I. BACTERIAL PATHOGENESIS

B. VIRULENCE FACTORS THAT PROMOTE BACTERIAL COLONIZATION OF THE HOST

2. The ability to adhere to host cells and resist physical removal

The overall purpose of this Learning Object is:
1) to learn how the ability to adhere to host cells and resist physical removal plays a role in bacterial pathogenicity by promoting colonization;
2) to learn three different ways bacteria may adhere to host cells and resist physical removal; and
3) to introduce several examples of medically important bacteria that use each of these mechanisms in order to adhere to and colonize host cells.

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.
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 adhere to host cells and resist physical removal.

Virulence Factors that Promote Bacterial Colonization of the Host

2. The Ability to Adhere to Host Cells and Resist Physical Removal

As we will see in Unit 4, one of the body's innate defenses is the ability to physically remove bacteria from the body through such means as the constant shedding of surface epithelial cells from the skin and mucous membranes, the removal of bacteria by such means as coughing, sneezing, vomiting, and diarrhea, and bacterial removal by bodily fluids such as saliva, blood, mucous, and urine. Bacteria may resist this physical removal by producing pili, cell wall adhesin proteins, and/or biofilm-producing capsules. In addition, the physical attachment of bacteria to host cells can also serve as a signal for the activation of genes involved in bacterial virulence. This process is known as signal transduction.

1. Using Pili (fimbriae) to Adhere to Host Cells (def)

As seen in Unit 1, pili enable some organisms to adhere to receptors on target host cells (see Fig. 1) and thus colonize and resist flushing by the body. Pili are thin, protein tubes originating from the cytoplasmic membrane and are found in virtually all gram-negative bacteria but not in many gram-positive bacteria. The pilus has a shaft composed of a protein called pilin. At the end of the shaft is the adhesive tip structure having a shape corresponding to that of specific glycoprotein or glycolipid receptors on a host cell (see Fig. 2). Because both the bacteria and the host cells have a negative charge, pili may enable the bacteria to bind to host cells without initially having to get close enough to be pushed away by electrostatic repulsion. Once attached to the host cell, the pili can depolymerize and enable adhesions in the bacterial cell wall to make more intamate contact.

 

You Tube animation showing Pseudomonas using motility, pili, and exotoxins to cause an infection. 3D Mecical Animations Library and Downloads, www.rufusrajadurai. wetpaint.com

Bacteria are constantly losing and reforming pili as they grow in the body and the same bacterium may switch the adhesive tips of the pili in order to adhere to different types of cells and evade immune defenses (see Fig. 3).

For example:


Highlighted Bacterium: Neisseria meningitidis

Click on this link, read the description of Neisseria meningitidis, and be able to match the bacterium with its description on an exam.

 

 

2. Using Adhesins to Adhere to Host Cells(def)

Adhesins are surface proteins found in the cell wall of various bacteria that bind to specific receptor molecules on the surface of host cells and enable the bacterium to adhere intimately to that cell in order to colonize and resist physical removal (see Fig. 5). Many, if not most bacteria probably use one or more adhesins to colonize host cells.

For example:

 

3. Using Capsules (biofilms) to Adhere to Host Cells

Many normal flora bacteria produce a capsular polysaccharide matrix or glycocalyx to form a biofilm (def) on host tissue (see Fig. 5). A biofilm(def) consists of layers of bacterial populations adhering to host cells and embedded in a common capsular mass. For example:

For further information on bacterial pathogenesis, see the online Microbiology Web Textbook at the University of Wisconsin-Madison.
 

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

 

 

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