Fundamentals of Microbiology

1.2.2 Structure and function of prokaryotes

Membrane systems

Both prokaryotic and eukaryotic membranes are similar in overall structure. Membranes of eukaryotic microorganisms serve to compartmentalize cell contents into organelles such as mitochondria, thus allowing concentration of specific metabolites at certain locations. Prokaryotic organisms contain only a single membranous structure, the cytoplasmic membrane or plasma membrane. The cytoplasmic membrane separates the cell contents from the environment and measures 4 – 5 nm thick. The cytoplasmic membrane also constitutes the permeability barrier of the cell and allows it to concentrate desired nutrients and excrete waste products. Membranes also involve in complex biochemical processes such as respiration. All membranes are formed of a lipid bilayer, made of phospholipids. Phospholipid molecules consist of two parts, a fatty acid portion which is hydrophobic (water-repelling) and a glycerol phosphate part which in hydrophilic (water loving).

In such a bilayer, the hydrophobic fatty acid portions point towards each other and produce a hydrophobic environment. The hydrophilic parts are exposed to the aqueous external environment.

The proteins of membrane may be associated with only one side of the membrane, or may be completely embeded in the phospholipid matrix. Because of this the inner and outer sides of the cytoplasmic membrane have different properties. This property of ‘sidedness’ is of great importance in membrane function.

The overall structure of a membrane is maintained by hydrogen bonds and hydrophobic interactions. Positive charged ions (Mg²⁺, Ca²⁺) help to stabilize the structure by forming ionic bonds with negatively charges phospholipids.

Eucaryotic membranes can be differentiated from those of prokaryotes with the presence of sterols. (Sterols are flat, rigid molecules, whereas lipids are flexible). Sterols help the membrane to withstand greater physical stress. The only group of prokaryotes that contain sterols in their membranes are mycoplasmas, as they lack cell wall and the cytoplasmic membrane has to withstand all stress.

Photosynthetic membranes of bacteria include:

a. Chlorobium vesicles in Green sulphur bacteria

b. Lamellae in Purple sulphur bacteria and

c. Chromatophores chlorosomes in Non-sulphur bacteria

Cell walls

Protozoans lack a cell wall, whereas bacteria, algae and fungi have one. Bacterial cell wall is unique and two broad categories can be recognized depending upon the appearance of cells upon staining, Gram positive and Gram negative. Gram positive bacteria have a thick, single layered wall where as the Gram negative have a complex multilayered wall which is relatively thin.

Peptidoglycan layer is present in the cell walls of both Gram-positive and Gram-negative eubacteria. In Gram positive bacteria, the bulk of the wall is peptidoglycan whereas in Gram-negative it accounts for only the innermost layer and is relatively thin.

Peptidoglycan consists of alternate layers of amino sugars N-acetylmuramic acid (NAM) and N-acetylgucosamine (NAG) which are linked by bonds described as β1-4 linkages (since carbon atoms 1 and 4 of adjacent residue are involved). Short peptide chains of four amino acids are linked to the N- acetylmuramic acid (NAM). Peptidoglycan forms a three dimensional network, single bag – shaped molecule (a murein sacculus). The peptidoglycan structure is completed by interpeptide cross – bridges which join together pairs of adjacent peptide chain (normally from 3^rd amino acid in one chain to the 4^th in another chain).

There is a group of bacteria (Archaea) which lack peptidoglycan, but their cell wall structure is made of any of the six different types namely pseudopeptidoglycan, polysaccharide, sulphated polysaccharides, glycoprotein, proteins and a unique cytoplasmic membrane. Under unfavourable conditions (eg. Nutrient depletion) structural changes take place inside the cell. It is characterized by thickening of the cytoplasm in a certain region and the formation of a forespore, which becomes surrounded by a thick poorly permeable multilayered wall. The rest of the cell gradually disappears and a spore is produced.

Gram positive cell walls also contain large amounts of another polymer called teichoic acid, made up of glycerol or ribitol joined by phosphate groups. Some genera including Mycobacterium, Corynebacterium contain waxy esters of mycolic acids, which are complex fatty acids.

Among eukaryotes, algal cell wall is made fibres of cellulose which form a strong wall surrounding the whole cell. Cellulose is a straight- chain polymer of glucose. In addition to cellulose, hemicellulose (polysaccharide of glucose and other sugars) and pectin may be present. Some algae contain a number of additional polysaccharides like xylans, mannans and alginic acids. Fungal haphae have a thick, multilayered cell wall composed of chitin (fibrillar carbohydrate-based polymer).

Fimbriae and Pili:

Some bacteria possess additional hair like structures on their cell surface, called fimbriae. These are shorter than flagella but are numerous. They enable the organism to stick to a surface. Pili are similar to fimbriae but are longer and fewer in number. The function of sex pilus is to bring together two cells during the conjugation process before the transfer of genetic material where it acts as a conjugation tube. Pili of a different type are involved with the attachment of pathogenic bacteria to human tissues.

Glycocalyx (Slime / capsule)

The glycoclyx consists of a number of polysaccharides, in association with glycoprotein. It is secreted on to the outer surface of the bacterial cell wall. It may aid invasion of a host organism by binding a bacterium to a specific tissue. It also hinders the engulfing (phagocytosis) of the bacterium by a host organism’s phagocytic immune defence cells. Glycocalyx also prevents desiccation.

Cytoplasm

Cytoplasm is the entire content of the cell. It has a gel – like consistency where small molecules move rapidly. The liquid component of cytoplasm is called cytosol and 80% of it is water. Cytoplasm consists of the following components: protein and enzymes, ribosomes, storage granules, bacterial chromosome and plasmids. Bacterial cytoplasm also contains helical actin-like proteins that contribute to cell shape.

Ribosomes

Ribosome is a cytoplasmic nucleoprotein particle with RNA accounting for 2/3 of the mass. It consists of two subunits denoted 30S and 50S. Together the ribosome has a sedimentation coefficient of 70S. Ribosomes serve as the site of mRNA translation and protein synthesis. The 30S subunit has 16S rRNA and 21 proteins. 50S subunit contains 5S and 23S rRNA and 31 proteins. A typical bacterium may have as many as 15,000 ribosomes.

Nucleoid

Prokaryotic DNA is in circular form. Prokaryotes lack a nuclear envelope. Bacterial DNA is not associated with proteins, therefore it is described as ‘naked DNA’. The DNA is highly coiled (super coiled). It may be concentrated in the centre of the cell as a nucleoid. In addition to the major DNA (bacterial chromosome) some accessory DNA materials called plasmids may be present in bacteria. They are extrachromosdmal circular DNA which confers certain specific characteristics to the cell such as resistance to antibiotics and other agents.

Eukaryotic organisms possess a nuclear envelope which contains the genetic material. In eukarytotes, the DNA exists as a single linear molecule to which histones and other proteins are attached. The number of chromosomes can vary from just a few to many hundreds.

Flagella

Bacteria move by means of hair-like structures called flagella. Bacterial flagella are thin (14 – 20 nm diameter) and rigid and rotate like a ship’s propeller. Flagella are made up a protein called flagellin, which is organized into flagellar subunits. At the base of the flagellum, there is a basal body which rotates the flagellum to cause movement of the cell. Through the action of the basal body, each flagellum is caused to rotate in an anticlockwise direction at about 12000 rpm giving a speed of 20 µm to 80 µm per second. This is about 10 lengths per second. It can move up to 60 cell length / second. The rotary motion is given from the basal body (which functions as motor). Mot complex drives rotation of the flagellum deriving energy through proton movement across the membrane.

The arrangement of flagella in bacterial cell is important in classification. Accordingly bacteria are designated as follows:

A. Atrichous: Bacteria without flagella are called atrichous.

B. Monotrichous: Bacteria with single polor flagellum. Eg. Vibrio cholerae

C. Amphitrichous : Single flagellum at each end of the cell. eg. Spirillum volutans

D. Lophotrichous: Two or more (a tuft) flagella at each end of the cell. eg. Alcalegenes faecalis

E. Peritrichous: Flagella all over the entire cell. eg. E.coli