2 - Bacterial Ultrastructure

Abstract

Bacteria are small, prokaryotic cells, generally of the size of mitochondria. A variety of bacterial shapes can be observed under the light microscope, including cocci, rods, spiral, and even cubes. The inner surface of the cell wall is in contact with a cytoplasmic membrane within which there is the cytoplasm. This contains a nuclear region of DNA, ribosomes and various inclusion bodies. Various surface appendages are present on the outer surface of the bacterial cell wall, which are concerned with the relationship between the bacterium and its external environment. Flagella are the key structures concerned with bacterial motility. Flagella can be located singly at one cell pole (monotrichous flagella), at both poles (amphitrichous flagella), in large numbers along the length of the cell (peritrichous flagella), or as a tuft of flagella at a polar end (lophotrichous flagella). S-layers are found in many types of bacteria, including Gram-positive and Gram-negative bacteria, cyanobacteria, and archaebacteria, and are composed of crystalline arrays of protein or glycoprotein subunits. In Gram-positive organisms, the S-layer lies on the outer surface of the peptidoglycan layer, while in Gram-negative bacteria it is present on the outer surface of the outer membrane.