Morphology: Anatomy of Bacterial Cell

Introduction to the Anatomy of Bacterial Cell:

Let us make an in-depth study of the anatomy of bacterial cell. The below given article will help you to learn about the following things:- 1. Bacterial Nucleus (DNA) 2. Bacterial Cytoplasm 3. Capsules & Microcapsules and 4. Application of Morphology of Bacteria to Nursing.

Cell is a unit of organised living material or protoplasm.

It consists of:

(a) The protoplast, surrounded by a thin semi-permeable which is known as cytoplasmic membrane or plasma membrane.

(b) Cell wall which is outermost and rigid.

The protoplast is distinguished into:

The cytoplasm, the nucleus. The nucleus is an inner body filled with the thread-like chromosomes in the genes which are the hereditary determinants of character. Microorganism is simple unicellular microscopic structure. On a solid medium, its progenies accumulate at the site of inoculation as colonies which are macroscopic.

The microorganisms are differentiated into: primitive (prokaryotic) cells and more advanced cells (eukaryotic) cells. Prokaryotic cells are — bacteria and related organisms (rickettsiae, chlamydiae and mycoplasmas). Eukaryotic cells are fungi, protozoa, helminths, plants and animals.

The characteristic features of prokaryotic cells:

1. Prokaryotic cell has a homogeneous, simple, nucleus without nuclear membrane separating it from the cytoplasm; the nucleus does not contain nucleotides, spindle and non-identical chromosomes.

2. It does not possess the internal membrane separating the respiratory and photosynthetic enzyme systems in specific organelles. Thus, the respiratory enzyme is located in the mesosome (convoluted structure) of the cytoplasmic membrane of the bacteria.

3. It has a rigid cell wall containing a specific mucopeptide which is strengthening element. The mucopeptide is the target of antibacterial action of antibiotics and lysozymes. It is absent in eukaryotic cell.

Bacteria can be defined briefly as small microorganism with a prokaryotic form of cellular organisation. They are usually unicellular, however they may grow attached to one another in clusters, chains, filaments (hyphae) or a mycelium (Actinomycetales; higher bacteria).

They usually measure from 0.4 to 1.5 µm and smaller than fungi and protozoa. They have rigid cell walls which are responsible for the maintenance of their characteristic shape; the shape may be spherical (coccus); rod shaped (bacillus); comma-shaped (vibrio); spiral (spirillum and spirochaetes) orfilamentous (Fig. 2.1 a).

The whole body of living material (protoplast or protoplasm) is surrounded by a very thin elastic and semi-permeable cytoplasmic membrane. A rigid, supporting, porous and relatively permeable cell wall covers very closely this membrane. The development of a transverse cytoplasmic membrane and a transverse cell wall or cross wall from the periphery to inwards is known as cell division.

The cytoplasm is made of watery sap packed with many granules known as ribosomes which are centres for protein synthesis and few convoluted membranes bodies known as mesosome which is site for respiratory enzyme. The nuclear body or chromatin is referred to the nuclear material; the bacterial nucleus cannot be seen under light microscope, though the word nucleus is now accepted.

The cytoplasm contains also inclusion granules of storage products such as metachromatic granules or volutin (polyphosphate), lipid (poly-β-hydroxy butyrate), glycogen or starch. A capsule, which is outside the cell wall, is a protective gelatinous covering layer; if it is too thin (less than 0.2 µ.m it is called as microcapsule. A loose slime is the soluble large molecular material disposed by the bacterium into the environment.

There are two filamentous appendages protruding outwards from the bacterial cell wall:

(a) Flagella:

Which are organs of locomotion and

(b) Fimbriae(Syn. pill,):

Which are organs of adhesion.

The cell wall, capsules or microcapsules, flagella and fimbriae have the special roles in the process of infection (Fig. 2.1 d).

Bacterial Nucleus (DNA):

Deoxyribonucleic acid (DNA) is double stranded long molecule occurring in the form of a closed circle of circular thread of about 1,000 µ m long, coiled and tightly packed up in a bundle resembling a skein of woolen thread, it is considered by the geneticist as chromosomes containing genetic information’s of bacterial cell.

As the chromosome is not bound to the protein, it is stained like eukaryotic chromosome. The nucleus cannot be differentiated from the cytoplasm under light microscope and there is no nuclear membrane separating the nucleus from the cytoplasm and there is no nucleus.

Bacterial Cytoplasm:

The bacterial cytoplasm is a viscous watery substance. It contains organic and inorganic solutes and small granules known as ribosomes. There is no endoplasmic reticulum or membrane bearing microsomes, no mitochondria. It shows no amoeboid movement.

Ribosomes:

Bacterial ribosomes can be seen under the electron microscope. They are strung together on strands of RNA to form polysomes and at this site that the code of mRNA is translated into peptide sequences. The antibacterial agent, like streptomycin, may interfere with bacterial metabolism at the ribosomal level without upsetting human ribosomal function.

Inclusion Granules:

In the cytoplasm of many bacteria, there are round granules which are not permanent or essential structures. They seem to be concerned with the cell metabolism, because they are in plenty when the energy-yielding nutrients are in abundance and disappear under energy source starvation. They are volutin (polyphosphate), lipid, glycogen, starch or sulphur.

Volutin and lipid granules, 0.1 – 1.0 µm in diameter are found in parasitic and saprophytic bacteria and very useful to identify certain bacteria; e.g., diphtheria bacillus, (pathogenic) can be differentiated mainly by its volutin from diphtherias (non-pathogenic) which are common in the throat.

Volutin granules (Syn. metachromatic granules) have an affinity for basic dyes (e.g., methylene blue) and stained red violet with contrasting blue stained cytoplasm of the bacteria. It seems that polymerised inorganic phosphate is responsible for the metachromatic staining and volutin granules are rich source of energy and phosphate for cell metabolism, slightly acid-fast and appear highly opaque under electron microscope.

Lipid granules are spherical, varying in size, and consist mainly of polymerised P-hydroxy butyric acid and act as a carbon and energy storage product and have an affinity for fat soluble dyes like Sudan black, so they are stained black in contrast to the bacterial cytoplasm which is counterstained pink with basic fuchsine.

Polysaccharide Granules:

They may be glycogen or starch. If it is glycogen, it is stained red-brown; if it is starch it is stained blue by iodine. In E. coli, the cytoplasmic polysaccharide is glycogen and appears as minute granules under electron microscope.

Mesosomes:

They are convoluted membranous bodies developed by complex invagination of cytoplasmic membrane into the cytoplasm which can be seen only under electron microscope. It is suggested that they are responsible for the compartmenting of DNA at cell division or at sporulation and for the excretion of material from the cytoplasm to the exterior.

Cytoplasmic Membrane:

It is 5-10 nm thick and is made of lipoprotein. It can be seen under electron microscope. It surrounds externally the cytoplasm and has little mechanical strength. The cell wall supports the cytoplasmic membrane. Cholesterol is absent in bacterial cytoplasmic membrane, but is normally present in animal cell cytoplasmic membrane. It acts as osmotic barrier which is permeable to many small molecular solutes like lipid soluble ones. It affects selective transport of specific nutrient solutes into the cell and that of the waste products out of the cytoplasm.

Besides the enzymes (permeases) which carry out the active uptake of nutrients, the cytoplasmic membrane contains other enzymes — respiratory enzymes and pigments (cytochrome system), certain enzymes of tricarboxylic acid cycle and polymerizing enzymes that manufacture cell wall and extra mural structure.

Cell Wall:

It is 10-25 nm thick, strong, rigid, porous and permeable to small solute molecules of 1 nm in diameter. It maintains the characteristic shape of the bacterium and supports the weak cytoplasmic membrane. Cell wall of bacteria may be compared to the outer cover and the cytoplasmic membrane to the inner tube of the pneumatic tyre of a motor car (Fig.2.1b,c).

If the cell wall is weakened or ruptured, the protoplasm may swell from osmotic imbibition of water and the weak cytoplasmic membrane will burst. The bacterium will disintegrate and will be dissolved —the process is known as lysis. If the cell wall ruptures partially, the protoplasm will protrude out of the cell wall — the phenomenon is known as plasmoptysis. Bacterial cell wall substances can be hydrolysed by their own enzymes, known as autolysis.

Abnormal Forms:

(Spheroplasts, free protoplasts, pleomorphic involution forms and L-forms or L-phage organisms) can be produced by weakening, removal or defective formation of the cell wall.

Pleomorphism:

A considerable variation in size and shape which differs from the normal (e.g., swollen, spherical and sphere shaped forms, filaments with localised swellings, elongated filaments) Strep to bacillus monili form is. Yersinia pestis show this pleomorphism in old culture on artificial media with substance (penicillin, sodium chloride high concentration and organic acids at low pH).

Involution Forms, or degenerate forms are seen generally in abnormal cells; some are not viable, others will grow and revert to normal forms when they are grown in a suitable environment. This abnormal form may be due to defective cell wall synthesis. The growing cytoplasm expands the weakened wall to produce a swollen cell similar to a spheroplast that later bursts and lyses.

Free Protoplasts and Spheroplasts:

If all cell wall material of bacteria is removed, the spheres are free protoplasts. If these spheres are enclosed by an intact but weakened residual cell wall, they are called spheroplasts. Bacillus megaterium can liberate protoplasts by dissolution of cell wall with egg white lysozyme. Escherichia colican produce spheroplasts, when they are grown in presence of a substance (penicillin, bacitracin, oxymycin or glycine). Protoplasts enlarge but do not multiply in an osmotically protected nutrient medium. Spheroplasts may multiply by fission in a nutrient medium with penicillin.

L-Forms of Bacteria (L-Phage Organisms):

These are abnormal forms derived from bacteria of abnormal morphology (cocci, bacilli, vibrio) due to variation in the laboratory. They lack rigid cell wall, they are viable, grow and multiply on suitable culture media. They are soft protoplasmic bodies, spherical or disk-like. They vary in size (0.1 µ.m – 20 µm).The smallest ones can pass through the bacteria stopping filters.

Some L-forms are devoid of a cell wall. Colonies of L-phage organisms on agar media are small and have a characteristic ‘fried egg’ appearance. Though they are similar to mycoplasma organism, L-forms should be regarded as laboratory artefacts, degenerate growths, that do not occur or survive in natural habitats, the reason of which is not yet clear.

Capsules and Microcapsules:

Outside and immediately in contact with the cell wall, there is a discrete covering layer of relatively firm gelatinous material which can be seen under light microscope in the wet preparation and can be called as capsules. When this capsule is narrower, it can be demonstrated indirectly by serological reaction (capsular swelling reaction) or directly by electron microscope, it is known as microcapsule. The capsule is made of complex polysaccharide in most species. In some species it consists of polypeptide or protein.

Capsules cannot be demonstrated by ordinary method (e.g., Gram stain and Leishman stain). The most reliable method is by negative staining in wet films with India ink. The carbon particles of the Ink make a dark background in the film and cannot penetrate into the capsule which appears as a clear halo around the bacterium.

Function of Capsule:

It protects the cell wall against attack by various antibacterial agents (e.g., bacteriophages, colicines, complement, lysozyme) and against ingestion by the phagocytes. In Bacillus anthracis, the capsule is a polymer of D-glutamic acid. It protects the bacteria against phagocytosis and the action of a bactericidal basic polypeptide of animal tissues. When capsulate pneumococci are treated with type specific antiserum, the sharpness of outline of the capsule is greatly enhanced, which is known as the “capsular swelling reaction”.

Loose slime or freeslimeis an amorphous viscid, colloidal material, that is secreted extra cellularly by some noncapsuiate bacterium and also, outside their capsules by many capsulate bacteria. In capsulated bacteria, the slime is chemically and antigenically similar to capsular substance, when slime forming bacteria are grown on a solid medium, they form watery “mucoid” colonies.

Flagella:

Motile bacteria possess long thin filamentous appendages known as flagella, which act as organs of locomotion. The flagellum originates in the bacterial protoplasm and is extruded through cell wall. It is made of a protein flagellin chemically similar to myosin which is the contractile protein of muscle. It can be easily demonstrated by electron microscope or by Leifson stain. The flagella may be peritrichous or lateral when they originate from the sides of the cell; polar when they originate from one or both ends (Fig. 2.1 d). Image Repetition

Motility of bacteria can be shown by hanging drop method or spreading growth on semi-solid agar medium.

Fimbriae:

Certain Gram-negative bacteria (saprophytic intestinal commensal and pathogenic species) in the family enterobacteriacae possess filamentous appendages which are different from the flagella. These are called fimbriae or pili and occur in some non-motile bacteria; they are more numerous than flagella. Fimbriae act as organs of adhesion.

Bacterial Spores:

Some species, Bacillus and Clostridium develop a highly resistant resting phase or endospore, whereby the organism can survive in a dormant state, through a long period of starvation and other adverse environmental condition. Each vegetative cell forms only one spore. This phenomenon is known as sporulation. Each spore gives rise to a single vegetative cell known as germination.

The appearance of the mature spore varies according to the species, being spherical, ovoid, elongated occupying a terminal, sub-terminal or central position. In aerobic spore-forming bacilli, Bacillus anthracis, the spores are within the cell, not bulging, spherical or oval central. In anaerobic spore-forming bacilli, Clostridia, the spores are bulging, spherical, terminal, oval sub-terminal (Fig. 2.2).

Application of Morphology of Bacteria to Nursing:

In the daily practice of professional nursing, knowledge of morphology, structure and physiology of bacteria, is of very great importance—to associate the name of bacterium with the fact that this particular bacterium is resistant to heat as it forms spores; hence it has to be exposed to a higher temperature of hot air oven or autoclave; and is also responsible for a particular clinically diagnosed disease.

The knowledgeable and professional nurse can understand the scientific names of microorganisms and the associated diseases appearing in the professional literatures.

An experienced professional nurse should be able to recognise and understand the scientific names of microorganisms in the microbiological laboratory reports with the specific disease, so that she can bring it immediately to the attention of the physician, if the organism isolated in the laboratory is responsible for the severe infectious disease. The prompt transmission of the laboratory report to the physician may be quite useful to the physician to switch over to specific treatment and to save from further damage to the patient’s health.