Gram-negative Bacterial Fimbriae
Most Gram-negative bacteria have these short, fine appendages surrounding the cell. In contrast, Gram-positive bacteria never have fimbriae.
These protein projections are not used to help the cell move about, but instead make it possible for bacteria adhere to each other, surfaces and to host cells. Basically fimbriae help bacteria stick to things.
How Fimbriae Help Bacteria Cause Disease
Fimbriae are a major factor in bacterial virulence (the ability of a bacterium to cause disease), since these structures enable some bacteria to colonize human epithelial cells (cells of mucous membranes).
At the end of each fimbria are special proteins called adhesins. The specific type of adhesin varies by type of bacteria, but regardless of the type, adhesin molecules allow bacteria with fimbriae to adhere to host cells by docking, like a lock and key, with receptor proteins on the surface of host epithelial cells.
This ability of fimbriae to stick to epithilial cells leads to many diseases transmitted via mucous membranes, including gonorrhoeae, bacterial meningitis and infections of internal medical devices and indwelling catheters. The following are descriptions of three species of pathogenic bacteria that rely on their fimbriae to cause disease.
Fimbriae and Neisseria gonorrhoeae
Gram-negative bacteria, such as Neisseria gonorrhoeae, have fimbriae that allow them to attach to host cells and cause gonorrhea. When this bacterium infects the eye, is can result in potentially serious eye infections. This is the reason that all babies born in US hospitals have antibiotic ointment put in their eye shortly after birth.
Neisseria gonorrhoeae is never considered normal flora. It is a full-time pathogen and it takes only about 100 cells to cause infection. One of the most communicable diseases in the United States, N. gonnorhoeae infects approximately 125 per 100,000 people. In 15 – 19 year olds the rate of infection skyrockets at 534 per 100,000.
Fimbriae and Neisseria meningiditis
Neisseria meningitidis can be normal flora of the nasal pharynx. It is not clear how N. menigitidis crosses the blood-brain barrier, but when it does, it can cause meningiococcal meningitis, the leading cause of meningitis in young adults. The meningiococcal vaccine helps keep the incidence of this type of bacterial meningitis at low numbers (approximately 1 in 100,000).
Changes in weather patterns, thought to be caused by global warming, are causing the incidence of meningiococcal meningitis to skyrocket in many increasingly arid parts of the world, as this bacteria can be carried in dust storms.
Fimbriae and Pseudomonas aeruginosa
Pseudomonas aeruginosa is a widely distributed opportunistic pathogen, meaning that it rarely causes disease in healthy individuals, but given the opportunity, can infect just about any tissue. This opportunist is estimated to be the causative agent of 10% of nosocomial infections (hospital acquired infections) and is particularly a problem for cystic fibrosis patients, burn victims and those with compromised immune systems. P. aeruginoa also contributes to infections associated with internal medical devices and indwelling catheters.
P. aeruginosa has fimbriae as well as a number of other virulence factors, including toxins, enzymes and a bacterial capsule. The capsule contributes to the bacteria's ability to adhere to cells and also prevents it from being destroyed by phagocytic cells of the host’s immune system.
The fimbriae of P. aeruginosa are similar in structure to those of N. gonnorhoeae, but also include a bacterial enzyme, called neuraminidase that enhances its ability to attach to host cells.
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