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 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 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 intimate contact.There is also evidence that the binding of pili to host cell receptors can serve as a trigger for activating the synthesis of some cell wall adhesins.

 

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). E. coli, for example,  is able to make over 30 different types of pili.

One class of pili, known as type IV pili, not only allow for attachment but also enable a twitching motility. They are located at the poles of bacilli and allow for a gliding motility along a solid surface such as a host cell. Extension and retraction of these pili allows the bacterium to drag itself along the solid surface. This might enable bacteria with these types of pili to move around a cellular surface and find an optimum area on that cell for attachment once they have initially bound. Type IV pili are used by Pseudomonas aeruginosa, Neisseria gonorrhoeae, Neisseria meningitidis, and Vibrio cholerae.

Examples of bacteria using pili to colonize:



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.

  • Uropathogenic strains of Escherichia coli (def) can produce pili that enable the bacterium to adhere to the urinary epithelium and cause urinary tract infections. They also produce afimbrial adhesins (see below) for attachment to epithelial cells. Enteropathogenic E.coli (EPEC) use pili to adhere to intestinal mucosal cells.

 

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 Biofilms to Adhere to Host Cells

Many normal flora bacteria produce a capsular polysaccharide matrix or glycocalyx to form a biofilm on host tissue (see Fig. 5).Biofilms (def) are groups of bacteria attached to a surface and enclosed in a common secreted adhesive matrix, typically polysaccharide in nature.

Many pathogenic bacteria, as well as normal flora and many environmental bacteria, form complex bacterial communities as biofilms.

Bacteria in biofilms are often able to communicate with one another by a process called quorum sensing and are able to interact with and adapt to their environment as a population of bacteria rather than as individual bacteria. By living as a community of bacteria as a biofilm, these bacteria are better able to:

Biofilms are, therefore, functional, interacting, and growing bacterial communities. Biofilms even contain their own water channels for delivering water and nutrients throughout the biofilm community.

For example:

Many chronic and difficult-to-treat infections are caused by bacteria in biofilms. Within biofilms, bacteria grow more slowly, exhibit different gene expression than free planktonic bacteria, and are more resistant to antimicrobial agents such as antibiotics because of the reduced ability of these chemicals to penetrate the dense biofilms matrix. Biofilms have been implicated in tuberculosis, kidney stones, Staphylococcus infections, Legionnaires' disease, and peridontal disease. It is further estimated that as many as 10 million people a year in the US may develop biofilm-associated infections as a result of invasive medical procedures and surgical implants.

 

 

 

 

 

 

 

 

 

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

 

 

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