3. Bacteria

the basic facts

Questions

Which cell structure differs between Gram-negative and Gram-positive bacteria?

Which structure is used for conjugation?

Bacteria are divided into two main groups on the basis of chemical staining (with the Gram stain) and light microscopy: Gram positive and Gram negative. The differential staining is based on a major difference in the cell wall (Fig. 3.3.1). Both have a lipid bilayer cytoplasmic membrane with various inserted proteins, the most important of which are membrane-spanning permeases. These control active transport of nutrients and waste products in and out of the cell. A complex web of cross-linked peptidoglycan outside the cytoplasmic membrane provides the cell with mechanical strength. It is particularly abundant in Gram-positive organisms, when it also contains strands of teichoic and lipoteichoic acid. Gram-negative organisms have an additional outer membrane enclosing a thin layer of peptidoglycan and enzymes within the periplasm, an environment controlled by the movement of substrates through membrane porins. Some bacterial species may have a loosely adherent polysaccharide capsule exterior to the cell wall and possibly additional structures which project from the cell surface (Fig. 3.3.2):

flagellae: whip-like cords, typically 0.02 μm thick, 10 μm in length; cells move by rotating the flagellae

fimbriae (pili): hair-like structures typically 6 nm thick, 1 μm long; they carry adhesin proteins that enable the specific attachment of the bacterium to its target