Bacteria [Plural of ‘Bacterium’]

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Chapter: Pharmaceutical Microbiology : Characterization, Classification and Taxonomy of Microbes

Exhaustive historical evidences based on the survey of literatures, amply stress and reveal the fact that ‘bacteria’ predominantly share with the ‘blue-green algae’ a unique status and place, in the ‘world of living organisms’.

Bacteria [Plural of ‘Bacterium’]


Exhaustive historical evidences based on the survey of literatures, amply stress and reveal the fact that ‘bacteria’ predominantly share with the ‘blue-green algae’ a unique status and place, in the ‘world of living organisms’.


A bacterium may be regarded as a one-celled organism without true nucleus or functionally specific components of metabolism that essentially belongs to the kingdom Prokaryotae (Monera), a name which means primitive nucleus. However, all other living organisms are termed as Eukaryotes, a name that precisely implies a true or proper nucleus.


It has been duly observed and established that ‘bacteria’ are exclusively responsible for the causation of several painful ailments in humans, namely : tonsillitis, pneumonia, cystitis, school sores, and conjunctivitis.


Alternatively, one may define bacteria as microscopic single-celled organisms that can pen-etrate into healthy tissues and start multiplying into vast numbers. Interestingly, when they do this, they invariably damage the tissue that they are infecting, causing it to break down into the formation of pus.** Due to the damage they (bacteria) cause, the affected and involved area becomes red, swollen, hot and painful. In this manner, the waste products of the damaged tissue, together with the bacteria, rapidly spread into the blood stream, and this virtually stimulates the brain to elevate the body tempera-ture so as to fight off the contracted infection; and this ultimately gives rise to the development of ‘fever’ (normal body temperature being 37°C or 98.4°F).


Salient Features


The various salient features of ‘bacteria’ are as stated under:


(1) The body is invariably invaded by millions of organisms every day, but very few surprisingly may ever succeed in causing serious problems by virtue of the fact that body’s defence mechanisms usually destroy the majority of the invading microbes.

In fact, the white-blood cells (WBCs) are the main line of defence against the prevailing infections. Evidently, the WBCs rapidly migrate to the zone of ‘unwanted bacteria’ and do help in engulfing them and destroying them ultimately. Importantly, when these defence mechanisms get overwhelmed, that a specific infection develops and noticed subsequently.


(2) Nomenclature: Each species of organisms or bacteria (and fungi but not viruses) has two names: first — a family name (e.g., Staphylococcus) that essentially makes use of a capital initial letter and comes first always; and secondly — a specific species name (e.g., aureus) which uses a lower case initial letter and comes second.

Example: The golden staph bacteria that gives rise to several serious throat infections is therefore termed as Staphylococcus aureus, but should be normally abbreviated to S. aureus.


(3) As different types of bacteria invariably favour different segments of the body and thereby lead to various glaring symptoms; therefore, it is absolutely necessary to choose and pick-up an appropriate educated guess about the antibiotic(s) to be administered by a ‘physician’. In the event of any possible doubt it is always advisable to take either a ‘sample’ or a ‘swab’ being sent to a ‘microbiological laboratory’ for an expert analysis, so that the precise organ-ism may be identified, together with the most suitable antibiotic to destroy it completely.


(4) Obviously, there are a plethora of organisms (bacteria), specifically those present in the ‘gut’, observed to be quite useful with respect to the normal functioning of the body. These organisms usually help in the digestive process, and prevent infections either caused by fungi (e.g., thrush) or sometimes by viruses. Importantly, antibiotics are capable of killing these so called ‘good bacteria’ also, which may ultimately give rise to certain apparent side-effects due to the prolonged usage of antibiotics, such as : diarrhoea, fungal infections of the mouth and vagina.


The most commonly observed ‘bacteria’ that invariably attack the humans and the respective diseases they cause, or organs they attack, are as listed under :

Bacteria : Diseases or Place of Infections

Bacteroides : Pelvic organs

Bordetella pertussis : Whooping cough

Brucella abortus : Brucellosis

Chlamydia trachomatis : Vineral disease, pelvic organs, eye

Clostridium perfringens : Gas gangrene, pseudomembranous colitis.

Clostridium tetani : Tetanus

Corynebacterium diphtheriae : Diphtheria

Escherichia coli : Urine, gut, fallopian tubes, peritonitis

Haemophilus influenzae : Ear, meningitis

Helicobacter pylori : Peptic ulcers

Klebsiella pneumoniae : Lungs, urine

Legionella pneumophilia : Lungs

Mycobacterium leprae : Leprosy

Mycobacterium tuberculosis : Tuberculosis

Mycoplasma pneumoniae : Lungs

Neisseria gonorrhoea : Gonorrhoea, pelvic organs

Proteus : Urine, ear

Pseudomonas aeruginosa : Urine, ear, lungs, heart

Salmonella typhi : Typhoid

Shigella dysenteriae : Gut infections

Staphylococcus aureus : Lungs, throat, sinusitis, ear, skin, eye, gut, meningitis, heart, bone, joints

Streptococcus pyrogens : Sinuses, ear, throat, skin

Streptococcus viridans : Heart

Treponema pallidum : Syphillis

Yersinia pestis : Plague


Structure and Form of the Bacterial Cell:


These characteristic form of the bacterial cell may be sub-divided into two heads, namely:

(i) Size and shape, and

(ii) Structure

These two categories shall now be dealt with separately in the sections that follows :


Size and Shape:


The size and shape of bacteria largely vary between the dimensions of 0.75 - 4.0 μm. They are invariably obtained as definite unicellular structures that may be essentially found either as spherical forms (i.e., coccoid forms) or as cylindrical forms (i.e., rod-shaped forms). How-ever, the latter forms, in one or two genera, may be further modified into two sub-divisions, namely:

(a) With a single twist (or vibrios), and

(b) With several twists very much akin to ‘cork screw’ (or spirochaetes).

In actual practice, there prevails another predominant characteristic feature of the bacterial form i.e., the inherent tendency of the coccoid cells to exhibit growth in aggregates. It has been duly observed that these ‘assemblies’ do exist in four distinct manners, such as:


(i) As ‘pairs’ (or diplococci),


(ii) As ‘groups of four systematically arranged in a cube’ (or sarcinae),


(iii) As ‘unorganized array like a bunch of grapes’ (or staphylococci), and


(iv) As ‘chains like a string of beads’ (or streptococci).


In general, these ‘aggregates’ are so specific and also characteristic that they usually assign a particular generic nomenclature to the group, for instance :


(a) Diplococcus pneumoniae — causes pneumonia,


(b) Staphylococcus aureus — causes ‘food-poisoning’ and boils, and


(c) Streptococcus pyogenes — causes severe sore throat.




There exists three essential divisions of the so called ‘bacterial cell’ that normally occur in all species, such as: cell wall or cytoplasmic membrane and cytoplasm.


Based upon the broad and extensive chemical investigations have evidently revealed two funda-mental components in the structure of a bacterial cell, namely :


(a) Presence of a basic structure of alternating N-acetyl-glucosamine, and


(b) N-acetyl-3-0-1-carboxyethylglucosamine molecules. In fact, the strategic union of the said two components distinctly give rise to the polysaccharide backbone.


Salient Features: The salient features of the structure of a bacterial cell are as stated under:


(1) The two prominent and identified chemical entities viz., N-acetyl glucosamine (A), and N-acetyl-3-0-1- carboxymethylglucosamine (B) are usually cross-linked by peptide chains as shown under:


(2) The combined structure of [A] and [B] as shown in (1) above basically possesses an enor-mous mechanical strength, and, therefore, essentially represents the target for a specific group of ‘antibiotics’, which in turn via different modes, categorically inhibit the biosynthesis that eventually take place either in the course of cell growth or in the cell division promi-nently.


(3) The fundamental peptidoglycan moiety (also known as murein or mucopeptide) besides contains other chemical structures that particularly gets distinguished by the presence of two kinds of bacteria, namely :


(a) Gram-negative organism, and


(b) Gram-positive organisms.


However, these two variants of organisms may be identified distinctly and easily by treating a thin-film of bacteria, duly dried upon a microscopic slide with a separately prepared solution of a basic dye i.e., gentian violet, and followed soon shape after by the application of a solution of iodine. Thus, we may have:


Gram-negative bacteria — by alcohol washing the dye-complex from certain types of cells, and Gram-positive bacteria — by retaining the dye-complex despite the prescribed alcohol-washing.

Further, the prevailing marked and pronounced differences in behaviour, just discovered by a stroke of luck, are now specifically recognized to be a glaring reflection of wall structure variants in the two kinds of cell as illustrated in Fig. 3.10.


X = Generalized structure of a Bacterial Cell;

Y = Gram + ve Structure;

Z = Gram – ve Structure

A = Cytoplasm;

E = Lipopolysaccharide;

B = Cytoplasm membrane;

F = Lipoprotein;

C = Cell-Wall peptidoglycan;

G = Covalent-Bond;

D = Teichoic acid;

H = Flagellum;


Gram-positive Cell Wall [Y] : In this particular instance, the walls of bacteria essentially com-prise of the molecules of a polyribitol or polyglycerophosphate that are found to be strategically attached by means of covalent bonds (G) to the prevailing oligosaccharide backbone; and these chemical entities are nothing but teichoic acids [D]. It is, however, pertinent to mention here, that the said teichoic acids do not give rise to any sort of additional rigidity upon the ensuing cell wall, but as they are acidic in character, they are capable of sequestering essential metal cations derived from the culture media upon which the bacterial cells are growing. Importantly, this could be of immense value in such circum-stances wherein the ‘cation concentration’ in the environment is apparently at a low ebb.


Gram-negative Cell Wall : Interestingly, the Gram-negative cell wall is observed to be much more complex in character by virtue of the presence of the lipoprotein molecules (F) strategically at-tached covalently to the respective vital oligosaccharide backbone. Besides, on its outer region, a layer of lipopolysaccharide (E) along with the presence of protein critically attached by hydrophobic interac-tions and divalent metal cations e.g., Ca2+, Fe2+, Mg2+, Cu2+, whereas, in its inner side is a layer of phospholipid.


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