Introductory Microbiology

Bacterial structures

 
• Examination of bacterial cells reveals various component structures. Some of these are external to the cell wall. Others are internal to the cell wall. Some structures are present in only certain species; some are more characteristic of certain species than of others.

Structures external to the cell wall
Flagellar arrangement
Flagella are arranged differently on different bacteria. In polar flagellation, the flagella are attached at one (e.g, Psendousona saerugenosa) or both the ends of the cell. Occassionally a tuft (group) of flagella may arise at one end of the cell (e.g. Psendomonas fluorescens), an arrangement called lophotrichous. Tufts of flagella of this type can be seen in living cells by dark field microcopy, where the flagella appear light and attached to light colored cells against a dark back ground. In extremely large prokaryotes, tuft of flagella can also be observed by phase contrast microscopy (e.g, Aquaspirillum serpens). In peritrichous flagellation, the flagella are inserted by many locations around the cell surface (e.g., Salmonella typhi). The type of flagellation, polar or peritrichous, is used as one characteristic in the classification of bacteria as shown in the figure.

Flagella and Motility Bacterial
Bacterial flagella are hair like helical appendages that protrude through the cell wall and are responsible for swimming motility. They are much thinner than eukaryotes flagella and or cilia. Their location on the cell varies depending on the bacterial sp. and may be polar (at one end or both the ends of the bacterium) or lateral (along the sides of the bacterium).
A flagellum is composed of 3 parts, a basal body associated with the cytoplasmic membrane and the cell wall, a short hook and a helical filament which is usually several times as long as the cell. Some gram-ve bacteria have a sheath surrounding the flagellum; this sheath continuous with the outer membrane of the gram –ve bacterial cell wall. The chemical composition of the basal body is unknown but the hook and filament are composed of protein subunits arranged in helical fashion. The protein of the filament is known as flagellin.
Unlike a hair, a flagellum grows at its tip rather than at the base. Flagellin monomers synthesized within the cell are believed to pass along the hollow center of the flagellum and are added to the distal end of the filament.
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