Crash Course: Biology

Bacteria: Structures, etc.

Peptidoglycan (sometimes called murein) surrounds and protects the bacterial cell. Its chemical structure is a repeating disaccharide, which is attached with polypeptides to form a lattice.

There are two main classifications of bacteria, gram-positive and gram-negative.

        Gram-positive bacteria produce exotoxins when they die, and in the Gram Staining Test, the final result is purple because the peptidoglycan cell wall holds in the crystal violet stain better than Gram-negative bacteria, which produce endotoxins when they die, and in the Gram Staining Test, they end up pink after the counter-stain safranin is applied due to the outer layer on the cell wall outside of the peptidoglycan layer. 

The Gram Staining Test is a test that determines major characteristics of a species of bacteria based on the cell wall structure.

        First, a Crystal Violet stain is applied.
        Then, a Mordant, most commonly Gram's Iodine is applied to help the stain stick.
        After that, the stain is washed with ethanol alcohol, and
        Finally, a counter-stain, such as safranin is applied.

If the bacteria was gram-positive, then the bacteria would be clearly stained purple after the alcohol wash. If it was gram-negative, it will appear pink under the microscope after the conter-stain is applied.

The fimbriae, often numerously distributed across the bacterial cell, is an appendage used for attachment, since it allows the cell to adhere to the surface of other cells. To cause disease, fimbriae are often present to attach to cells and cause discomfort, such as in the case of strep throat.

The pili are used most often in conjugation, the process of transferring DNA from one bacterial cell to another. The pili that do this are often called conjugation pili or sex pili.

Both pili and fimbriae consist of the protein pilin.

In terms of flagellum, bacteria can be classified into four types:

        Peritrichous, which has flagellum coming out from every side of the bacteria,
        Monotrichous, which has a single flagellum out of one side,
        Amphitrichous, which has a single flagellum out of both sides, and
        Lophotrichous, which is a tuft of flagellum, either out of one or both ends.

A flagellum (in a prokaryotic cell) has three basic parts: the Filament, the Hook, and the Basal Body.

The filament is composed of the protein flagellin and is a hollow tube that is 20 nanometers thick. It's a helical structure.
The hook is the sharp bend just outside the outer membrane which allows the helix to point directly away from the cell.
The basal body (also know as the kinetosome or the blepharoplast), is  a rod and a system of rings spinning at 100,000 rpm, which propels the bacterium.

In spirochetes (corkscrew shapes), the structure for motility is the axial filament.

When a bacteria moves, it often follows a "run" and "tumble" cycle. When a bacterium moves in one direction (straight), the movement is called a "run". When it abruptly stops to turn into another direction, the movement is called a "tumble". A series of "runs" and "tumbles" are often required for a bacterium to reach its intended destination, often provoked by taxis, often called a stimulus.

A few common types of taxis are phototaxis (light), chemotaxis (chemicals), magnotaxis (Earth's Magnetic Field), and oxytaxis (oxygen; if aerobic).

a/n: still technically a work in progress, I probably won't be adding too much to it, though. If I made any errors, feel free to comment and tell me, and ask any questions you might have. I kind of glossed over most of it because I'm fairly comfortable with the material.