Re-posting: Fine structure of Bacteria

Focus topics: Structure present outside the cell wall structure of bacteria

Bacteria (The True Bacteria or Eubacteria)

The bacteria constitute a very wide group of micro-organisms that exhibit a fascinating diversity in morphology, habitat, nutrition, metabolism, and reproduction. Although they are not very complex morphologically, the tiny bacteria nevertheless have highly complex physiological, biochemical, cytological, and genetical characteristics making them a valuable tool for understanding the various intricacies of life. Due to their extreme simplicity in structure, small size favoring rapid cell division, highly resistant nature and diversified mode of nutrition, bacteria are of universal occurrence. They are present in our mouth and flourish in intestine. They are present in air we breathe and in food we eat; they abundantly occur in fresh and salt water, soil water and even in ice. Their most favorable habitat is soil, where they occur in abundance mainly in the upper half feet. In a handful of garden soil, the bacterial population may outnumber the human population on the earth.

They live in all conditions not fatal to living beings and are among the most numerous of all living beings present in almost every conceivable environment. Some bacteria are deadly parasites of plants, animals and human beings; some live as mutualists with plants or as commensals in the alimentary canals of animals. Some bacteria may remain viable when cooled upto -190°C, while others may remain viable when boiled upto 78°C.

Morphology of Bacteria

The following categories of bacteria are recognized on the basis of diversity in their morphologi­cal features

1. Unicellular Bacteria

2. Filamentous Bacteria

3. Myxobacteria

4. Sprichaete

Unicellular Bacteria

Cocus

These -unicellular bacteria are spherical, varying 0-1.25 μ in diameter and existing either singly (Micrococcus), in pairs (Diplococcus), in chains (Streptococcus), in clusters (Staphylococcus), or in cubical masses of 8 or more cells (Sarcina).

Bacillus

These unicellular bacteria are hyphen (-) or small rod-shaped ranging about 1.5 J1 in diameter and 10 μ in length. They occur either singly (Microbacillus), in pairs (Diplobacillus), in chains (Streptobacillus), or in palisade arrangement. Bacillus anthracis, B. subtilis, Lactobacillus and Clostridium are the common examples of bacilli bacteria.

Characteristic groupings of cocci; A. micrococcus; B. diplococcus; C. streptococcus; D. tetracoccus 

E. sarcina; F. staphylococcus.

Vibrio

When the bacilli bacteria are so curved that they look like a comma, they are called vibrios. They seldom exceed 10 J.1 in length and 1.5 to 1.7 J.1 in diameter, e.g., Vibrio comma.

Spirillum

When the bacilli bacteria are coiled like a cork-screw through 1-5 complete turns, they are referred to as Spirilla. They range from 10-50 μ  in diameter. e.g. Spirillum undulum and S. volutans.

Stalked Bacteria

These are unicellular bacteria having well defined stalks. In some cases, the stalk is a part of the cell (Caulobacter), in others it is formed as a result of secretion from the cell (Gallionella). Usually these bacteria have sticky, knob-like base that join each other forming a rosette-like structure.

Budding Bacteria

These unicellular bacteria are globose having a small, thin tube-like structure. The whole call looks like a foot-ball. The tubular structure elongates and swells forming a new cell. This process results in a network of globular cells, e.g., Rhodomicrobium.

Filamentous Bacteria

Some bacteria have filamentous and branched mycelial body and for a long time they were considered to be fungi. It was the prokaryotic nature of their cells that enabled the microbiologist to put them under bacteria. These filamentous bacteria are called 'Actinomycetes' and vary from 1-5μ in diameter. Some actinomycetes are pathogenic to man, while others cause important plant diseases. Mostly these are present in soil. Actinomycetes are very important bacteria as they are one of the most important sources of antibiotics, e.g., Mycobacterium, Actinomyces, Streptomyces, Actinoplanes etc.

Morphological types of bacteria, A. cocci; B. bacilli; C. vibrios; D. spirilla; E. stalked bacteria [(i) rosette-like; (ii) single bacterial cell)]; F. budding bacteria; G. filamentous bacteria; H. myxobacteria [(i) fruiting bodies bearing cysts; (ii) germinating cysts releasing cigar-shaped cells)]; I. and J. spirochaetes (helical and spiral) .

Myxobacteria

The myxobacteria love mostly soil though they are also present in dung and water. The cells of myxobacteria are cigar-shaped and usually form a colony in a common slimy mass. They are peculiar for their 'gliding movement' as they lack flagella and hence also called gliding bacteria. Though some of the gliding bacteria do not form any fruiting body, others form. At the time of fruetification the cells come closer and heap-up. The mucilaginous covering around them becomes hard and the whole structure looks a tree with the branches hearing brightly coloured oval or spherical cysts. In each cyst hundreds of bacterial cells are present which glide out when the cyst-wall ruptures. Examples of myxobacteria are, Beggiatoa, Chondromyces.

Spirochaete

Spirochaetes are the bacteria having spiral-shaped body but lacking a rigid cell wall. They measure 3-495μ in length; are flexuous and motile, lacking flagella and the movement is spinning or whirling brought about by flexions of the body. The flexion is caused by contraction of fibrils called crista. Each end of crista is anchored in the cytoplasm. Spirochaetes are found in fresh, sea and polluted waters. They divide by binary fission and do not produce resting spores. Examples of spirochaetes are, Cristispira, Treponema, Spirochaeta etc.

Nanobacteria

Nanobacteria are very minute bacteria existing in nature. The size of putative nanobacteria is considered to be of 0.1 mm diameter for coccus-shaped structures. Opponents of the existence of nanobacteria claim that these are simply the artifacts of chemical or geochemical reactions of non-living materials and that even the smallest bacterial cells are significantly larger than reported nanobacteria. Opponents also argue that if one considers the space needed to store all of the essential biomolecules of life, it is highly unlikely that these could arrange themselves within the volume available to a structure of 0.1 μm or less. Therefore, the very existence of nanobacteria is controversial. If such very small cells do exist, they would represent the smallest known living structures.

Size of Bacteria and the Significance of Their Being Small

Bacteria vary in size from cells as small as 0.1-0.2 μm in diameter to those more than 50 μm in diameter. The dimensions of an average rod-shaped bacterium, Escherichia coli, for example, are about 1 x 3 μm. For comparison, typical eukaryotic cells may be 2 µm to more than 200 μm in diameter. Bacteria are thus extremely small in comparison to eukaryotes.

Surface area and volume relationships in speres

Small size of bacteria (and almost all other prokaryotes) affects a number of their biological properties. For convenience, the rate at which the nutrients are taken in and wastes area passed out of a cell is in general inversely proportional to cell size. This is because transport rates are to some degree a function of the membrane surface area available; small cells have more surface area available than do large cells. As a cell increases in size, its surface area-to-volume (SN) ratio decreases. The surface area-to-volume (SN) ratio of a sphere (cell) is expressed as 3/r.A small cell having a smaller r (radius) value has a higher SN ratio than a larger cell and, therefore, can enjoy more efficient exchange of nutrients with its surroundings than can a large cell, show more rapid growth rates and the formation of larger cell populations. The parameters of rapid growth and larger cell populations greatly affect microbial ecology. It is so because high numbers of rapidly metabolizing cells can cause major physio-chemical changes in an ecosystem even over very short periods of time.

The Largest Known Bacterium

Heidi Schulz (1997) has discovered the largest of all known bacteria, Thiomargarita namibiensis from the ocean sediment off the cost of Namibia. This bacterium is spherical often forming chains of cells and ranging between 100-750 mm in diameter. It is considered to be over 100 times larger in volume than Epulopiscium fishelsoni (a bacterium related to. the gram positive genus Clostridium about a million times larger in volume than Escherichia coli and discovered by Fishelsion et al. in 1985). T. namibensis possess a vacuole that occupies 98% of the cell volume and is surrounded by a 0.5-2.0 mm layer of cytoplasm filled with sulphur granules

Flagellation
The pattern of flagellar arrangement (flagellation) is a good identification mark in bacteria. Flagella are either confined to the pole or poles or it may be present alround the body of the bacterium. However, bacteria can be grouped as under on the basis of flagellation:

Bacterial Flagellation

A. Atrichous	B.Monotrichous	C. Flagellation	D. Amphitrichous	E.Lophotrichous	F.Peritrichous

Atrichous - Bacteria that lack flagella. E.g. Staphylococcus aureus

Monotrichous - Single flagellum on either of the poles of the bacterial cell. E.g. Pseudomonas aeruginosa

Amphitrichous - One flagellum or more on each pole of the bacterial cell. E.g. Spirillium volutans

Lophotrichous - Flagella in groups present on one pole of the bacterium. E.g. Pseudomonas florescens

Peritrichous - Flagella present all around the body of the bacterial cell. E.g. Salmonalla typhi

Flagellar Movement

Bacterial (prokaryotic) flagella operate in different manner when compared to eukaryotic flagella. Each individual flagellum is a semi-rigid helix and moves by rotation like propellers on a boat (Fig. 3.4). The rotatory motion of the flagellum is imparted from the motor at the base. A rod or shaft extends from the hook and ends in the M-ring which can rotate freely in the plasma-membrane. Though the exact mechanism of flagellar rotation still is not clear, it is believed that the S-ring is attached to the cell wall in gram-positive bacterial cell and does not rotate. The P and L-rings of gram­-negative bacteria act as bearings for the rotating rod.

The energy required for the rotation of the flagellum comes from the proton motive force. (PMF), not directly by ATP as is the case with eukaryotic flagella. The plasma membrane becomes energetically charged and during this state protons (H+) are separated from hydroxyl ions (OH-) across its surface. This charge separation is a form of energy. This energized state of plasma membrane is called proton motive force. Calculations have shown that about 1000 protons must be translocated per single rotation of the flagellum.

Monotrichous and lophotrichous polar flagella rotate counter-clockwise and make the bacterial cell spring around and run forward from place to place with the flagellum trailing behind. These bacteria stop and tumble randomly by reversing the direction of flagellar rotation. The flagella of peritrichous bacteria rotate counter clockwise, like montrichous and lophotrichous ones, to move forward. The flagella bend at their hooks to form a rotating bundle that propels them forward. Clockwise rotation of the flagella disrupts the bundle and the cell tumbles

A bacterial cell can run in water from 20 to almost 90 μm/second which is equivalent to travelling from 2 to over 100 cell (body) length/second. In contrast, an exceptionally fast 6 ft man can run around 5 body length/second and a cheetah (the fastest animal) can run about 25 body lengths/second. Thus, when size is accounted for, bacterial cells run actually much faster than larger organisms:

Flagella (Sing. Flagellum)

These are long filamentous organs of locomotion that arise from the cytoplasmic membrane and pass out through the cell wall. A flagellum of bacteria cell consists of three distinct parts-the basal body, the hook, and the filament. The basal body constitutes the extreme basal part of the flagellum attached with the plasma membrane; the hook represents a somewhat broader and thicker basal region of a flagellum and passes out through the cell wall; and the filament is the thinner, elongated, terminal part. However, the structure of the bacterial flagellum allows it to spin like a propeller and thereby helps moving the bacterial cell

Ultrastructure of a typical bacterial cell (diagrammatic)

1.	Flagellum	9.	RNA
2.	Pilus	10.	Nucleoid
3.	Slime Layer	11.	Gas Vacuole
4.	Cell Wall	12.	Poly-β-Hydroxy-Butyric Acid
5.	Cyoplasmic Membrane	13.	Thylakoids (lamellae)
6.	Chromatophores (vesicles)	14.	Polyribosome
7.	RiBosomes	15.	Plasmid
8.	Mesosome	16.	Metachromatin Granules (Volutin Granules)

A. Motion of Monotrichous Polar Bacterium	B. Motion of Peritrichous Bacterium

Figure:  Flageller motility. A. motion of monotrichous polar bacterium, B. motion of peritrichous bacterium

Figure: Bacterial Flagellum. A. morphological views, longitudinal view of flagellin molecule chains arranged in 8- rows. C. cross-section of the flageller filament

Filament

The filament is a fine, cylindrical, helical hollow structure, about 120-200Ǻ in diameter. It is composed of a fibrin protein called 'flagellin' structurally similar to keratin and myosin proteins and ranging in mol. wt. from 30,000 to 60,000 dalton. A cross section of the filament reveals that there are eight flagellin molecules surrounding a central hollow cylinder. Actually, the 8 flagellin molecules seen in the cross section are the parts of flagellin molecule-chain eight in number and running longitudinally around the central hollow cylinder. Each chain contains approximately 1,000 spherical, smaller flagellin molecules each of 40 Ǻ diameter. In this way, the bacterial flagellum fundamentally differs from the flagellum of an eukaryotic cell, which has 9 + 2 type of arrangement in its filament.

Hook
As said earlier, hook is a somewhat broader and thicker basal region of a flagellum and passes out through the cell wall. It is made up of a single type of protein and functions to connect the filament to the basal body of the flagellum

Basal Body (The Motor Portion of the Flagellum)

The basal body is the most complex part of a flagellum. In gram-negative bacteria, the basal body has four rings connected to a central rod. The outer L and P rings remain embedded in the lipopolysaccharide and the peptidoglycan layers respectively. The inner Sand M rings are located within the cytoplasmic membrane. In gram-positive bacteria, which lack the outer lipopolysaccharide layer, only the inner pair of rings is present. There are a pair of proteins called Mot A and Mot B that surround the inner ring and are associated to the cytoplasmic membrane. In addition to Mot proteins, there is a final set of other proteins called Fli proteins.

Actually, the portion of the basal body that rotates (the motor) is consisted of the rod, the M-ring, and the Mot and Ph proteins. The Mot proteins actually drive the motor causing a torque that rotates the filament whereas the Fliproteins function as the motor switch reversing rotation of the flagella in response to signals send by the bacterial cell.

Details of the flagellum of gram-negative bacterium showing mechanism of flageller movement

1.	Filament	6.	Peptidoglycan Layer	11.	Mot A
2.	Hook	7.	Periplasmic Space	12.	Fli Proteins
3.	L-Ring	8.	Plasma Membrane	13.	Mot Proteins
4.	P-Ring	9.	S-Ring	14.	M - Ring
5.	Outer Membrane	10.	Mot B	15.	H

Pili, Fimbriae and Spinae

Some bacteria possess fine hair-like projections on their surface. These fine projections are called pili or fimbriae and spinae, and originate from the cell membrane. The term pili was introduced by Brinton (1950) and fimbriae by Duguid et al. (1955). Both these are structurally similar to flagella, but are not involved in motility. They have been observed mostly in gram-negative bacteria, and measure 3-25 nm in diameter and 0.5-20 (mu) m in length. Both are made up of protein (pilin) molecules with a molecular weight of about 17,000

Pili and fimbriae

Fimbriae are considerably shorter than flagella and are more numerous. The functions of fimbrae are not known with certainty, some evidences suggest that they enable microbes to stick to surfaces of host in the case of pathogenic bacteria, or to form pellicles or biofilms on their surfaces.

Pili are generally longer than fimbriae and only one or a few pili are present on the surface. They genetically determined and their number and type vary in different strains. Some are used for attachment; pathogenic bacteria, for example, use them to identify and to attach to their host cells.

Others, the F or sex pili, are involved in bacterial conjugation and are found exclusively on the cells that donate DNA during this process. If these pili are absent or if the bridge established by sex-pili between the donor and recipient cell is interrupted, the conjugation process is not completed. Pili, however, also act as receptor sites and provide site for attachment for some bacteriophages on the bacterial cells.

In some gram-positive bacteria, spinae have been reported. These are tubular, pericellular, non-­prosthecate rigid hairy-appendages said to help adjust bacterial cells to some environment conditions such as pH, salinity, temperature etc. Spinae are made up of a single protein, namely, spinin.

The Glycocalyx : Slime Layers And Capsules

At the time of their active growth, many bacteria produce polysaccharide-containing substances of high molecular weight. These substances collect on the surface of the cells and form a gelatinous covering around them. This covering is called glycocalyx. When the glycocalyx does not form a persistent layer, but is present more diffusedly forming a loose mass around the bacterial cell, it is called slime layer. The slime layer can very easily be removed by washing the bacterial cells. When the gelatinous covering forms a well-defined persistent layer, it is called capsule.

Capsules are composed generally of polysaccharides (e,g., Streptococcus mutans, S. salivarious. Xanthomonas. Corynebacteria) that contain, apart from glucose, aminosugars, rhamnose, uronic acids of various sugars, 2-keto-3-deoxygalactonic acid, and organic acids such as pyruvic acid and acetic acid. However, the capsules of some bacilli bacteria (e.g., Bacillus subtilis, B. anthracis) consist of polypeptides, mainly poly glutamic acid. Capsules can be seen by negative staining with dyes that do not penetrate the capsular material, such as India ink, Chinese ink, nigrosin or Congo red. In negative staining, the capsule appears light against a dark background. Slime layers and capsules may protect the bacterial cell against dehydration and a loss of nutrients. In addition, the capsule is especially important in protecting bacterial cells against phagocytosis by various protozoa and white blood cells and, therefore, capsulated bacteria usually prove to be virulent pathogens in comparison to non capsulated ones which are easily subjected to phagocytosis by WBCs.