In this video, we're going to talk about the 3rd step in bacterial pathogenesis, which is invasion into the host cell or into the host tissues. Once a pathogen has adhered to the host, it will then either find a way to invade the inside of a host cell or just enter into the surrounding host tissues. Invasion of the inside of a host cell allows a pathogen to reduce competition for nutrients and to hide in order to avoid the immune system. Invading the inside of a host cell can be beneficial for the pathogen. The invasion into most types of host cells involves one of two potential mechanisms of induced endocytosis, which is when the bacterial cell tricks, "host cells" to perform endocytosis or to endocytose even when that host cell typically would not perform endocytosis. We'll talk a little more about this as we go along. This induced endocytosis can occur via the pathogen coming into contact with epithelial cells and secreting effector proteins that cause membrane ruffling, which occurs with the cell's actin, part of the cell's cytoskeleton. Membrane ruffling can be defined as a rearrangement of the actin filaments. This rearrangement in the host cell results in membrane ruffles, which are essentially wrinkles that ultimately enclose the bacterial cell and bring the bacterial cell into the host cell, resembling endocytosis, but not quite exactly the same. The other way is by standard endocytosis where the pathogen expresses surface proteins that bind to receptors on the host cell and the pathogen is then engulfed into the host cell.
If we take a look at this image below, we can get a better idea of how invasion into the host cell can occur. On the left, we have some pathogens that have adhered, and on the right, pathogens that have invaded the host cells. Here we're showing you two different pathways. We have the top pathway here in red and the bottom pathway in blue, both facilitating the invasion inside the host cell. The first path that the pathogen could take is through membrane ruffling. The pathogen will secrete effector proteins which are taken in by the host cell. These effector proteins ultimately cause the actin filaments to rearrange in this membrane ruffling. You can see that the membrane starts to wrinkle up and ruffle, and ultimately, what happens is that the pathogen is brought towards the inside of the host cell. Once the pathogen is inside, it has less competition for nutrients allowing it to survive and thrive better and also to avoid the host's immune system. It can be advantageous for pathogens to invade inside host cells.
The other way the pathogen could invade inside the host cell is by being engulfed through regular endocytosis. The surface proteins on the surface of the bacteria bind to host cell receptors. Upon binding to the bacterial surface proteins, the host cell receptors induce standard endocytosis and the pathogen is engulfed. The pathogen then makes its way into the host cell where it has less competition for nutrients and can avoid the host's immune system. Ultimately, the host cells have been invaded and the cells are now inside the host cell. If they're not going to remain inside the host cell, they can then continue to make their way into the host tissues.
This concludes our brief lesson on the invasion of the pathogen into the host cell or the host tissues and we'll be able to get some practice applying these concepts and then talk about the 4th and final stage of bacterial pathogenesis as we move forward. So I'll see you all in our next video.
