3) Surviving Inside Phagocytic Cells: Study with Video Lessons, Practice Problems & Examples

Pathogens can evade the immune system by surviving within phagocytic cells through three mechanisms. First, some bacteria escape the phagosome before it fuses with the lysosome, avoiding degradation. Second, they can inhibit the fusion of the phagosome and lysosome using specific proteins. Lastly, certain pathogens endure even within the phagolysosome, resisting the lysosomal enzymes. Understanding these survival strategies is crucial for developing effective treatments against infections caused by these resilient pathogens.

Surviving Inside Phagocytic Cells

In this video, we're going to talk about yet another method that some pathogens can use to evade or avoid the immune system, and that is through surviving inside of phagocytic cells. So recall from some of our previous lesson videos that we describe the process of phagocytosis in a stepwise manner. And so recall from those videos that when a pathogen is engulfed by a phagocyte, that pathogen will enter the phagocyte within a phagosome. And the phagosome will generally fuse with a lysosome, which will allow for the pathogen to be digested and eliminated. Now some bacteria have evolved mechanisms to be able to survive inside of a phagocytic cell even after they have been engulfed. And they can do this in 3 different ways that we have down below in our image. And so notice in this image on the outside right here what we have is our pathogen. And of course, our phagocytic cell is going to be this big blue cell that you see here. This is going to be our macrophage. And so notice that the pathogen, here has opsonins on its surface and those opsonins allow for the process of opsonization that makes the pathogen easier to be engulfed through the process of phagocytosis. And so when the pathogen is engulfed, notice that it is brought into the cell, within a phagosome. Okay? Now the first mechanism that some bacteria have evolved to survive inside of phagocytes is, they have evolved mechanisms to escape from the phagosome before the phagosome fuses with the lysosome. And so what you'll notice here in this image, right here in this region, we are showing you how some bacteria have evolved the mechanism to escape the phagosome. And when they escape the phagosome, they escape their destiny to be degraded and destroyed. And so when they escape, they are able to survive within the macrophage. Now another mechanism that some bacteria have been able to evolve over time to survive in phagocytes is that they can avoid or block the fusion of the phagosome with the lysosome by producing inhibitory proteins. And so notice here that, sometimes right here, step number 2, where the phagosome would usually fuse with lysosomes, this step can be blocked and prevented. And so that will allow the pathogen to survive within the phagosome and not have to worry about the digestive enzymes from the lysosome. Now the third possibility here that some bacteria have evolved is the amazing ability to be able to survive inside of the phagolysosome even after the lysosomes have fused with the phagosome. And so, even when the lysosomes fuse with the phagosome and all of these digestive hydrolytic enzymes are there, trying to degrade the pathogen. In some cases, the pathogens have been able to survive those conditions and avoid being destroyed by the contents of the lysosome. And so notice this pathogen here is saying 'can't kill me' because they are able to survive even inside microbes that are able to survive inside of phagocytes by using each of these 3 different methods. And so this here concludes our brief lesson on how some pathogens can survive inside of phagocytic cells, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

Which of the following answers is not a mechanism pathogen use to survive within a phagocytic host cell?

• A. Withstanding the digestive and hydrolytic enzymes of the phagolysosome.
• B. Secreting digestive enzymes to lyse the phagocytic cells from the inside.
• C. Forming pores in the phagosomal membrane and escaping the phagosome.
• D. Creating proteins which blocks the fusion of the phagosome and lysosome.

Some encapsulated bacterial pathogens are said to be serum-resistant. Serum resistant pathogens avoid the effects of the complement system proteins. How are encapsulated bacteria able to be serum-resistant and avoid phagocytosis?

• A. The capsule ensures that C3b cannot bind the pathogen and trigger phagocytosis or opsonization.
• B. The capsule ensures that C3 convertase cannot trigger the activation of phagocytic immune cells.
• C. The capsule ensures that the pathogen will be able to escape the phagosome inside the phagocytic cell.

Here’s what students ask on this topic:

What are the mechanisms by which bacteria can survive inside phagocytic cells?

Bacteria can survive inside phagocytic cells through three main mechanisms. First, some bacteria escape the phagosome before it fuses with the lysosome, thus avoiding degradation. Second, they can inhibit the fusion of the phagosome and lysosome by producing specific inhibitory proteins. Lastly, certain pathogens can endure even within the phagolysosome, resisting the lysosomal enzymes. These survival strategies allow bacteria to evade the immune system and persist within the host, making infections more difficult to treat.

How do bacteria escape from the phagosome before it fuses with the lysosome?

Bacteria escape from the phagosome before it fuses with the lysosome by employing various mechanisms. Some bacteria produce enzymes that degrade the phagosomal membrane, allowing them to escape into the cytoplasm. Others may use specialized secretion systems to inject proteins that disrupt the phagosomal membrane. By escaping the phagosome, these bacteria avoid the degradative environment of the lysosome, allowing them to survive and replicate within the host cell.

What role do inhibitory proteins play in bacterial survival within phagocytic cells?

Inhibitory proteins play a crucial role in bacterial survival within phagocytic cells by preventing the fusion of the phagosome with the lysosome. These proteins interfere with the normal cellular processes that facilitate this fusion, thereby creating a safe environment for the bacteria within the phagosome. By blocking this fusion, the bacteria avoid exposure to the lysosomal enzymes that would otherwise degrade them, allowing them to persist and potentially cause chronic infections.

How do some bacteria survive within the phagolysosome despite the presence of lysosomal enzymes?

Some bacteria have evolved mechanisms to survive within the phagolysosome despite the presence of lysosomal enzymes. These bacteria can resist the harsh conditions by producing protective proteins or enzymes that neutralize the lysosomal enzymes. Additionally, they may alter the pH of the phagolysosome to create a less hostile environment. These adaptations enable the bacteria to withstand the degradative processes and continue to live and replicate within the phagocytic cell.

Why is understanding bacterial survival mechanisms inside phagocytic cells important for developing treatments?

Understanding bacterial survival mechanisms inside phagocytic cells is crucial for developing effective treatments because these mechanisms allow bacteria to evade the immune system and persist within the host. By identifying and targeting these survival strategies, researchers can develop new therapeutic approaches to combat infections. For example, drugs that inhibit bacterial escape from the phagosome or block the production of inhibitory proteins could enhance the effectiveness of the immune response, leading to better outcomes for patients with bacterial infections.