Question

In detail, describe the mechanisms by which intracellular pathogens enter their host cell. What would be the fate of the bacterium once it is inside?

Answer 1

The pathogens infected humans usually use the cells of the host organism. This provide the protected site for the replication. The endocytosis machinery help them to reach the intracellular compartment. Bacteria are considered to be more complex than the viruses. Bacteria are single celled and a typical bacterium is around 1-5 µm in length.

The bacteria use endocytosis to for the entry into the host cell and this process starts with the irreversible interaction. There are two major mechanism that take place. The first mechanism is the "Zipper mechanism" and the second mechanism is the "trigger mechanism".

Zipper mechanism: the adhesion between the ligand of the bacteria ad surface receptor of the specific host cause the receptor clustering. Also, around the bacteria there is a zippering of the plasma membrane and the initiation of signaling events help in the entry.

The trigger mechanism: during this mechanism there is a transient event in the initial contact that allow the direct delivery of active T3SS effectors into the cell cytosol. This process trigger the rearrangements of the cytoskeleton and remodeling of the membrane that lead to the event like macropinocytosis. Diagram given below is showing both the mechanisms and some additional mechanism of entry of the bacteria into the host cell.

The bacteria usually secrete proteins including toxins through the variety of secretion systems. The effector proteins transported from the interior of the bacteria into the host cell. The effector protein can be the enzyme that use components of the host as substrate that functionally or structurally mimic the proteins of eukaryotes and interfere with the cellular mechanisms.

There are two possible scenarios depending upon the type of host cell with which, bacteria interact. Phagocyte cells are the active players, they used to engulf the bacteria to destroy. The immune cells specifically B cells get active as they encounter the bacterial pathogens and produce the antibody against the bacteria.