Microbiology, 2021

626 15 • Microbial Mechanisms of Pathogenicity
Jump to the next Clinical Focus box. Go back to the previous Clinical Focus box.
Adhesion
Following the initial exposure, the pathogen adheres at the portal of entry. The term adhesion refers to the
capability of pathogenic microbes to attach to the cells of the body using adhesion factors, and different
pathogens use various mechanisms to adhere to the cells of host tissues.
Molecules (either proteins or carbohydrates) called adhesins are found on the surface of certain pathogens
and bind to specific receptors (glycoproteins) on host cells. Adhesins are present on the fimbriae and flagella of
bacteria, the cilia of protozoa, and the capsids or membranes of viruses. Protozoans can also use hooks and
barbs for adhesion; spike proteins on viruses also enhance viral adhesion. The production of glycocalyces
(slime layers and capsules) (Figure 15.7), with their high sugar and protein content, can also allow certain
bacterial pathogens to attach to cells.
Biofilm growth can also act as an adhesion factor. A biofilm is a community of bacteria that produce a
glycocalyx, known as extrapolymeric substance (EPS), that allows the biofilm to attach to a surface. Persistent
Pseudomonas aeruginosa infections are common in patients suffering from cystic fibrosis, burn wounds, and
middle-ear infections (otitis media) because P. aeruginosa produces a biofilm. The EPS allows the bacteria to
adhere to the host cells and makes it harder for the host to physically remove the pathogen. The EPS not only
allows for attachment but provides protection against the immune system and antibiotic treatments,
preventing antibiotics from reaching the bacterial cells within the biofilm. In addition, not all bacteria in a
biofilm are rapidly growing; some are in stationary phase. Since antibiotics are most effective against rapidly
growing bacteria, portions of bacteria in a biofilm are protected against antibiotics. 7
Figure 15.7
Glycocalyx produced by bacteria in a biofilm allows the cells to adhere to host tissues and to medical devices such as the
catheter surface shown here. (credit: modification of work by Centers for Disease Control and Prevention)
Invasion
Once adhesion is successful, invasion can proceed. Invasion involves the dissemination of a pathogen
throughout local tissues or the body. Pathogens may produce exoenzymes or toxins, which serve as virulence
factors that allow them to colonize and damage host tissues as they spread deeper into the body. Pathogens
may also produce virulence factors that protect them against immune system defenses. A pathogen’s specific
virulence factors determine the degree of tissue damage that occurs. Figure 15.8 shows the invasion of H.
pylori into the tissues of the stomach, causing damage as it progresses.
7 D. Davies. “Understanding Biofilm Resistance to Antibacterial Agents.” Nature Reviews Drug Discovery 2 (2003):114–122.
Access for free at openstax.org.