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What are bacteria?
The origin of the term ‘bacteria’ can be traced to 19th
century German botanist ,Ferdinand Cohn (1828-98),
though initially they were described as microscopic
animalcules by Antony van Leeuwenhoek (1632-1723) in
the 17th century. Bacteria comprise a group of single-
celled prokaryotic microorganisms, existing either
independently, or as parasites virtually in all
environments, including those which apparently are
inhospitable for rest of the life forms. They are the most
abundant and diversified group of differently shaped
micro-organisms on Earth, adapted for different living
conditions (air, soil, water), ranging from the most
ancient lineage of extreme thermophilic chemosynthetic
autotrophs to the lineages of photosynthetic autotrophs
represented by cyanobacteria. They are immensely
important for their life-sustaining ecological and
economic roles in different aspects of human well being.
Phylogenetic analysis through molecular approaches
reveals 12 major lineages or kingdoms of Bacteria. A
pinch of rich soil in your palm contains billions of
bacteria, representing around 10,000 different species.
However, this immensely diverse world still remains
largely shrouded in mystery because of cultural,
technological and taxonomic impediments. Nevertheless,
recent molecular techniques and metagenomic
approaches have helped in getting useful insights about
their structural organization and community profiling.
Bacteria: Shape and size of bacteria
Due to the presence of a rigid cell wall, bacteria maintain
a definite shape, though they vary as shape, size and
structure. When viewed under light microscope, most
bacteria appear in variations of three major shapes (Fig.
1). the rod (bacillus), the sphere (coccus) and the spiral
type (vibrio). In fact, structure of bacteria has two
aspects, arrangement and shape. So far as the
arrangement is concerned(Fig. 2), it may Paired (diplo),
Grape-like clusters (staphylo) or Chains (strepto). In
shape they may principally be Rods (bacilli), Spheres
(cocci), and Spirals (spirillum).
Bacillus: The length of the rod or bacillus may be from
as long as 20µm to as short as 0.5 µm. Rods vary in
shape too, from slender (e.g. Typhoid fever causing
bacterium), rectangular (e.g. Anthrax agents), to club
shaped (e.g. Diptheria bacillus
). While most rods occur
singly, some form long chains called Streptobacilli, e.g.
the bacterium causing a particular form of the rat bite
fever.
Fig. 1. Common shapes of bacteria
Coccus: The diameter of the spherical bacteria is
approximately of the order of 0.5µm, and the spheres
are usually round in shape, though they may vary from
oval, elongated or indented on one side. Cocci may be of
three types:
Diplococci :{cocci that remain in pairs after
reproduction}, e.g. bacteria causing gonorrhoea.?
Streptococci :{cocci consisting of chains of diplococcic},
e.g. those found in intestines or involved in strep throat.
Sarcina type :{a cube like packet of 8 cocci}, e.g
Micrococcus luteus.
Staphylococcus :{irregular grape like cluster}, e.g.
Staphylococcus aureus.
Fig. 2: Different arrangements of cocci bacteria.
Vibrio :{spiral bacteria which may either be curved rods,
e.g. Vibrio cholrea, or corkscrew shaped, e.g.
Spirochetes}. Spiral bacteria may be from 1-3 μm in
length and 0.3-0.6 μm in width. (Fig. 3)
Fig. 3. A view of the spiral bacteria.
Ultra-structural details of bacteria
The simple structural organization of a typical bacterial
cell essentially includes cell wall, cell membrane,
cytoplasm, and chromosome with the genetic material.
The other special components of a bacterial cell include
capsule, flagella, and pili. In addition, bacterial cells also
possess some specialized inclusion bodies varying in
composition and function.
Cell walls
It is situated as the outermost covering of the bacterial
cell, ranging in thickness from 15-30nm and comprising
about 10%-25% of dry weight of the bacterium. Cell wall
is characteristically composed of a common
peptidoglycan, a huge polymer of interlocking chains of
identical monomers (Fig. 4a). N-acetylglucosamine (NAG)
and N-acetlymuramic acid (NAM), basically the two
derivatives of glucose molecule that are connected by
interpeptide bridges, comprise the backbone of
peptidoglycan layer. Peptidoglycan occurs in multiple
layers, connected by side chains of 4 amino acids, and
forms a supporting net around the bacterium (Fig. 4b) .
However, different bacteria can have different amino
acids in the tetrapeptide chain, as well as different cross
links. While there is a set of identical tetrapeptide side
chain attached to N-acetyl-muramic acid, but there may
be different components and binding modes in Gram
positive and Gram negative bacteria. In Gram positive
bacteria,the third amino acid is lysine, while in Gram
negative bacteria, it is diaminopalmelic acid. Important
differences between the cell wall of Gram positive and
Gram negative bacteria are given in Table 1.
a.
b.
Fig.4a,b: N-acetylglucosamine (NAG) and N-acetlymuramic acid (NAM), the backbone of peptidoglycan layer connected by interpeptide bridges.? Table 1: Differences between cell wall of Gram positive and Gram negative bacteria.
Gram positive bacteria
Gram negative bacteria
Peptidoglycan layer very thick (25 nm)
Peptidoglycan layer thin (3 nm)
Peptidoglycan contains Tiecholic acid, an additional polysaccharide
Tiecholic acid absent
About 60-90% of cell wall is peptidoglycan
Only 10-20% of cell wall is peptidoglycan
Cell wall contains very little lipids and proteins
Cell wall contains many lipids and
proteins They retain crystal
violet iodine complex in Gram staining due to
plenty of peptidoglycan and high thickness
Gram stain is lost due to thinnes of cell wall
and abundance of lipo-proteins and lipopolyaccharides
Outer membrane absent
Outer membrane present
Periplasmic space absent
Periplasmic space present
Fig.5. Depiction of the outer membrane and periplasmic space in the cell wall. Functions of Cell Wall The most important functions of the cell wall in bacteria
include:
a. Maintaining the characteristic shape of the cell. In
fact, the rigid wall compensates for the flexibility of
the phospholipid membrane, and keeps the cell from
assuming a spherical shape ,
b. Countering the effects of osmotic pressure,
c. Providing attachment sites for bacteriophages,
d. Providing a rigid platform for surface appendages.
Flagella, fimbriae, and pili all emanate from the wall
and extend beyond it,
e. Plays an essential role in cell division,
f. Offers sites of major antigenic determinants of the
cell surface,and
g. Impart resistance to antibiotics, except those which
are wall
specific
in
nature.
Gram positive bacteria
Gram negative bacteria
Peptidoglycan layer very thick (25 nm)
Peptidoglycan layer thin (3 nm)
Peptidoglycan contains Tiecholic acid, an additional polysaccharide
Tiecholic acid absent
About 60-90% of cell wall is peptidoglycan
Only 10-20% of cell wall is peptidoglycan
Cell wall contains very little lipids and proteins
Cell wall contains many lipids and
proteins They retain crystal Gram stain is lost due
Flagella
A
flagellum (plural- flagella) is a locomotory tail-like
projection, emanating from the cell body of certain
bacteria and facilitates specific types of movement in
them. While flagella are also found in some eukaryotes,
bacterial flagella differ markedly from them in respect of
their composition, structure, and mechanism of
propulsion. The prokaryotic flagellum is about 1/10th of
the eukaryotic flagellum and is about 10-20µm in length.
Bacterial flagella also differ from eukaryotic flagella in
lacking microtubules with (9+2) arrangement and a
plasma membrane. About half of all the known bacteria
are motile due to the presence of flagella. Flagella vary in
number and placement, and their arrangement is an
important basis for bacterial classification. Depending
upon the flagellar arrangements (Fig. 6), bacteria can be:
violet iodine complex in Gram staining due to
plenty of peptidoglycan and high thickness
to thinnes of cell wall and abundance of
lipo-proteins & lipopolyaccharides
Outer membrane absent
Outer membrane present
Periplasmic space absent
Periplasmic space present
Monotrichous: with just one flagellum at one end, e.g.
Pseudomonas aeruginosa
Lophotrichous: with tuft of flagella at one end, e.g.
Pseudomonas fluorescence
Amphitrichous: with tufts of flagella at both ends, e.g.
Aquaspirillum serpens
Peritrichous
: with flagella all around bacteria, e.g.
Salmonella typhi.
Fig. 6. Different flagellar arrangements in bacteria.
Bacterial flagella are composed of long, rigid strands of
protein called Flagellin, arranged in chains and wound
around a triple helix, with a hollow central core. Each
flagellum is attached to cell membrane by a basal region
consisting of a protein other than flagellin. A flagellum is
composed of 3 parts (Fig 7):
a. basal body (associated with the cell membrane
and the cell wall), consisting of a central rod or
shaft surrounded by a set of rings.
b. A short hook,and
c. A helical filament (several times as long as the
cell), and
Gram negative bacteria have two pairs of rings (one in
cell membrane & one in cell wall). Gram positive bacteria
have just one pair (one in cell membrane & 1 in cell
wall). When flagella bundle together, they rotate counter
clock wise and allow bacteria to run in a straight line.
when flagella rotate clockwise, the flagellar bundles come
apart, causing the bacterium to tumble randomly.
Fig. 7. Structure of the flagellar components. Pilli
Pilli are hollow, non-helical, tiny, hair like filamentous
appendages, composed of protein subunits called pilin.
They are thinner, shorter and more numerous than
flagella. Pili are not generally involved in the movement,
but allow attachment to other bacteria and surfaces,
though some pilli called as IV pili, generate some motile
forces. They facilitate passage of genetic material
between bacteria (e.g. F-pilus), or help in their flotation
to increase buoyancy and reach oxygen rich surface
waters to form the Pellicle (scum on water). Pilli that help
in attachment are called as fimbrae, which are spread
over the surface ,or may be located at poles of the cell.
Fig. 8: Pilli around bacteria and facilitating bacterial copulation. Capsule and glycocalyx The cell wall in bacteria is generally surrounded by a
sticky, gelatinous, and protective layer, called as capsule,
which is formed mainly from polysaccharides and
polypeptides, or the both. It is found only in certain
bacteria, e.g. mostly in Bacilli and Cocci, but not in Spiral
bacteria. Composition of the capsule is specific to
bacteria that secretes it. A relatively thinner layer bound
less tightly to cell the wall, is called as Slime layer.
However, the currently accepted inclusive term for all
the polysaccharide containing substances found external
to the cell wall from the thinnest slime layer to the
thickest capsule is the Glycocalyx.
Functions of Glycocalyx The main functions of the glycocalyx include:
i. Serves as a buffer between the cell and its external
environment
ii. Prevents the cell against drying and dehydration due
to high water content,
iii. Helps bacteria trap nutrients and protects nutrients
from flowing away
iv. Sticky sugars allow attachment of bacteria to host
cells or tissues, rock surfaces, plant root hairs, etc.
v. Facilitates disease establishment, because
encapsidated bacteria can not be easily
phagocytosed, and
vi. It may block the attachment of the bacteriophages.
Cell Membrane
All bacteria do possess a lipoproteinaceous, bilayer
membrane (Fig. 9), which is selectively permeable,
flexible and dynamic. While the relative proportion of
lipids and proteins may vary between species, generally
membranes comprise of 60% proteins and 40% lipids.
However, there is a striking difference between
phospholipids of eubacterial and archaeobacterial
membranes, which mainly influences their differential
ability to thrive in different environmental conditions.
While in eubacteria phospholipids are phosphoglycerides,
in which straight chain fatty acids are ester linked to
glycerol. in case of archaeobacteria, the lipids are
polyisoprenoid, branched chain lipids, in which long
chain, branched alcohols (phytanols) are ether linked to
glycerol.
Fig. 9: Structure and components of the bacterial cell
membrane.
Functions of the cell membrane
The most important functions of the membrane include:
a. It regulates movement of materials into and out
of the cells ,
b. It is a seat of cellular respiration ,
c. Thylakoids in photosynthetic bacteria occur here,
d. It provides location for enzymes used in cell wall
synthesis,
e. It serves as anchor for the attachment of DNA
during replication, and
f. Flagellar appendages are based in membrane.
There is also an extensive internal membrane system in
bacteria, discernible in the form of mesosomes.
Mesosomes are invaginations of the plasma membrane,
in the shape of vesicles, tubules or lamellae, present in
both gram positive and gram negative bacteria, being
more prominent in the former. So far as the functions of
mesosomes are concerned, they are ,not still exacrly
known though a few functions attributed to them are as
given below:
a. May be involved in cell wall formation during division,
b. May play a role in chromosome replication and
distribution to daughter cells,and
c. May be involved in some secretory processes
It is pertinent to mention here that some
bacteriologists consider mesosomes just as artifacts
Cytoplasm
The cytoplasm of a bacterial cell generally comprises
about 80% water and 20% salts, proteins,
carbohydrates, lipids, and nucleic acids, etc. Cytoplasmic
streaming as exhibited by eukaryotic cells, is
characteristically absent in bacteria. Being prokaryotes,
there are no organelles, such as mitochondria, golgi
bodies, etc, but ribosomes of 70S types are attached
either to plasma membrane or present in the matrix of
cytoplasm. There are also gas vesicles, especially in
aquatic bacteria, for buoyancy and optimal utilization,
and inhabitation of appropriate micro-environments.
Some inclusion bodies (avariety of small bodies, including
granules and vesicles) are found in bacteria. Granules
contain very densely packed, compact substances, which
do not dissolve in cytoplasm; each granule contains
specific substances, such as glycogen (for energy),
polyphosphates or volutin granules (for use in
starvation), polyhydroxy butyrate granules (PHBs) for
carbon storage, and carboxysomes ( the sites for CO2
fixation). Specific kinds of proteins, called Chaperones
(heat shock proteins) help bacteria withstand heat shocks
and even osmotic shocks.
Bacterial Genome Bacteria do not have membrane bound nucleus; instead
they have a nucleoid, the part where the genetic material
is generally concentrated in the cytoplasm. Bacterial
genome (Fig. 10) is haploid, and there are certain
advantages of having this, as manifested in terms of
more efficiency, quicker growth and fast rate of
mutations. In contrast to the linear chromosomes as
found in eukaryotic cells, bacteria generally have single
circular chromosomes in addition to Plasmids, the extra
circular DNA capable of independent replication.
However, not all bacteria do have a single circular
chromosome, some do have multiple circular
chromosomes, and many bacteria even have linear
chromosomes and linear plasmids. In case of a circular
DNA molecule there are no free ends (as seen in most
eukaryotes), which may be problematic to cells for their
DNA replication and stability. Plasmids vary in number as
well as size. In size they vary from 1 to over 1,000
kilobase pairs and in number they can range in the single
cell from one to even thousands in some cases. While
plasmids have assumed incredible significance in
biotechnology and genetic engineering as vectors, they
are mainly associated with horizontal gene transfer
during conjugation in bacteria.
Fig. 10. Bacterial genome
Endospores
These are the specialized round or oval structures
resistant to heat, irradiation, cold, and remain viable
even after boiling for one hour or more;they are formed
by bacteria generally to overcome harsh conditions. If
rest of cell dies because of harsh conditions, endospore
will survive and make a new cell when environment is
favourable. This is because endospores have a tough and
resistant outer covering made of keratin, (Fig. 11) which
also resists staining; so specialized procedures are
necessary to stain bacterial endospores. Endospores are
formed only by a few genera of bacteria, such as Bacillus
and Clostridium. The location (central, sub-terminal,
terminal) of endospores is an important taxonomic
character, and used in classification of bacteria.
Fig. 11. Endospores of bacteria