Anatomy of Bacterium

ANATOMY OF BACTERIA CELL

Any bacterial cell whether it is a coccus or a bacillus will have some structures common. These structures are cell wall, cell membrane, cytoplasm, ribosomes and the chromosome. Other intra-cellular structures such as plasmid, inclusion bodies and extra-cellular structures such as capsule, fimbriae and flagella are possessed only by some bacteria.

Glycocalyx/Capsule/Slime:

A gelatinous polysaccharide or polypeptide outer covering of certain bacteria is called glycocalyx. These are the structures that surround the outside of the cell envelope. The glycocalyx is referred to as a capsule if it is firmly attached to the cell wall, or as a slime layer if loosely attached.

The chemical nature of bacterial capsules is diverse but majority of them are polysaccharides. These polymers are composed of repeating oligosaccharide units. However, the capsule of Bacillus anthracis is composed of a polypeptide (polyglutamic acid). Yersinia pestis produces a capsule of mixed amino acids. Capsules may be weakly antigenic to strongly antigenic, depending on their chemical complexity. Capsules may be covalently linked to the underlying cell wall or just loosely bound to it. They have no net charge and will not bind charged dye particles, hence they can’t be stained. Bacteria with capsules form smooth (S) colonies while those without capsules form rough (R) colonies. A given species may undergo a phenomenon called S-R variation whereby the cell loses the ability to form a capsule. Some capsules are very large and absorb water (e.g., Klebsiella pneumoniae) forming

mucoid (M) colonies. Capsules are often lost during in vitro culture. They are not essential to cell viability and some strains within a species will produce a capsule, while others do not. Capsules are sometimes referred as K antigens (in Enterobacteriaceae) or as VI antigen (in Salmonella typhi). Capsular antigens may either be specific to a species or may be shared by few different...