Capsules & Slime layers

Capsule

Capsules are the well organized, not easily washed off layer lying outside the bacterial cell. They are tightly bound to the cell wall and they are found in both Gram positive and Gram negative bacteria. They are generally made up of polysaccharide but they may contain other materials like Bacillus anthracis has D-glutamic acid (a polypeptide); Escherichia coli contains peptidoglycan and muramic acid.

A capsulated bacteria is resistant to phagocytosis by host phagocytic cell. They help cells to adhere to solid surfaces and host tissues. Capsules contain water and hence protect bacteria against desiccation. They also protect bacteria against viruses and hydrophobic toxic materials like detergents.

Slime layer

A slime layer is a less compact, unorganized, easily removable film that surrounds the bacterial cell wall. They are usually made up of exopolysaccharides, glycoproteins and glycolipids. They help bacteria to adhere to smooth surface such as catheters. They provide resistance to phagocytosis as well. They act as a protective layer against antibiotics, desiccation and chemicals like chlorine, iodine and many others. Flushing with boiling water or autoclaving are the only means to remove slime layer containing bacteria.

Difference between capsule and slime layer

Characteristics	Capsule	slime layer
Composition	polysaccharide	exopolysaccharide, glycolipids and glycoproteins
Thickness	thick	thin
Binding	tightly bound to the cell wall	loosely bound to the cell wall
Organization	well organized; not easily washed off	unorganized layer; can be easily washed off
Function	mainly evades phagocytosis	mainly helps in adherence

Ribosomes

Prokaryotic ribosome

Ribosomes are small but complex molecular machine that contains both protein and ribonucleic acid (RNA). They are generally found in the cytoplasmic matrix of a bacterial cell; and some of them are bound to the plasma membrane. They are the site of protein synthesis i.e. translation (links amino acids together in the sequence specified by mRNA to form protein). Those proteins that remain within the cell are synthesized by matrix ribosomes, whereas plasma membrane ribosomes synthesize those proteins that are transported outside.

Prokaryotes have 70S ribosome, consisting of a large (50S) and a small (30S) subunit. They are about 20 nm in diameter and molecular weight is about 2.7 million. The S in 70S stands for Svedberg unit. This unit is a measurement of sedimentation in centrifugation i.e. greater the Svedberg value, faster the particle travel during centrifugation. Also, heavier and compact particles have higher Svedberg number. The 50S subunit consists of 5S rRNA (ribosomal RNA), 23S rRNA and 31 proteins whereas 30S subunit contains 16S rRNA and 21 proteins.

Eukaryotic ribosome

Eukaryotic ribosomes are larger than prokaryotic ribosomes; they are 80S ribosomes and are about 22nm in diameter with 60S and 40S subunit. Their molecular weight is about 4 million. The 40S subunit contains 18S rRNA and 60S subunit contains 28S rRNA, 5.8S rRNA and a short 5S rRNA. They can be free in the cytoplasmic matrix or can be associated with endoplasmic reticulum. Endoplasmic reticulum bound to ribosomes are called rough endoplasmic reticulum (RER) and those without ribosomes are called smooth endoplasmic reticulum (SER). In RER, ribosome is attached through its 60S subunit.

Proteins are synthesized by both free and endoplasmic bound ribosomes. Nonsecretory and nonmembranous proteins are synthesized by free ribosomes. Ribosomes bound to ER synthesize protein that either inserts into ER as integral membrane protein or enters into lumen for transport and secretion. These synthesized proteins are mostly functionally inactive and they require proper folding. Molecular chaperone aids this process and make them functionally active and also prevents them from aggregating into nonfunctional structures. Chaperones also help in the transport of protein into eukaryotic organelles like mitochondria. Generally during translation, multiple ribosomes are linked to a single mRNA at different locations and polypeptide synthesis occurs. This complex of mRNA and multiple ribosomes is known as polysome or polyribosome.

Fluid mosaic model

Fluid mosaic model was proposed in 1972 by S. Jonathan Singer and Garth L. Nicholson. It is a widely accepted model to explain the structural and physiological nature of the cell membrane. The model states that (1) Membranes are formed from both lipids and proteins and (2) Membrane proteins are of two types: peripheral protein and integral protein. The structure-forming unit is a lipid bilayer. Integral proteins are buried in the bilayer and peripheral proteins are attached on the surface.

Peripheral proteins are one that are loosely connected to the membrane surface and can be easily removed by mild treatment. They makes up 20 to 30 % of total membrane protein and are soluble in aqueous solutions. Integral proteins are buried in the lipid and they makes up 70 to 80 % of membrane protein. They can’t be extracted easily from the membrane and are insoluble in aqueous solutions. They are amphipathic in nature; with hydrophobic region buried in the lipid and hydrophilic region projects from the membrane surface.

S-Layer

S-layer is a regularly structured layer that is found in many Gram positive and Gram negative bacteria. S-layers are present on the surface of the bacteria and in almost all archaea and they are composed of protein or glycoprotein. In Gram positive bacteria, it is associated with peptidoglycan; and in Gram negative bacteria, it is linked to the outer membrane. It maintains the shape and rigidity of bacterial cell and provides protection against osmotic stress, enzymes (for instance: lytic enzyme), ions and pH fluctuations and predacious bacteria like Bdellovibrio. It also provides adhesion property to the cell and helps them to attach to the surfaces. S-layer is responsible for the virulence of some pathogens by protecting them against phagocytosis and complement attack.

Flagella

The motility of the bacteria is due to flagella (singular flagellum). They are found in both Gram positive and Gram negative bacteria. Flagella are long, filamentous surface appendages that are 15 to 20 μm long and about 20 nm thick.

Bacterial species are characterized on the basis of the number and pattern of flagella distribution on their surface. Based on this, bacteria may be one of the following:

Monotrichous (mono means single; trichous means hair): They have one flagellum for example Vibrio cholera. If flagellum is located at an end, it is called polar flagellum.

Amphitrichous (amphi means on both sides): They have a single flagellum attached at each end for example Alkaligens faecalis.

Lophotrichous (lopho means tuft): They have cluster of flagella at one or both the ends for example Spirillum.

 

Peritrichous (peri means around): Flagella are spread all over the surface for example Salmonella typhi.

Atrichous: Having no flagella for example Diptheriae bacillus.

Structure of flagella

A bacterial flagellum is composed of three parts: filament, basal body and hook. The long filament lies external to the cell surface; basal body is embedded in the cell; and hook connects filament and basal body.

The flagellar filament is a rigid cylindrical structure with a hollow core and is made up of a single protein called flagellin. A capping protein is found at an end of filament. In some bacteria, the flagella are surrounded by sheath like Vibrio cholera has lipopolysaccharide sheath.

The hook are short, curved and are flexible unlike filament. The basal body comprises of four rings, which are connected to a central rod as seen in E.coli and other Gram negative bacteria. These four rings are M (for membrane), S (for supramembranous), P (for peptidoglycan layer) and L (for lipopolysaccharide). But later study has shown that M and S ring contains different domains of same protein and they function as a unit (MS ring or M ring). A fourth ring is present inside the cell i.e. C ring (for cytoplasmic). Gram positive bacteria contains only two rings; one inner ring that is connected to the plasma membrane and an outer ring that is attached to the peptidoglycan.

Mechanism of Flagella movement

Several studies have shown that flagella acts like a propeller on a boat. Bacteria generally moves when the rigid filament rotates and the kind of bacterial movement is determined by the flagellar motion. Flagella can rotate clockwise or counterclockwise. When flagella rotates counterclockwise, bacterial cell will slowly rotate clockwise and they run forward. Whereas, when flagella rotates clockwise, cells tumble and forward motion ceases.

When flagella rotates counterclockwise, they basically bends at their hooks and forms rotating bundle that help them to propel forward. On the contrary, clockwise motion disrupts the bundle and leads to the tumbling of cell.

A motor is present at the base that helps in the rotation of rigid flagella. A rod can rotate freely in the plasma membrane and it extends from the hook and ends in the M ring. The rotor portion of the motor consists of a rod, M ring and C ring. C ring is joined to the rotor on the cytoplasmic side of the basal body. These two rings are made up of several proteins; in particular Fli G. Fli G is involved in flagellar rotation. Mot A and Mot B are two membrane proteins that are present at the stator part of the motor. Mot A and Mot B forms a proton channel through plasma membrane. Mot B also attaches the Mot complex to the peptidoglycan. The free energy that is required for flagellar rotation is not derived directly from ATP but is derived from proton or sodium gradient across the plasma membrane. ATP is used as an energy source in eukaryotic flagella. 

The S ring does not rotate and is attached to the cell wall in Gram positive bacteria. P and L rings acts as bearings for the rotating rod in Gram negative bacteria. 

In this way, flagella helps in the movement of bacteria and bacteria can swim from about 20 to 90 m per second. Besides flagella rotation, there are other mechanisms also that can aid in bacterial movement. In spirochetes, flagella reside inside the cell within periplasmic space and are called as periplasmic flagella. Spirochetes travel in viscous media by flexing and spinning movements, caused by special axial filament. Another type of non-flagellar movement is gliding motility; which is generally seen in Cyanobacteria, myxobacteria and some mycoplasmas and helps bacteria to move on solid surfaces.