Question

EXPLAIN different mechanisms used by specific pathogens to attach to their hosts and cause diseases, including examples of a pathogen that use each attachment mechanisms ( 40 marks)

Answer 1

The answer to your query is as follows:

Different mechanisms are used by specific pathogens to attach to their hosts and cause diseases.

1. Bacteria use pili or fimbriae to attach to their host cells

Pili are thin protein thread-like structures made up of pilin protein that arise from the cytoplasmic membrane. It is found in all Gram-negative bacteria.

The shaft of the pilus has the adhesive tip structure which corresponds to specific glycoprotein receptors on a host cell on which it binds. After attachment with the host cell, the pili depolymerizes and activates the release of adhesions in the bacterial cell wall.

Apart from attachment some pili like type IV pili also helps in twitching motility that helps them to perform gliding motility near the host cell.

Examples:

• Bacteria like Neisseria gonorrhoeae colonize the mucosal surface of columnar epithelial cells with the help of pili. This bacteria has the power to synthesize new structures and pilus adhesive tips. Thus, it can easily bind to different host cells sperm cells, the epithelial cells of the respiratory mucous membranes, urogenital tract, and the conjunctiva of the eye.
• The Type IV pili of Neisseria meningitidis allows it to cross the blood-brain barrier and adhere to meninges causing septicemia and meningitis.
• Urinary tract infections are caused by Escherichia coli pili that can attach to the urinary epithelium easily. Intestinal mucosal cells are also adhered to by the pili of Enteropathogenic E. coli (EPEC) that causes intestinal infections.
• Different wounds are easily colonized by the pili of Pseudomonas aeruginosa that allow it to grow easily on the wound.

2. Various pathogens use adhesins to adhere to different host cells

The surface proteins found in the cell wall of various pathogens like bacteria attach to specific receptors on the surface of host cells. It helps the pathogen to intimately attach to the host cell and avoid physical removal from it.

Example:

• A large number of adhesins are produced by Streptococcus pyogenes. They are as follows:

a. Protein F

b. M-protein

c. Lipoteichoic acid

They all adheres to the fibronectin on epithelial cells and causes streptococcal pharyngitis

• A spirochete bacterium Treponema pallidum has adhesins on its tip that bind to fibronectin on epithelial cells of the host cells.
• A specialized type 3 secretion system is used by Escherichia coli O157 to insert effector proteins into epithelial cells of the intestine.

3. Various pathogens use biofilms to attach to host cells

Biofilms are a complex polysaccharide capsule or adhesive matrix formed by a group of bacteria that are attached to the surface of the host cell.

Bacteria live as a community in biofilm and are better able to resist the attack of chemicals or antibiotics released by host cells. Bacteria in the biofilm can easily capture nutrients for growth and attach to environmental surfaces very effectively.

Example:

• Two pathogens responsible for causing dental caries are Streptococcus mutans, and Streptococcus sobrinus. Streptococcus mutans convert sucrose into dextran which is a sticky polysaccharide. Thus, forming a biofilm that enables the bacteria to attach to the enamel of the tooth and form plaque.The fermentation of glucose by these bacteria results in the production of lactic acid. This causes tooth decay.
• Chronic ear infection called otitis media involves the formation of a biofilm of bacteria in the middle ear. This biofilm is made up of a community of Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis .
• Pseudomonas aeruginosa is responsible for causing serious respiratory infections in people with cystic fibrosis. It forms a flagellum that can secrete a biofilm made up of polysaccharide slime alginate. It can also attach to a host cell by pili and cell wall adhesins.

Hope it helps!