I. BACTERIAL PATHOGENESIS
B. VIRULENCE FACTORS THAT PROMOTE BACTERIAL COLONIZATION OF THE HOST
3. The ability to invade host cells
The overall purpose of this Learning Object is:
1) to learn how the ability to invade host cells plays a role in bacterial pathogenicity by promoting colonization; and
2) to introduce several examples of medically important bacteria that are able to invade host cells in order to promote colonization.
LEARNING OBJECTIVES FOR THIS SECTION
In this section on Bacterial Pathogenesis we are looking at virulence factors that promote bacterial colonization of the host. The following are virulence factors that promote bacterial colonization of the host .
1. The ability to use motility and other means to contact host cells and disseminate within a host.
2. The ability to adhere to host cells and resist physical removal.
3. The ability to invade host cells.
4. The ability to compete for iron and other nutrients.
5. The ability to resist innate immune defenses such as phagocytosis and complement.
6. The ability to evade adaptive immune defenses.
We will now look at virulence factors that enable bacteria to invade host cells.
Virulence Factors that Promote Bacterial Colonization of the Host
3. The Ability to Invade Host Cells
Some bacteria produce adhesin molecules called invasins (def) that activate the host cell's cytoskeletal machinery enabling bacterial entry into the cell by phagocytosis. Advantages of entering a human cell include:
a. providing the bacterium with a ready supply of nutrients.
b. protecting the bacteria from complement, antibodies, and other body defense molecules.
In addition, some pathogenic bacteria:
a. invade phagocytic cells, neutralize their killing ability, and turn them into a safe haven for bacterial replication (see Fig. 2C3).
b. kill phagocytic dendritic cells (def) once they are engulfed and prevent those dendritic cells from activating the T4-lymphocytes (def) and T8-lymphocytes (def) required for adaptive immunity.
Invasins of Shigella (inf), and enteroinvasive strains of Escherichia coli (EIEC) (inf) , for example, allow these bacteria to enter epithelial cells of the colon. These bacteria, like many involved in infection, have the ability to co-opt the functions of the host cell for the bacterium’s own benefit. This is done by way of bacterial secretions systems that enable the bacterium to directly inject bacterial effector molecules into the cytoplasm of the host cell in order to alter its cellular machinery or cellular communication.
The most common type is the type 3 secretion system. A secretion apparatus in the cytoplasmic membrane and cell wall of the bacterium polymerizes a hollow needle that is lowered to the cytoplasmic membrane of the host cell and a translocon protein is then delivered to anchor the needle to the host cell. Effector proteins in the bacterium can now be injected into the cytoplasm of the host cell. The delivery system is sometimes called an injectosome.
When these bacteria contact the epithelial cells of the colon, the type III secretion system delivers proteins into the epithelial cells enabling them to polymerize and depolymerize actin filaments (def). This cytoskeletal rearrangement is a key part of the pseudopod formation in phagocytic cells and is what enables phagocytes to engulf bacteria and place them in a vacuole. Thus the bacterium with its invasins is able to trick the epithelial cell into behaving like a phagocyte and engulfing the bacterium. The bacteria then replicate within the host cell.
We will now look at several examples of bacteria that use invasions to invade host cells.
- It is thought that Shigella first transit the mucous membrane of the colon by passing through M cells. (M cells are phagocytic cells in the mucous membrane whose function is to sample microbes from the intestinal lumen and pass them on to the lymphoid tissue of the Peyer's patch in order to activate the immune defenses against intestinal microbes). Once across the mucosa, the Shigella use a type 3 secretion system to inject invasins into the underside of the epithelial cells to induce phagocytic uptake of the bacterium (see Fig. 2A).
Once inside they escape from the vacuole into the cytoplasm and multiply. Once inside, Shigella produces a protease that cleaves tubulin, a major component of the microtubule cytoskeleton. The microtubules represent a barrier to bacterial movement within the infected cell and the protease breaks down this barrier.
Now they move through the host cell and spread to adjacent host cells by a unique process called actin-based motility whereby actin filaments (def) polymerize at one end of the bacterium producing comet-like tails that propel the Shigella through the cytoplasm of the host cell. When they reach the boundary of that cell, the actin filaments push the Shigella across that membrane and into the adjacent cell (see Fig. 2A). Actin-based motility enables the bacteria to spread from cell-to-cell without having to encounter defense cells and antibodies. As the Shigella grow and spread within the epithelial cells, those epithelial cells die and provoke a strong inflammatory response leading to the symptoms of dysentery.
In addition, Shigella can induce the host cells to produce signaling molecules that attract phagocytic, antigen-presenting dendritic cells to the area. It enters the dendritic cells and uses them to carry the Shigella through the intestinal wall to the underside. It then uses its type 3 secretion system to inject effector proteins from the phagosome into the cytoplasm. These proteins trigger apoptosis or cell suicide of the dendritic cell. Killing the dendritic cells prevents them from presenting Shigella to T4-lymphocytes, a step required for the production of antibodies against the Shigella (see Fig. 2B).
- Salmonella (inf) use a type 3 secretion system to inject intestinal epithelial cells with effector proteins that stimulate actin (def) re-arrangement and cause the epithelial cell's cytoplasmic to "ruffle" up and engulf the bacteria Fig. 2C1 - Fig. 2C2. The Salmonella pass through the epithelial cell where they are engulfed by phagocytic macrophages. Once in the phagosome of the macrophage the bacterium uses its type 3 secretion system to inject proteins that prevent the lysosomes from fusing with the phagosomes, thus providing a safe haven for Salmonella replication within the phagosome and protecting the bacteria from antibodies and other defense elements (see Fig. 2C3).
By injecting flagellin into the cytoplasm of the macrophage the Salmonella can also eventually kill the macrophage by inducing apoptosis, a programmed cell suicide.
Molecules injected into the intestinal epithelial cells also stimulate diarrhea. Advantages of inducing diarrhea include:
- Flushing out normal flora bacteria so there is less competition for nutrients; and
- Better enabling Salmonella that are not attached to host cells to be transmitted to a new host via the fecal-oral route.
For a movie showing Salmonella invading a human cell, see the Theriot Lab Website at Stanford University Medical School. Click on"Greatest Hits" and then on "Salmonella typhimurium invading a fibroblast cell."
- Listeria monocytogenes (inf) is another bacterium that enters intestinal cells via invasins and spreads to adjacent cells by actin-based motility. Its actin-based motility enables it to moves approximately 1.5 µm per second within the host cell.
- For movies showing Listeria entering host cells and being propelled by actin-based motility within a cell, see the Theriot Lab Website at Stanford University Medical School. Click on "Greatest Hits" and then on "Life history of a single infecting Listeria monocytogenes" and "Listeria monocytogenes moving in PtK2 cells."
- Although enteroinvasive Escherichia coli (EIEC) (inf) don't have actin-based motility, they invade and kill epithelial cells of the colon in a manner similar to Shigella.
- Legionella pneumophila (inf), after being ingested by macrophages and placed in a phagosome, uses a type 4 secretion system to inject effector proteins that prevent the lysosomes from fusing with the phagosomes and turning the macrophage into a safe haven for bacterial replication. The same mechanism allows the Legionella to survive inside amoebas in nature. These amoebas serve as a reservoir for the bacterium in the environment.
- Streptococcus pneumoniae (inf) produces phosphorycholine, an invasin that enables the bacterium to enter host cells where it can resist phagocytosis. The phosphorylcholine is also thought to aid the bacterium in entering the blood and the meninges.
- F protein and M-protein of Streptococcus pyogenes (Group A beta streptococci) enables the bacterium to invade epithelial cells. This is thought to help maintain persistent streptococcal infections and enable the bacterium to spread to deeper tissues.
- The spirochete Borrelia bergdorferi (inf) probably uses a combination of invasins and motility to penetrate host cells. In this case the host cell doesn't phagocytose the bacterium. Instead, one tip of the spirochete attaches to the host cell and some form of invasin apparently causes the host cell to release digestive enzymes that enable the spirochete with its corkscrewing motility to penetrate the host cell membrane. Once in the host cell the bacteria may remain dormant for years and hide from the immune system and antibiotics.
- Another spirochete, Treponema pallidum (inf), is thought to enter cells in a similar fashion. Motility also helps B. bergdorferi and T. pallidum to invade and leave blood vessels by passing between and through endothelial cells, thus enabling the spirochetes to dessiminate to other locations in the body. Electron micrograph of Treponema pallidum invading a host cell.
E-Medicine article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.