Bacteria are small, microscopic, unicellular
organisms that exhibit a great diversity in habitat, morphology, nutrition,
metabolism and reproduction. The term bacterium was first used by Ehrenberg
which comes from a Greek word bacterion which means little rod or stick. The
study of bacteria is known as bacteriology.
A Dutch man, Anton van Leeuwenhoek, was the first to
discover the bacteria in 1675, but he thought them to be animalcules. For about
two centuries, thereafter, the bacteria did not receive much scientific
attention. Later Louis Pasteur in 1864 and Robert Koch in 1876 reported that
these microorganisms could cause disease.
Louis Pasteur declared that all putrefaction
(decomposition of organic substances) is due to action of microorganisms. He
also established that fermentation is also caused by microorganisms and
developed the method of preservation of liquids (milk, wine etc) called the
pasteurisation. Heating of liquids upto less than 100o C temperature
to kill the microorganisms is called the pasteurisation.
Robert Koch showed that anthrax disease is
transmitted through spores. Koch proposed that all bacterial disease must
follow some set conditions: i) that the organisms must be present in the
lesions of the disease, ii) that it must be possible to grow these organisms in
pure cultures outside the host body, iii) that the culture should be able to
produce the typical symptoms of the disease, iv) that the same organism must
invariably be present in the disease caused by the inoculum.
General Characters of Bacteria:
1. They are omnipresent, found in all possible
habitats, which can support life.
2. They are unicellular and morphologically least
complex of all living organisms.
3. Cell is prokaryotic – a well organised nucleus is
absent. Membrane bound cell organelles
like endoplasmic reticulum, Golgi bodies, Mitochondria etc are absent.
4. Most of the bacteria have heterotrophic mode of
nutrition i.e., they obtain their food directly from external source. They may
live as saphrophytes, parasities or symbionts. Some bacteria are autotrophic,
as they possess bacteriochlorophyll, the photosynthetic pigment.
5. Binary fission is the most common method of
reproduction.
6. True sexual reproduction is absent. However,
recombination of genetic material occurs by conjugation, transformation and
transduction.
Occurrence and Distribution:
Bacteria are among the most numerous of all living
organisms being present in almost very conceivable environment. There are about
2,000 known species.
Bacteria are ubiquitous organisms and occur in all
natural habitats. They occur in abundance in both fresh waters of rivers,
ponds, reservoirs and lakes and alo in saline waters of Oceans and Seas. They
are found in all soil and above the earth.
Some thrive well in hot water springs (75oC)
and can also survive below freezing point in the Atlantic ice. Similarly, their
tolerance to hydrogen concentration ranges from pH near to 0 to pH 11.
Many species of bacteria are free-living, and others
are symbiotic, parasitic or saphrophytic. They are found within and upon the
bodies of plants and animals. They flourish well on our skin, mouth and
intestine.
The features which contribute to their universal
distribution are:
1. Extremely simple structure
2. Small size, cell division occur rapidly.
3. Resistance of vegetative cells to adverse
environmental factors
4. Formation of highly resistant endospores
5. Diversity of their modes of nutrition.
Size:
Bacteria are very small microscopic, unicellular
organisms. The size of bacteria varies from 0.2µ (1 micron = 0.0001mm) in
diameter and less than 2-10 microns in length.
Thiomargarita namibienxsis and Epulopischum
fishelsoni are upto ½ a millimetre long and are visible to the unaided eye.
Forms or Shapes of Bacteria:
The bacteria vary in their shape which is usually
constant for each species and helps in their identification.
I. Coccus Bacteria
Spherical or ellipsoidal bacteria are called cocci
bacteria. Almost all coccal forms lack flagella (non-motile). The spherical
cells may occur singly but often tend to stick together and occur in pairs,
chains etc. On the basis of arrangement of cells in a cluster, the cocci are
classified into the following six groups:
1. Monococcus: single spherical shaped cells.
E.g., Micrococcus luteus
2. Diplococcus: a pair of spherical shaped
cells (result when cocci divide and remain together to form pairs) e.g., Neisseria,
Diplococcus pneumonia
3. Streptococcus: Cocci cells occur in long
chains. Result when cells adhere after repeated divisions in one plane. E.g.,
Streptococcus lactis (milk bacterium).
4. Staphylococcus: Irregular group of many
spherical bacteria arranged in bunches(grapes like). Result when cocci divide
in radom planes. E.g., Staphylococcus albus (causes abscesses, wound
infections).
5. Sarcinia: cuboidal arrangement of eight
cocci cells. Results when cocci cells divide regularly in three planes. E.g.,
Sarcinae lutea
II. Bacillus
Bacteria
This is the probably the most common form of
bacteria. These are rod-shaped or hyhens (-) like bacteria are called the
bacillus bacteria. Some are motile and some non-motile.
Based on the number of cells and their arrangement
bacillus are classified into following :
1. Monobacillus: single rod shaped cells.
E.g., Pseudomonas, Clostridium.
2. Diplobacillus:a pair of rod shaped cells. E.g.
Corynebacterium diphtheria, Yersinia pestis (plague bacterium)
3. Streptobacillus: chain of bacillus
bacteria appearing like straws. E.g., Bacillus tuberculosis, Lactobacillus.
III. Helical Bacteria
: the helical forms are of two types – vibrioid and spirillum..
1. Vibrioid: rod shaped bacteria are slightly
curved and look like a comma. A vibrio bears a single flagellum at its tip.
E.g., Vibrio cholearae
2. Spirillum: the bacteria having the body
twisted spirally or into helix. The spirilla forms usually bear two or more
flagella at one or both poles. They occur as single cells or in short chains
but do not form colonies. E.g., Spirillum volutans.
IV. Filamentous Bacteria:
Some bacteria are filamentous and branched. E.g.,
Beggiatoa, Thiothrix.
V. Pleomorphic Bacteria:
Some bacteria are able to change shape and size temporarily
in response to changes in the environment. For example, Acetobacter, a group of
vinegar bacteria occus as small rods, long rods, ellipsoids or a chain of small
rods. Such bacteria are called pleomorphic.
Staining of Bacteria:
Physician Christian Gram, in 1884 devised a
differential staining procedure for identification of bacteria called the Gram
staining.
A thin smear of bacterial cell is prepared on a
glass slide. To the smear, Crystal Violet solution is applied for 30 seconds –
step I. The slide is then rinsed in clean water. Then an Iodine solution is
applied for 30 seconds – step II. This, inturn, is rinsed off with clean water.
Examination of the smear with the microscope reveals
that all the bacteria have taken the purple colour. The next step in the
staining procedure is to apply 95% Ethyl alcohol to the smear until the all the
except the thickest layers of the smear have ceased to give off the dye.
Examination of the smear at this stage reveals that
many of the bacteria still retain the original purple colour, whereas from
others it has been washed off.
The bacteria that retain the original purple stain
are called gram-positive bacteria. Those which loss the purple colour and
become decolourised are called gram-negative bacteria.
The decolourised gram-negative bacteria are nearly
invisible. To study their details counter stains like eosine or safranine are
added to the smear.
Structure of Bacterial Cell
Bacteria are unicellular organisms. The ultra
structure of bacteria is a typical example for the prokaryotic cell that lacks
membrane bound nucleus and other cell organelles.
Like other living plant cells, the bacterial cell
comprises of a cell wall and protoplast. Structurally, bacterial cell consists
of following components:
1. Structures external to cell wall – flagella,
fimbriae, pilli, capsule or slime layer
2. The cell Wall
3. Structures internal to cell wall – cell membrane,
mesosomes, ribosomes, nucleoid
4. Special cell structures – storage granules, gas
vacuoles, carboxysomes, magnetosomes, endospore
Structures External to Cell Wall
Slime layer:
The gelatinous layer present on the outer surface of
the cell wall. It is secreted by the cell protoplast. The slime diffuses
through the cell wall and deposits in form of a thin, extracellular, viscous
layer external to the cell wall.
It is usually composed of polysaccharides or
polypeptides or both. When it is made up of only polysaccharides, which forms a
viscous layer and loosely attached to the cell wall, it is called Slime layer.
Under certain conditions of growth, the slime
accumulates in form of a well-defined, thick , persistent layer, firmly attached to the cell wall. It
is called the sheath or capsule. E.g., Streptococcus pneumonia, Bacillus
anthracis.
The sheath protects the bacteria against dessication
and antibodies. The encapsulated pathogenic bacteria can with stand
phagocytosis.
Some bacteria produce diffuse slime which spreads
into the environment and encloses bacterial colonies. This is known as loose
slime.
The bacterial cell with a capsule normally does not
bear flagella.
Functions:
Provides protection under unfavourable conditions.
Function in the storage of food substances, and as
storage site for the disposal of waste material.
Helps bacterium in attaching to various surfaces in
order to survive in its natural environment such as streams, roots, teeth and
tissues.
Flagella:
Mobility is universal among the spirillum bacteria,
common among the bacillus bacteria but lacking or rare in coccal forms.
The motile bacteria have small whip or hair-like
appendages called the flagella. They are about 120A0 – 150A0
thick and 4 0r 5 µ long. They are cytoplasmic in nature. Each flagellum arises
within the cytoplasmic membrane and passes out through the cell wall.
Structure
Bacterial flagellum is composed of three parts –
basal body, the hook and the filament.
The outermost, long, hair-like, cylindrical, hollow
structure of the flagellum is called filament or shaft. It has a constant
diameter of 120-200A0. It is made up of globular proteins, the
flagellin.
The flagellum is composed of several chains of
flagellin subunits forming a hollow cylindrical thread. Unlike the Eukaryotes,
the filaments are not covered by a membrane or sheath.
Hook is short, curved, thicker segment of the
flagellum. It is made up of a single type of protein. It connect the basal
bodies to the filament of the flagellum. It passes out through the cell wall.
The basal body attaches the flagellum to the cell
wall and plasma membrane. It is composed of a small central rod inserted into a
series of rings. In Gram negative bacteria two pairs of rings, the proximal
ring and the distal ring, are connected by a central rod.
The outer pair of rings i.e., L-ring
(Lipopolysaccharide ring) and P- ring (Peptidoglycon ring) are attached to
respective polysaccharide and
peptidoglycan layers of the cell wall. The inner pair of rings i.e.,
M-ring (Membrane ring) and S-ring (Super membrane ring) are attached with the
cell membrane.
In Gram positive bacteria only inner pair of rings
are present. The S ring is attached to the peptidoglycan layer and M-ring is
attached to the cell membrane.
Flagellation:
The number, position and arrangement of flagella
varies with the species. The flagella may be restricted to one or both ends of
the bacterium cell (polar flagellation) or may be distributed uniformly
all over the body surface (non-polar flagellation).
On the basis of number and arrangement of flagella,
bacteria are classified into the following types:
Polar Flagellatoin:
This type of flagellation is found in bacilli and
spirillum bacteria. They are all gram-negative. Polar flagellation are
following types:
I. Monontrichous: bacteria with a single
flagellum located at one end or pole. All Vibrios are monotrichous. Eg.,
V.cholerae
II. Amphitrichous: bacteria with one
flagellum at each pole or end of the cell.
III. Cephalotrichous : two or more flagella
at one pole of the bacterium cell. Eg., Pseudomonas fluorescenes. The flagellum
shows more than two curves.
IV. Lophotrichous: Bacteria with two or more
flagella at both the poles of the cell. Eg., Spirillum volutans.
Non-Polar Flagellation:
Flagella are found all over the cell surface. It is
of one type only.
Peritrichous:
bacterial cell with flagella evenly distributed all over the cell surface. Eg.,
Bacillus tylphosus, Proteus vulgaris.
Atrichous:
all bacteria which lack flagella are called atrichous. Eg., Diptheria bacilli,
Lactobacillus.
Pili:
Minute, straight, hair-like, non-flagellar
appendages found on the cell wall of many gram negative bacteria. They are
rigid and composed of a protein called pilin. They may be several microns long
but are smaller in diameter around 30-50 A0.
There may be 100-400 pilli distributed over the
surface of the bacterium cell.
The structures which are shorter than the pili are
called the fimbriea.
Function:
Some pili are used for the attachment.
Pathogenic bacteria use them to identify and to
attach their host cells.
F or sex pili are involved in the transfer of the
DNA during bacterial conjugation.
Cell Wall:
Enveloping the protoplast of bacterial cell, there
is a thin, firm cell wall. It gives rigidity and shape to bacterial cell.
The bacterial cell wall is composed of 4 layers. The outer most layer is L4,
it is wavy and electron dense. Within this is a lighter layer called the L3.
Next is the inner dense layer – L2 made up of mucopeptide. The inner
most layer is the L1.
The most important component of bacterial cell wall
is peptidoglycon or murein (mucopeptide or muramic acid, Gr. Murus=wall). This
polymer is made up of chains of alternating molecule of N-acetyl glucosamine
(NAG) and N-acetyl muramic acid (NAM). NAG and NAM are linked by 1, 4, β-glycosidic
bonds.
The cell wall of Gram +ve bacteria (Clostridium,
Bacillus) is thick, homogenous, mainly consists of peptidoglycon (90%). The
remainder being made up of proteins, polysaccharides and teichoic acid. Lipid
content is very less in Gram +ve bacteria. The cell wall of Gram +ve bacteria is
more sensitive to pencillin.
The cell wall of Gram –ve bacteria is chemically
more complex and heterogenous. Peptidoglycan comprises up to 10% of the cell
wall. The outer layer that surrounds peptidoglycan is two layered and consists
of phospholipids, proteins and lipopolysaccharide mixture. Gram –ve bacteria
possess more lipid. Teichoic acid is absent. The cell wall is less sensitive to
pencillin.
The protoplast:
Within the cell wall is the living liquid called the
protoplast. The protoplast is a clear water or slightly viscous substance.
It is differentiated into the cell or cytoplasmic
membrane, cytoplasm and chromatin or nuclear body.
Cell or Cytoplasmic or Plasma Membrane:
At the surface of protoplast is a differentiated,
delicate, fine membrane known as cell membrane or cytoplasmic membrane. It is
about 75A0 thick. It lies close to the cell wall. Chemically it
consists of phospholilpids with proteins and polysaccharides.
It is a selective permeable membrane which controls
the passage of dissolved substances in an out of the cell. The cytoplasmic
membrane also contains enzymes involved in enzymes and synthesis of capsule and
cell wall material. It is also the site of ATP synthesis and the centre for
control of flagellar motility.
Mesosomes
The plasma membrane of bacteria, at certain places,
forms infoldings to form vesicle or tubular structures called the mesosomes. The
term mesosomes was coined by Fitzjames. They increase the surface area of the
membrane. They have respiratory enzymes, but they are not analogous to mitochondria.
During division, they play a role in the DNA replication, septa formation, cell
wall synthesis and spore formation.
Cytoplasm:
The bacterial cytoplasm is a complex mixtures
composed of 70-80% of water and proteins, carbohydrates, lipids, inorganic
ions. Organic matter is in the colloidal state.
The cytoplasm shows no streaming movement and
contain no vacuoles. Cytoplasm is dense and transparent.
Bacterial cytoplasm contains nucleoid, plasmid, and
ribosomes. Other membrane bound cell organelles like mitochondria, Endoplasmic
reticulum, Golgi bodies are absent. Although a well organised chloroplast is
absent, the photosynthetic bacteria have chromatophores.
The non-living inclusions are the storage granules
of volutin, glycogen, lipid globules, poly-β-hydroxybutyrate (PHB).
Ribosomes:
In electron micrographs the bacterial cytoplasm is
granular and contains numerous, minute, nearly spherical, hollow bodies called
the ribosomes. They are composed of rRNA and proteins. Their number varies from
10,000 to 15,000 in a cell, which account upto 30% of total dry weight of the
cell.
Bacterial ribosomes are of 70S type and is made up
of two sub-units – 50S and 30S. Ribosomes are functional only when the two
subunits are together. The association and dissociation of two subunits of
ribosomes depends on the concentration of Mg2+ ions.
The 50S subunit has one molecule of 23S rRNA (3200
nucleotides), one molecule of 5S rRNA (120 nucleotides) and 34 different
proteins. 30s subunit contains one molecule of 16s rRNA (1600 nucleotides) and
23 different proteins.
The ribosomes lie free in the cytoplasm but
sometimes they occur in small groups called polyribosomes held by a strand of
messenger RNA.
The ribosomes are sites for protein synthesis. Several
antibiotic like streptomycin, neomycin and tetracycline inhibit protein
synthesis on the ribosomes.
Nucleoid:
Area in the bacterial cytoplasm where the genetic
material or DNA is present is called as nucleoid or genophore. Usually it is
centrally located in bacterial cells and is equivalent to the nucleus in
eukaryotes. However, it lacks nuclear membrane and nucleoli.
The shape of the nucleoid varies, often it is
irregular, sometimes spherical or oval but elongated. The nucleoid is composed
of about 60% DNA, 30% RNA and 10% proteins.
The DNA is single, long, circular, double stranded
approximately 5X109 base pairs and molecular weight of about 3X109
daltons. The nucleoid can be stained with Feulgen stains.
Plasmids:
In addition to the nucleoid, bacterial cytoplasm
normally contains one or more molecules of circular, double stranded DNA. Small,
circular, self replicating and double stranded DNA molecule present in
bacterial cell, in addition to nucleoid are called as plasmids. Lederberg
coined the term plasmid.
They replicate autonomously, and inherited by both
the daughter cells during the cell division. Some plasmids get integrated into the bacterial DNA or chromosome. They are
called episomes by Jacob Schaeffer and Wollman.
The number of plasmids ranges from one to hundred or
more per bacterial cell. A plasmid contains 5-100 genes that govern several
biological functions. Plasmids are not essential for the growth and survival of
the bacterial cell, but they confer special characteristics like drug
resistance, toxigenecity to the bacteria.
On the basis of functions, plasmids are divided into
several types:-
i. F-factor (fertility) plasmids,
ii. R-factor (Resistance) plasmids
iii. col-factor (calicinogeny) plasmids
iv. Virulance plasmids conferring pathogenecity to
mammals
v. Ti- plasmids of Agrobacterium tumifacines
vi. Ri-plasmids (root inducing) if Agrobacterium
rhizogenes