Structure and Function of Bacterial Cells

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Chapter: Pharmaceutical Microbiology : Structure and Function of Bacterial Cells

The present section shall encompass briefly the major cellular structures usually encountered in the bacteria. Nevertheless, the various functional anatomy of these cell types would throw an ample light upon the various special activities that such cells perform normally.


THE BACTERIAL CELLS

 

The present section shall encompass briefly the major cellular structures usually encountered in the bacteria. Nevertheless, the various functional anatomy of these cell types would throw an ample light upon the various special activities that such cells perform normally.

 

The cellular structure should essentially provide the following three cardinal objectives, namely :

 

(a) a specific container to support the internal contents and to segregate it totally from the exter-nal medium,

 

(b) to store and replicate the genetic information, and

 

(c) to synthesize energy and other necessary cellular components for the replication of the cell.

 

In general, the bacterial cells grossly fulfil these requirements completely ; besides, they have distinguishable characteristic features to help differentiation one from the other.

 

It is, however, pertinent to state here that extensive hurdles and difficulties were encountered by the microbiologists across the globe in carrying out the detailed cytological studies** of bacteria on account of the following vital factors, such as :

 

(i) the extremely small size (dimension) of the microorganism, and

(ii) almost optically homogeneous nature of the cytoplasm.

 

As to date, the advent of the development of complex and precisely selective staining techniques amalgamated with the magnificent discovery of electron microscope and phase-contrast microscope have contributed enormously in obtaining a far better in-depth knowledge and understanding of the ‘internal structures of bacterial cells’.

 

The various important aspects referring to the domain of the ‘bacterial cells’ shall be adequately dealt with under the following heads stated as under :

 

(i) Typical bacterial cell

 

(ii) Capsules and slimes

 

(iii) Flagella and fimbria

 

(iv) Cell envelope

 

(v) Gram-positive and gram-negative bacteria

 

(vi) Significance of teichoic acids

 

(vii) The cell membrane

 

(viii) Bacterial cytoplasm

 

(ix) Ribosomes, and

 

(x) Cellular reserve materials

 

1. Typical Bacterial Cell

 

Bacteria being prokaryotic in nature are much simpler in comparison to the ‘animal cells’. In addition to this, they have three distinct characteristic features, namely : (a) an extensive endoplasmic reticulum* ; (b) essentially lack a membrane-bound nucleus ; and (c) mitochondria.

 

Nevertheless, bacteria do possess a rather complex surface structure having a rigid cell wall that surrounds the cytoplasmic membrane, as shown in Fig. 2.7, which essentially serves as the osmotic barrier as well as the ‘active transport’ necessarily needed so as to sustain and maintain a suitable intracellular concentration of the specific ions and the metabolites.


 

Infact, the bacterial cell wall has two major roles to play :

 

(a) to protect the cell against osmotic rupture particularly in diluted media, and also against certain possible mechanical damage(s), and

 

(b) to assign bacterial shapes, their subsequent major division into Gram positive and Gram negative microorganisms and their antigenic attributes.

 

2. Capsules and Slimes

 

Invariably certain bacterial cells are duly surrounded by a viscous material that essentially forms a covering layer or a sort of envelope around the cell wall. In the event this specific layer may be visualized by the aid of light microscopy employing highly sophisticated and specialized staining tech-niques, it is known as a capsule; in case, the layer happens to be too thin to be observed by light microscopy, it is called as a microcapsule. If the layer does exist in an absolute abundance such that quite many cells are found to be embedded in a common matrix, the substance is termed as a slime.

 

In other words, the terminology capsule usually refers to the layer both intimately and tightly attached to the cell wall ; whereas, the slime coating (layer) is contrarily the loose structure which often gets diffused right into the corresponding available growth medium as depicted in Fig. 2.8 below :


 

Salient features : The salient features of capsule and slime are enumerated as under :

 

(1) These structures are not quite necessary and important for the normal growth and usual survival of the bacterial cells but their very presence grants some apparent advantages to the bacterial cells that contain these structures.

 

(2) A plethora of bacteria are incapable of producing either a capsule or a slime ; and those which can do so would certainly lose the ability to synthesize legitimately these two compo-nents devoid of any adverse effects.

 

(3) The prime interest in these amorphous organic exopolymers i.e., capsules and slimes, was to assess precisely their actual role in the pathogenicity by virtue of the fact that majority of these pathogenic microorganisms do produce either a capsule or a slime.

 

It is worthwhile to mention here that the composition of such amorphous organic exopolymers varies according to the particular species of bacteria. In certain instances these are found as homopolymers essentially of either carbohydrates (sugars) or amino acids, whereas in other cases these could be seen as heteropolymers essentially of carbohydrates/substituted carbohydrates e.g., heteropolysaccharides.

 

A few typical examples of specific microorganisms (bacteria) having a varied range of amor-phous organic exopolymers are as given below :


 

Further investigative studies on different types of organisms (bacteria) have revealed, the precise composition of a few selective capsular polymers (i.e., amorphous organic exopolymers) along with their respective subunits and chemical substances produced at the end, as provided in Table 2.6.

 

Table : 2.6. Precise Composition of Certain Capsular Polymers : Their Subunits and Chemical Substances


 

Important Points : There are five important points that may be noted carefully :

 

(i) It is still a mystery to know that on one hand in certain bacteria the exopolymers are seen in the form of capsules ; whereas, on the other they are observed in the form of slimes.

(ii) Mutation* of capsular form to the corresponding slime forming bacteria has been well established.

(iii) Structural integrity of both the capsule as well as the slime are meticulously estimated by the critical presence of distinct chemical entities.

(iv) In many cases, the capsular material is not extremely water-soluble ; and, therefore, fails to diffuse rapidly away from the cells that eventually produce it.

(v) In certain other instances the capsular material is highly water-soluble ; and hence, either gets dissolved in the medium instantly or sometimes abruptly enhancing the viscosity of the broth in which organisms are cultured respectively.

 

Functions of Capsules : In reality capsules may serve five cardinal functions exclusively de-pending upon their respective bacterial species as described under:

(a) They may afford adequate protection against temporary drying by strategically bound to water molecules.

(b) They may cause absolute blockade of attachment to bacteriophages.

(c) They may be antiphagocytic* in nature.

(d) They may invariably promote attachment of bacteria to surfaces, such as : Streptococcus mutans — a bacterium that is directly linked to causing dental caries, by means of its ability to adhere intimately onto the smooth surfaces of teeth on account of its specific secretion of a water-insoluble capsular glucan.

(e) In the event when the capsules are essentially made up of compounds bearing an ‘electrical charge’, for instance: a combination of sugar-uronic acids, they may duly help in the pro-motion of the stability of bacterial suspension by preventing the cells from aggregating and settling out by virtue of the fact that such cells having identical charged surfaces would have a tendency to repel one another predominently.

 

3. Flagella and Fimbria

 

3.1. Flagella

 

Flagellum [Pl : Flagella] refers to a thread like structure that provides motility for certain bacte-ria and protozoa (one, few or many per cell) and for spermatazoa (one per cell).

 

It has been observed that the presence of flagella strategically located on certain bacteria (miroorganisms) has been known ever since the beginning of the nineteenth century ; besides, the actual form of flagellation and motility have been exploited judiciously as a taxonomic tool in the logical classification of bacterial variants.

 

Filaments : The ‘flagella’ are nothing but surface appendages invariably found in motile bacte-ria, and appear generally as filaments having diameter ranging between 12–20 nm and length between 6–8 μm. Importantly, the diameter of the individual flagellum in a culture is normally constant ; how-ever, the length may vary accordingly.

 

Location of Flagella : The exact location of the flagella in various bacteria varies widely and specifically ; and could be either polar monotrichous or polar or bipolar or polar peritrichous as shown in Fig. 2.9 ; and the number of flagella per cell also changes with the various bacterial species.

 

Flagellar Apparatus : Basically the flagellar apparatus consists of three distinct parts, namely : (a) filament ; (b) hook ; and (c) basal granule. Importantly, the outermost structural segment of bacteria is the filament which is a fibre essentially comprised of a specific protein termed as flagellin (a subunit having molecular weight 20,000), and this is securedly attached to the basal granule with the help of the hook.

 

Interestingly, both the basal granules and the hook essentially contain certain specific proteins that are antigenically distinct from the flagellin (i.e., the protein of the filament).


 

In fact, the particular structure of the basal body comprises of a small central rod inserted strate-gically into a system of rings as illustrated in Fig. 2.10 below. However, the entire unit just functions fundamentally as a ‘simple motor’. It has been amply demonstrated and established that the meticulous growth of the flagella invariably takes place by the careful addition of the flagellin subunits at the distal end after being drifted through from the cytoplasm, obviously via the hollow core of the very flagellum.


 

Functioning of Flagella : The modus operandi of flagella are as given under :

 

(1) Flagella are fully responsible for the bacterial motility.

 

(2) Deflagellation by mechanical means renders the motile cells immotile.

 

(3) The apparent movement of the bacterial cell usually takes place by the critical rotation of the flagella either in the clockwise or anticlockwise direction along its long axis.

 

(4) Bacterial cell possesses the inherent capacity to alter both the direction of rotation [as in (3) above] and the speed ; besides, the meticulous adjustment of frequency of ‘stops’ and ‘starts’ by the appropriate movement of the flagella.

 

(5) Evidently, the flagellated peritrichal* bacteria usually swim in a straight line over moderate distances. In actual practice, these swim-across straight line runs are interrupted frequently by abrupt alterations in the direction that ultimately leads to tumbling. Therefore, the move-ment of the bacteria is believed to be zig-zag.

 

(6) It has been observed that the phenomenon of smooth swimming in a fixed direction is invari-ably mediated by the rotation of flagella in an anticlockwise direction ; whereas, the process of tumbling in a zig-zag direction is usually caused by the rotation of flagella in a clockwise direction.

 

(7) The presence of ‘polar flagella’ in bacteria affords a distinct change in the direction that usually takes place by the reciprocal alteration in the direction of rotation.

 

3.2. Fimbriae [or Pili*]

 

Fimbriae or Pili are hollow, non-helical, filamentous hair-like structures that are apparently thinner, shorter, and more numerous than flagella. However, these structures do appear on the surface of the only Gram negative bacteria and are virtually distinct from the flagella.

 

Another school of thought rightly differentiates the terminology ‘fimbriae’ exclusively reserved for all hair-like structures ; whereas, other structures that are directly and intimately involved in the actual transfer of genetic material solely are termed as ‘pili’. Likewise, the bacterial flagella that may be visualized conveniently with the help of a light microscope after only suitable staining ; and the bacterial pili can be seen vividly only with the aid of an electron microscope.

 

Salient Features of Fimbriae : Some of the important salient features of ‘fimbriae’ are as enumerated under :

 

(1) At least 5 to 6 fimbriae variants have been recognized besides the sex pili.

 

(2) Type I fimbriae has been characterized completely.

 

(3) They contain a particular protein known as pilin having molecular weight of 17,000 daltons.

 

(4) The fimbriae are found to be spread over the entire cell surface. These have a diameter of 7 nm and a length ranging between 0.5 to 2 μm ; besides, an empty core of 2 to 2.5 nm.

 

(5) The pilin subunits are duly arranged in a helical manner having the pitch of the helix** almost nearly at 2.3 μm.

 

(6) In addition to the Type-I fimbriae, the Gram-negative bacteria invariably own a special category of pili termed as the sex pili (or F-pili), the synthesis of which is predominently directed by the sex factor (or F-factor). It has been observed that the sex pili do have a uniform diameter of approximately 9 nm, and a length almost nearing between 1-20 μm.

 

(7) Very much akin to the flagella, both fimbriae and pili are observed to originate from the basal bodies strategically located within the cytoplasm. Interestingly, neither fimbriae nor pili seem to be essential for the survival of the bacteria.

 

Human Infection : It has been demonstrated that certain pili do play a major role in causing and spreading human infection to an appreciable extent by permitting the pathogenic bacteria to get strategically attached to various epithelial cells lining the genito urinary, intestinal, or respiratory tracts specifically. It is worthwhile to mention here that this particular attachment exclusively checks and pre-vents the bacteria from being washed away critically by the incessent flow of either mucous or body fluids thereby allowing the infection to be established rather firmly.

 

4. Cell Envelope

 

Extensive morphological investigations have adequately revealed that the cell envelope of the Gram-positive bacteria* is much more simpler with regard to the structure in comparison to that of the Gram-negative bacteria.**

 

For Gram-positive Bacteria : In this instance the cell envelope contains chiefly the peptidoglycan and the teichoic acids.

 

Interestingly, the peptidoglycan represents a substituted carbohydrate polymer found exclusively in the prokaryotic microorganisms.

 

It essentially comprises of two major chemical entities namely :

 

(a) Two acetylated aminosugars e.g., n-acetyl glucosamine ; and n-acetylmuramic acid ; and

 

(b) Amino acids e.g., D-glutamic acid ; D- and L-alanine ;

 

In fact, the long peptide chains containing the two amino sugars that essentially constitute the ‘glycan strands’ comprise of alternating units of n-acetyl glucosamine and n-acetyl muramic acid in β-1, 4-linkage ; besides, each strand predominently contains disaccharide residues ranging from 10 to 65 units as shown in Fig. 2.11.


 

Nevertheless, the short peptide chains consisting four amino acids are found to be strategically linked to the corresponding muramic acid residues ; and invariably the most commonly encountered sequence being L-alanine, D-glutamic acid, meso-diamino pimelic acid, and D-alanine, as depicted in Fig. 2.12.


 

Salient Features : The various important and noteworthy salient features with regard to the formation of peptide chains are as enumerated under :

 

(1) The 3rd amino acid i.e., meso-diamino pimelic acid (Fig. 2.12) has been observed to vary with different organisms (bacteria) by any one of the three such amino acids as : lysine, diamino pimelic acid, or threonine.

 

(2) Besides, the adjacent peptide chains occurring in a peptidoglycan could be duly cross-linked by short peptide chains essentially comprising of a varying number of amino acids.

 

(3) An important characteristic feature viz., the variations in the structure of the peptidoglycan constituents usually take place ; and, therefore, it has been exploited and utilized judiciously as a wonderful taxonomic tool.

 

(4) The exact number of amino acids that eventually form the cross link prevailing between the two n-acetyl muramic acid residue i.e., the interpeptide bridge variation, may also vary from 2 to 5, as given in Table 2.7, depending upon the various species of microorganisms. Varia-tions in the n-acetyl muramic acid are also known and these alterations ultimately do affect the compactness of the peptidoglycan to an appreciable degree.


 

5. Gram-Positive and Gram-Negative Bacteria

 

The various characteristic features of Gram-positive and Gram-negative bacteria shall be dis-cussed at length in this particular section.

 

For Gram-negative bacteria. There are two distinct layers that have been duly recognized in the cell envelopes of Gram-negative bacteria, namely :

 

(a) An uniform inner layer approximately 2–3 mm wide, and

 

(b) A thicker outer layer nearly 8–10 nm wide.

 

Importantly, the peptidoglycan is prominently confined to the inner layer ; whereas, the outer layer (membrane) essentially comprises of proteins, lipoproteins, and lipopolysaccharides.

 

The principal chemical differences that predominently occur between the cell walls of Gram-positive bacteria and the inner rigid wall layer and outer wall layer(s) of Gram-negative bacteria have been duly summarized in Table 2.8 given below :

 

Table 2.8. Principal Chemical Differences Existing Between Cell Walls of Gram-positive and Gram-negative Bacteria


 

Cardinal characteristic features of component variants in Gram +ve and Gram –ve micro-organisms : The various important characteristic features of component variants in Gram +ve and Gram –ve microbes are as stated under :

 

(1) Peptidoglycans belonging to the Gram –ve microorganisms exhibits a rather low extent of cross linkages within the glycan strands.

 

(2) Outer-membrane. The fine structure of the outer membrane, very much akin to cell mem-brane, essentially comprises of a lipid bilayer wherein both phospholipids and lipopolysaccharides are definitely present. Besides, the lipopolysaccharide generates the major component of the outer membrane, and represents an extremely complex molecule varying in chemical composition within/between the Gram –ve bacteria.

 

(3) Outer surface. The peptidoglycan of the wall has particular kinds of lipoproteins residing on its outer surface, that are strategically linked by peptide bonds to certain diaminopimelic acid residues present in the peptidoglycan.

 

(4) Lipoproteins evidently serve as a sort of bridge right from the peptidoglycan upto the outer-wall-layer.

 

(5) The total number of proteins definitely present, unlike in the inner membrane, are quite a few in number (approx. 10) ; and, therefore, these are markedly distinct from those invari-ably found in the inner membrane.

 

Typical Example : It has been observed that the lipopolysaccharides belonging to either E. coli or Salmonella sp. necessarily comprise of subunits, and each subunit consists of three vital compo-nents, namely : (a) a lipid ; (b) core region ; and (c) O-side chain respectively, as given in Fig. 2.13.


 

Explanations : The proper explanations for the various transformations occurring in Figure : 2.13 are as given below :

(i) The various subunits in lipopolysaccharide are duly linked via pyrophosphates with the ‘lipid zone’.

(ii) The ‘lipid zone’ comprises of a phosphorylated glucosamine disaccharide esterified adequately with long chain fatty acids.

(iii) The ‘core region’ comprises of a short-chain of carbohydrates, and the O-side chain consists of different carbohydrates and is much longer in comparison to the R-core region.

(iv) Lipopolysaccharides represent the major antigenic determinants, and also the receptors for the active adsorption of several bacteriophages.

 

Comparative Activities of Gram-negative and Gram-positive Bacteria

 

The various glaring comparative activities of both Gram-negative and Gram-positive bacteria are enumerated below :

 

(1) It has been duly demonstrated that the outer membrane of Gram-negative bacteria promi-nently behaves as a solid barrier to the smooth passage of certain critical substances, for instance : antibiotics, bile salts*, and dyes into the cell. Hence, the Gram-negative organisms are comparatively much less sensitive to these substances than the Gram-positive ones.

 

(2) Adequate treatment of Gram-negative bacteria with an appropriate chelating agent, such as : ethylenediaminetetra acetic acid (EDTA), that eventually affords the release of a substantial amount of lipopolysaccharides renders ultimately the cells more sensitive to the drugs and chemical entities. Thus, the presence of lipopolysaccharide on the surface of the cell also helps the bacteria to become resistant to the phagocytes** of the host.

 

(3) The resistance acquired in (2) above is almost lost only if the host enables to synthesize the antibodies that are particularly directed against the O-side chain (Figure 2.13). There exists a vast diversification in the specific structure of the O-side chain ; and, therefore, gives rise to the somatic*** antigenic specificity very much within the natural bacterial populations. Evidently, the ensuing antigenic diversity exhibits a distinct selective advantage specifi-cally for a pathogenic bacterial species, because the animal host is not in a position to pos-sess higher antibody levels strategically directed against a relatively large number of varie-ties of O-side chains.

 

(4) In general, the prevailing lipids are invariably found to be phosphatidylethanolamine, and apparently to a much smaller extent phosphatidylserine and phosphatidylcholine, present duly in Gram-negative and Gram-positive bacteria.

 

6. Significance of Teichoic Acids

 

The teichoic acid is a polymer invariably found in the wall of certain bacteria. It has been re-ported that the walls of two Gram-positive organisms belonging to the genus of micrococci being a member of the family Micrococcaceae, order Eubacteriales, namely : Staphylococcus aureus, and Staphylococcus faecalis usually comprise of teichoic acids i.e., the acidic polymers of ribitol phos-phate and glycerol phosphate, that are covalently linked to peptidoglycan, and which can be conven-iently extracted with cold diluted acids, as given below :


 

In actual practice, however, the teichoic* acids may be duly grouped chiefly into two categories, namely : (a) wall teichoic acids, and (b) membrane teichoic acids.

 

Characteristic Features : Most teichoic acids do possess certain inherent characteristic fea-tures as stated here under :

 

(1) They usually get bound to Mg2+ ions specifically, and there is quite a bit of evidence to suggest that they do aid in the protection of bacteria from the thermal injury by way of providing an adequate accessible pool of such cations for the stabilization of the cytoplasmic membrane exclusively.

 

(2) Importantly, the walls of a plethora of gram-positive organism contain almost any lipid, but those which distinctly belong to Mycobacterium, Corynebacterium, and certain other genera are conspicuously excepted.

 

7. The Cell Membrane

 

Literally, ‘membrane’ designates a thin, soft, pliable layer of tissue that virtually lines a tube or cavity, covers an organ or structure, or separates one part from another specifically. The cell membrane refers to the very fine, soft, and pliable layer of tissue that essentially forms the outer boundary of a cell ; and it is made of phospholipids, protein, and cholesterol, with carbohydrates on the outer surface e.g., plasma membrane, as shown in Fig. : 2.14.


 

In other words, the cell membrane is the bounding layer of the cytoplasmic contents, and repre-sents the principal osmotic and permeability barrier. It is a lipoprotein (having a ratio of protein and lipid, 70 : 30), devoid of any polysaccharide, and on being examined via an electron microscope shows up with a distinct three-layer unit with a prominent unit membrane structure.

 

The actual thickness of the two outer layers are approximately 3.5 nm, and the middle layer is nearly 5 nm thick. The lipids observed in the cell membrane are largely phospholipids, for instance : phosphatidylethanol amine, and to a lesser extent phosphatidylserine and . The other three vital regions in the cell membrane are, namely :

 

(a) Polar head regions — of the phospholipids are strategically positioned at the two outer surfaces,

 

(b) Centre of membrane — contain the extended hydrophobic fatty acid chains, and

 

(c) Middle protein layer — is duly intercalated into the phospholipid bilayer.

 

Importance of Cell Membrane. The importance of the cell membrane lies in monitoring the three vital functions of immense utility to the cell, namely :

 

(1) It mostly acts as an ‘osmotic barrier’, and usually contains permeases that are solely respon-sible for the viable transport of nutrients and chemicals both in and outside the cell ;

 

(2) It essentially contains the enzymes that are intimately involved in the biosynthesis of membrane-lipids together with a host of other macromolecules belonging to the bacterial cell wall ; and

 

(3) It predominently comprises of the various components of the energy generation system.

 

It is, however, pertinent to state here that besides these critically important features there is an ample evidence to demonstrate and prove that the cell membrane has particular ‘attachment sites’ exclusively meant for the replication and segregation of the bacterial DNA and the plasmids.

 

Mesosomes. It has been duly observed that in certain instances of microorganisms, more specifi-cally and precisely in the Gram-positive bacteria, solely depending upon the prevailing growth factors as well as parameters the cell membrane vividly seems to be ‘infolded’ at more than one point. Such infoldings* are known as mesosomes as depicted in Fig. : 2.15.

 

Habitats. The actual presence of such folded structures in large quantum have also been found in microorganisms that do possess a relatively higher respiratory role to play (activity) ;

 

Examples : (a) Logarithmic phase of growth, and

(b) Azotobacter i.e., the nitrogen fixing bacteria.


 

In addition to the above, the mesosomes are also found in the following two types of microor-ganisms, such as :

 

(i) Sporulating bacteria in these the critical appearance of such infolding (i.e., mesosome formation) is an essential prerequisite for the phenomenon of ‘sporulation’ ; and

 

(ii) Photosynthetic bacteria — in these the actual prevailing degree of ‘membrane infolding’ has been intimately related to two important aspects, namely: firstpigment content, and second photosynthetic activity.

 

8. Bacterial Cytoplasm

 

Based upon various intensive and extensive investigations carried out on the bacterial cell, one may observe that the major cytoplasmic contents of it essentially include not only the nucleus but also ribosomes, proteins, water-soluble components, and reserve material. It has also been observed that a plethora of bacteria do contain extrachromosomal DNA i.e., DNA that are not connected to the chromosomes.

 

It has also been revealed that the ‘bacterial nucleus’ is not duly enclosed in a well-defined mem-branous structure, but at the same time comprises of the genetic material of the bacterial cell. Interest ingly, several altogether sophisticated meticulous and methodical investigations pertaining to the actual status/content(s) of the bacterial nucleus reveal amply that :

 

(a) Electron microscopy : Electron micrographs of the bacterial nucleus under investigation evidently depict it as a region very tightly and intimately packed with fibrillar DNA i.e., consisting of very small filamentous structure.

 

(b) Cytological, biochemical, physical, and genetic investigations : Such investigations with respect to a large cross-section of bacterial species revealed that the ‘bacterial nucleus’ essentially contains a distinct singular molecule of definite circular shape, and having a double-stranded DNA.

 

The genome size of DNA i.e., the complete set of chromosomes, and thus the entire genetic information present in a cell, obtained painstakingly from a variety of bacterial species has been deter-mined and recorded in Table 2.9 below :


 

Specifications of E. coli: The size of DNA in E. coli together with certain other specifications are as given below :

 

Average length : Approx. 1000 μm

Base pairs : 5 × 103 kilo base pairs

Molecular weight : 2.5 × 109 Daltons (± 0.5 × 109)

 

The ensuing DNA happens to be a highly charged molecule found to be dissociated with any basic proteins as could be observed in higher organisms.

 

Neutralization of charge is duly caused either by polyamines e.g., spermine, spermidine, or by bivalent cations e.g., Mg2+, Ca2+.

 

Plasmid DNA : Besides, the apparent and distinct presence of the bacterial ‘nuclear DNA’, they invariably contain extrachromosomal* DNA termed as plasmid DNA that replicates autonomously. It has been duly observed these plasmid DNAs exhibit different specific features, such as :

·        confer on the bacterial cell,

·        drug resistance,

·        ability to generate bacteriocins i.e., proteinaceous toxins.

·        ability to catabolize uncommon organic chemical entities (viz., in Pseudomonas).

Nevertheless, the actual size of plasmid DNA usually found in these specific structures may be nearly 1/10th or even less in comparison to that invariably found in the bacterial nucleus ; however, the exact number of copies may change from one to several. Besides, these structures are not enclosed in a membrane structure. Importantly, the plasmid DNA is mostly circular in shape and double stranded in its appearance.

 

9. Ribosomes

 

Ribosome refers to a cell organelle made up of ribosomal RNA and protein. Ribosomes may exist singly, in clusters called polyribosomes, or on the surface of rough endoplasmic reticulum. In protein synthesis, they are the most favoured site of messenger RNA attachment and amino acid assem-bly in the sequence ordered b the genetic code carried by mRNA.

 

In other words, the specific cytoplasmic area which is strategically located in the cell material bound by the cytoplasmic membrane having granular appearance and invariably rich in the macromolecular RNA-protein bodies is termed as ribosome.

 

Characteristic Features : Following are some of the cardinal characteristic features of the ‘ribosomes’, namely:

 

(1) Contrary to the animal or plant cells, there exists no endoplasmic reticulum to which ribosomes are bound intimately.

 

(2) Interestingly, there are certain ribosomes that are found to be virtually ‘free’ in the cyto-plasm ; whereas, there are some, particularly those critically involved in the synthesis of proteins require to be transported out of the cell, get closely linked to the inner surface of the cytoplasmic membrane.

 

(3) The number of ‘ribosomes’ varies as per the ensuing ‘rate of protein synthesis’, and may reach even upto 15,000 per cell. In fact, greater the rate of proteins synthesis, the greater is the rate of prevailing ribosomes.

 

(4) Ribosomes represent ribonucleoprotein particles (comprising of 60 RNA ; 40 Protein) hav-ing a diameter of 200 Å, and are usually characterised by their respective sedimentation physical properties as depicted in Fig. 2.16.

 

(5) Prokaryotic Ribosome. In the event when the ribosomes of the prokaryotes undergo ‘sedi-mentation’ in an ultra-centrifuge, they normally exhibit a sedimentation coefficient of 70 S (S = Svedberg Units), and are essentially composed of two subunits i.e., a 50 S and a 30 S subunit (almost fused as shown in Figure 2.16). Consequently, these two subunits get dis-tinctly separated into a 50 S and a 30 S units*. As a result the 50 S unit further gets segre-gated into a RNA comprised of two daughter subunits of 5 S and 23 S each together with thirty two (32) altogether different proteins [derived from 50 – (5 + 23) = 22 sub-units].

Likewise, the 30 S gets fragmented into two segments i.e., first, a RNA comprised of only one subunit having 16 S plus twenty one (21) precisely different proteins [derived from 30 – 16 = 14 sub-units], (see Fig. : 2.16).

 

(6) Eukaryotic Ribosome : This is absolutely in contrast to the ribosomes of the corresponding prokaryotic organisms, that do possess a sedimentation coefficient of 80 S, and are essen-tially comprised of two subunits each of 60 S and 40 S, respectively.


 

7. Polysomes. In a situation when these ‘ribosomes’ are specifically associated with the mRNA in the course of active protein synthesis, the resulting product is termed as ‘polysomes’.

It is, however, pertinent to mention here that there are a plethora of ‘antibiotics’ viz., chloramphenicol, erythromycin, gentamycin, and streptomycin, which exert their predomi-nant action by causing the inhibition of ‘protein synthesis’ in ribosomes.

 

10. Cellular Reserve Materials

 

It has been duly observed that there exist a good number of ‘reserve materials’ strategically located in the prokaryotic cells and are invariably known as the granular cytoplasmic inclusions. The three most vital and important organic cellular reserve materials present in the prokaryotes are namely : (a) poly-β-hydroxybutyric acid; (b) glycogen; and (c) starch (see Table : 2.10).


 

Salient Features. The salient features of the organic cellular reserve materials present in the prokaryotes are as stated under :

 

(1) Poly-β-hydroxybutyric acid. It is found exclusively in the prokaryotes and invariably ca-ters as an equivalent of lipoidal content duly stored in the eukaryotic cells. It is observed in several species of Azotobacter, bacilli, and pseudomonads. Interestingly, certain specific or-ganisms viz., purple bacteria has the ability to synthesize even two types of reserve materi-als (e.g., glycogen and poly-β-hydroxybutyrate) simultaneously.

 

(a) Visibility — These organic cellular reserve materials are found to be deposited almost uniformly very much within the cytoplasm ; however, they may not be detected under a light microscope unless and until these are duly stained.

 

(b) Cellular content — The actively ‘growing cells’ do have these reserve materials present in rather small quantum in the cellular content ; whereas, they get usually accumulated exclusively in the C-rich culture medium under the influence of restricted amounts of nitrogen.

 

(c) Availability — These reserve materials may sometimes represent even upto 50% of the total cellular content on dry weight basis.

 

(d) Utility — These reserve materials are fully utilized when the prevailing cells are ad-equately provided with a suitable source of N and the growth is resumed subsequently.

 

(2) Glycogen and Starch — It has been duly established that the synthesis of glycogen and starch is usually accomplished via a proven mechanism for storing C in a form which is osmotically inert ; whereas, in the particular instance of poly-β-hydroxybutyric acid it pre-cisely designates a method of neutralizing an acidic metabolite.

 

(3) Cyanophycine (a copolymer of arginine and aspartic acid) :

In general, prokaryotes fail to store particularly the organic nitrogenous materials, but the blue-green bacteria is expected which essentially accumulate a nitrogenous reserve material termed as cyanophycine. It invariably represents as much as 8% of cellular dry weight; and may be regarded as a copolymer of arginine and aspartic acid.

 

(4) Volutin (metachromatic) Granules. A plethora of prokaryotes acquire more and more of volutin granules that may be stained meticulously with a ‘basic dye’, for instance : methyl-ene blue. In fact, these prokaryotes appear as red on being stained with a ‘blue-dye’. Impor-tantly, the prevailing metachromatic nature of the ensuing ‘red complex’ is on account of the very presence of a substantial quantum of ‘inorganic phosphates’. Evidently, the actual accumulation of these substances in the prokaryotes takes place under critical parameters of starvation specifically during ‘sulphate starvation’. It has been observed that these instantly generated volutin granules disappear as soon as the cells are adequately made available with a ‘sulphur source’, and subsequently the phosphate moiety [PO43–] is incorporated strategi-cally into the nucleic acids i.e., DNA and RNA. From the above statement of facts one may vividly infer that the ‘volutin granules’ definitely represent particularly the ‘intracellular phosphate reserve’ when the desired nucleic acid synthesis fails to materialize.

 

(5) Sulphur Bacteria [e.g., photosynthetic purple sulphur bacteria ; and filamentous non-photosynthetic bacteria (viz., Baggiatoa and Thiothrix)]. The aforementioned two sulphur bacteria specifically help in the accumulation of ‘Sulphur’ transiently in the course of hydro-gen sulphide [H2S] oxidation.

 

(6) Thylakoids. These are solely present in the blue-green bacteria and are intimately involved in the phenomenon of photosynthesis. Besides, there are three prominent structures, namely : gas vesicles, chlorobium vesicles, and carboxysomes, that are critically bound by non-unit membranes have been reported to be present in certain photosynthetic organisms.

 

(7) Ribs. There are several aquatic prokaryotes essentially containing gas vacuoles that are intimately engaged in counter-balancing the prevailing gravitational pull appreciably. On being examined under a ‘light microscope’ the ensuing gas vacuoles do look like dense refractile structure having a distinct irregular peripheral boundary. Importantly, with a cer-tain surge in the hydrostatic built-up pressure the existing gas vacuoles collapse thereby the cells lose their buoyancy eventually. Precisely, each gas vesicle more or less has an appear-ance very much akin to a ‘hollow cylinder’ having an approximate diameter of 75 nm with distinct conical ends, and a length ranging between 200 and 1000 nm. These conglomerates of gas vesicles are usually surrounded by a layer of protein approx. 2 mm thick. These structures do possess several bands consisting of regular rows of subunits that almost run perpendicular to the axis, and are termed as ‘ribs’. The ribs are found to be impermeable to water.

 

(8) Photosynthetic Apparatus. The photosynthetic apparatus present specifically in the pho-tosynthetic green bacteria (chlorobium) possesses a distinct strategic intracellular loca-tion. It is usually bound by a series of cigar-shaped vesicles arranged meticulously in a corticle-layer which immediately underlies the cell membrane as illustrated in Fig. 2.17. Interestingly, these structures have a width nearly 50 nm, length varying between 100–150 nm and are delicating enclosed within a single layered membrane of thickness ranging be-tween 3–5 nm. They essentially and invariably contain the ‘photosynthetic pigments’.


 

(9) Carboxysomes. It has been amply demonstrated that a good number of photosynthetic and chemolithotrophic organisms, namely : blue-green bacteria, purple bacteria, and thiobacilli essentially comprise of polyhedral structures having a width of 50–500 nm and carefully surrounded by a single layer of membrane having a thickness of 3.5 nm approximately. These characteristic structures are known as carboxysomes. They are found to consist of certain key enzymes that are closely associated with and intimately involved in the critical fixation of carbon dioxide [CO2], such as : carboxy dismutase ; and thus, represent the precise and most probable site of CO2 fixation in the photosynthetic as well as chemolithotrophic organisms.

 

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