I. BACTERIAL PATHOGENESIS

B. VIRULENCE FACTORS THAT PROMOTE BACTERIAL COLONIZATION OF THE HOST

6. The ability to evade adaptive immune defenses

Fundamental Statements for this Learning Object:

1. There are various ways that the antibodies the body makes during adaptive immunity protect the body against bacteria.
2. Some antibodies such as IgG and IgE function as opsonins and stick bacteria to phagocytes (opsonization or enhanced attachment).
3. Antibodies, such as IgG, IgA, and IgM, can bind to bacterial adhesins, pili, and capsules and in this way block their attachment to host cells.
4. IgG and IgM can activate the classical complement pathway providing all of its associated benefits.
5. IgA and IgM can clump bacteria together enabling them to be more readily removed by phagocytes.
6. Antitoxin antibodies, mainly IgG, are made against bacterial
exotoxins. They combine with the exotoxin molecules before they can interact with host target cells and thus neutralize the toxin.
7.
Bacteria utilize a variety of mechanisms to resist antibodies made during adaptive immunity.
8. Some bacteria can vary their surface proteins or polysaccharides so that antibodies previously made against those proteins will no longer "fit."
9.
Some bacteria are able to coat themselves with host proteins and in this way avoid having antibodies being made against them because they are unable to be recognized as foreign
10. Some bacteria produce immunoglobulin proteases that degrade the body's protective antibodies (immunoglobulins) that are found in body secretions.

 

LEARNING OBJECTIVES FOR THIS SECTION


In this section on Bacterial Pathogenesis we will be 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 the ability of bacteria to evade adaptive immune defenses.


6. The Ability to Evade Adaptive Immune Defenses

One of the major defenses against bacteria is the immune defenses' production of antibody molecules (def) against the organism. The "tips" of the antibody, called the Fab portion (see Fig. 1 and Fig. 5A) have shapes that are complementary to portions of bacterial proteins and polysaccharides called epitopes (def). The "bottom" of the antibody, called the Fc portion (see Fig. 1) binds to receptors on phagocytes and NK cells (def)) and can activate the classical complement pathway (def).

There are various ways that the antibodies the body makes during adaptive immunity protect the body against bacteria:

a. As mentioned above under phagocytosis, some antibodies such as IgG and IgE function as opsonins (def) and stick bacteria to phagocytes (see Fig. 2).

b. Antibodies, such as IgG, IgA, and IgM, can bind to bacterial adhesins, pili, and capsules and in this way block their attachment to host cells.

c. IgG and IgM can also activate the classical complement pathway (def) providing all of its associated benefits.

d. IgA and IgM can clump bacteria together enabling them to be more readily removed by phagocytes (see Fig. 3).

These mechanisms will be discussed in greater detail in Unit 6.

 

 

Bacteria utilize a variety of mechanisms to resist antibodies made during adaptive immunity. These include the following:

a. Certain bacteria can evade antibodies is by changing the adhesive tips of their pili (def) as mentioned above with Escherichia coli (inf) and Neisseria gonorrhoeae (inf)   (see Fig. 4). Bacteria can also vary other surface proteins so that antibodies previously made against those proteins will no longer "fit." See Fig. 5A and Fig. 5B. For example, N. gonorrhoeae produces Rmp protein (protein III) that protects against antibody attack by antibodies made against other surface proteins (such as adhesins) and the lipooligosaccharide (LOS) of the bacterium.

Medscape article on infections associated with organisms mentioned in this Learning Object. Registration to access this website is free.

 

Doc Kaiser's Microbiology Home Page
Copyright © Gary E. Kaiser
All Rights Reserved
Updated: May, 2014

Please send comments and inquiries to Dr. Gary Kaiser