External Structure

A capsule is gel like compound  that surrounds the exterior of some bacterial cells; the gel compound is produced inside the bacterial cell and is excreted outside it and it attached firmly to the cell exterior; in some bacteria this material is not organized and is more loosely attached; in this case it is called a slime layer; a capsule can help bacterium to survive from harmful environmental changes and provide effective protection against attack by the host defense system; in addition of the sticky capsule helps the bacterium to adhere to the cell surfaces inside the host tissues.

Flagella are work like tales that specialized extensions of bacterial cell walls; a bacterium may have one, several or numerous flagella; Flagella can be located anywhere on the bacterium and give bacterial motility - the ability to move on her own-  motility allows the bacterium to move into more favorable environment; the response of senses located in the cell wall by specific stimuli produces taxis - movement toward or away from the stimulants - the majors responses of motile bacteria are chemotaxis (the response to chemicals stimuli) and phototaxis the (response to the light); these responses control the speed and direction of the flagellum rotation to create different movement paths; the cells can move ahead in constant rate or they can tumble, turn and continue on another path.

The cell walls of some bacteria have hale-like appendages called Fimbria and Pili these appendages are shorter and thinner than flagella and more like bristles and function in cellular attachment rather than cell movement; fimbria present in multiple numbers and found either in each end of the cell or distributed eventually along the entire cells surface; fimbria helps the bacterium to adhere to tissues surfaces within the host.

Pili are longer than fimbria and they are present either singly or in pairs on a single bacterium;

Their propose is to enable one bacterium to adhere to another during a specialized process called conjugation in which DNA is transferred between the cells.