament polymerization in $\mu M^{-1}{s}^{-1}$‍ for three different strains of L. monocytogenes.

Another peculiar property of L. monocytogenes is the ability to grow and multiply at lower temperatures relative to other bacteria. To demonstrate this ability, two different types of foodborne bacteria, L. monocytogenes and Escherichia coli, were cultured in a medium and refrigerated at $4^{\circ }$‍$C$‍. The colonies for the two species were kept separate to prevent any horizontal gene transfer. The amount of bacteria was then measured every few weeks and a growth curve was determined.

Figure 3. L. monocytogenes and E. coli bacteria growth curves.

Data adapted from: “Dortet L, Mostowy S, Louaka AS, Gouin E, Nahori M-A, et al. (2011). Recruitment of the Major Vault Protein by InlK: A Listeria monocytogenes Strategy to Avoid Autophagy. PLoS Pathog 7(8): e1002168.