Many bacteria, particularly those with Gram- cell wall structure, have external appendages; delicate strands of protein that extend beyond the surface of the cell. These external structures of bacteria can come in many forms. Long, thin flagella are used by some bacteria to move about. Shorter extensions, called fimbriae, enable bacteria to adhere to surfaces and potentially infect the cells of their host. Some bacteria have other extensions called sex pili, which allow them to share some of their genes with each other. This article focuses specifically on the structure and function of fimbriae (singular: fimbria).
Gram- Bacterial Fimbriae
Most Gram-negative bacteria have these short, fine appendages surrounding the cell. In contrast, Gram-positive bacteria never have fimbriae.
Like flagella, fimbriae may extend from only the poles (ends) of a bacterium or may be distributed over the entire surface of the cell. A bacterium may have just a few of these extension, or several hundreds.
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.
Sticky Bacterial Biofilms
Bacteria that are able to stick to each other and to surfaces can form biofilms. A biofilm is essentially a living layer of slime. Both fimbiae and glycocalyces can have a role in a bacterium’s ability to form biofilms. These slimy layers of bacteria are found widely throughout nature. Examples include the slimy, stinky yuck mouth that a person wakes up with in the morning, the slippery sheen that accumulates on river rocks, and the slimy build-up that can occur in infrequently cleaned toilet bowls.
How Fimbriae Help Bacteria Cause Disease
Fimbriae are also 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 enable 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.
Microbiology Sources
For more information on microbiology, see the Virtual Microbiology Classroom or the following sources:
Bauman, R. (2007). Microbiology with Diseases by Taxonomy. Pearson Benjamin Cummings.
Perry, J. and Stanley, J. (1997) Microbiology: Dynamics and Diversity. Saunders College Publishing.
Tortora, G., Funke, B. and Case, C. (2010) Microbiology: An Introduction. Tenth Edition. Benjamin Cummings
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