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5.5C: The Ability to Resist Phagocytic Destruction

Skills to Develop

  1. State at least 4 different ways bacteria might be able to resist phagocytic destruction once engulfed.

We will now look at the ability of bacteria to resist phagocytic destruction and complement serum lysis. Bacteria resist phagocytic destruction by a variety of means.

Preventing fusion of the lysosome with the phagosome

Once Salmonella is engulfed by macrophages and placed in a phagosome, 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 (Figure 1). 

Figure \(\PageIndex{1}\): Salmonella Surviving Inside 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.

Legionella pneumophila, 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. Neisseria gonorrhoeae produces Por protein (protein I) that prevents phagosomes from fusing with lysosomes enabling the bacteria to survive inside phagocytes.

Cell wall lipids of Mycobacterium tuberculosis, such as lipoarabinomannan, arrest the maturation of phagosomes preventing delivery of the bacteria to lysosomes. Some bacteria, such as species of Salmonella, Mycobacterium tuberculosis, Legionella pneumophila, and Chlamydia trachomatis, block the vesicular transport machinery that enables the lysosome to move to the phagosome for fusion.

Escaping from the Phagosome

Some bacteria, such as Shigella flexneri, Listeria monocytogenes, and the spotted fever Rickettsia, escape from the phagosome into the cytoplasm prior to the phagosome fusing with a lysosome (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Bacteria Escaping from a Phagosome. Some bacteria resist phagocytosis by escaping from the phagosome prior to its fusing with a lysosome.

Preventing Acidification of the Phagosome

Some bacteria, such as pathogenic Mycobacterium and Legionella pneumophilia, prevent the acidification of the phagosome that is needed for effective killing of microbes by lysosomal enzymes. (Normally after the phagosome forms, the contents become acidified because the lysosomal enzymes used for killing (acid hydrolases) function much more effectively at an acidic pH.)

Resisting killing by Lysosomal Chemicals

Some bacteria, such as Salmonella, are more resistant to toxic forms of oxygen and to defensins, the toxic peptides that kill bacteria by damaging their cytoplasmic membranes. The carotenoid pigments that give Staphylococcus aureus species its golden color and group B streptococci (GBS) its orange tint shield the bacteria from the toxic oxidants that neutrophils use to kill bacteria.

Resisting phagocytic destruction: killing the phagocyte

Some bacteria are able to kill phagocytes. Bacteria such as Staphylococcus aureus and Streptococcus pyogenes produce the exotoxin leukocidin that damages either the cytoplasmic membrane of the phagocyte or the membranes of the lysosomes, resulting in the phagocyte being killed by its own enzymes. Shigella and Salmonella, induce macrophage apoptosis, a programmed cell death.

Exercise \(\PageIndex{1}\): Think-Pair-Share Questions

  1. Some bacteria, such as pathogenic Mycobacterium and Legionella pneumophilia prevent the acidification of the phagosome within phagocytes. Why might this protect these bacteria from being killed within the phagocyte?
  2. Staphylococcus aureus and Streptococcus pyogenes both produce a toxin called leukocydin. How might this enable these bacteria to resist phagocytosis?

Summary

  1. Some bacteria resist phagocytic destruction by preventing fusion of the lysosome with the phagosome.
  2. Some bacteria resist phagocytic destruction by escaping from the phagosome before the lysosome fuses.
  3. Some bacteria resist phagocytic destruction by preventing acidification of the phagosome.
  4. Some bacteria resist phagocytic destruction by resisting killing by lysosomal chemicals.
  5. Some bacteria resist phagocytic destruction by killing phagocytes.

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)