Focus topics: Structure present outside the cell wall structure of bacteria
Bacteria (The True Bacteria or Eubacteria)
The bacteria constitute a very wide group
of micro-organisms that exhibit a fascinating diversity in morphology, habitat,
nutrition, metabolism, and reproduction. Although they are not very complex
morphologically, the tiny bacteria nevertheless have highly complex
physiological, biochemical, cytological, and genetical characteristics making
them a valuable tool for understanding the various intricacies of life. Due to
their extreme simplicity in structure, small size favoring rapid cell division,
highly resistant nature and diversified mode of nutrition, bacteria are of
universal occurrence. They are present in our mouth and flourish in intestine.
They are present in air we breathe and in food we eat; they abundantly occur in
fresh and salt water, soil water and even in ice. Their most favorable habitat
is soil, where they occur in abundance mainly in the upper half feet. In a
handful of garden soil, the bacterial population may outnumber the human
population on the earth.
They live in all conditions not fatal to
living beings and are among the most numerous of all living beings present in
almost every conceivable environment. Some bacteria are deadly parasites of
plants, animals and human beings; some live as mutualists with plants or as
commensals in the alimentary canals of animals. Some bacteria may remain viable
when cooled upto -190°C, while others may remain viable when boiled upto 78°C.
Morphology of Bacteria
The following categories of bacteria are
recognized on the basis of diversity in their morphological features
1. Unicellular Bacteria
2. Filamentous Bacteria
3. Myxobacteria
4. Sprichaete
Unicellular Bacteria
Cocus
These
-unicellular bacteria are spherical, varying 0-1.25 μ in diameter and
existing either singly (Micrococcus), in pairs (Diplococcus), in chains
(Streptococcus), in clusters (Staphylococcus), or in cubical masses of 8 or
more cells (Sarcina).
Bacillus
These unicellular bacteria are hyphen (-) or small rod-shaped ranging about 1.5
J1 in diameter and 10 μ in length. They occur either singly (Microbacillus), in
pairs (Diplobacillus), in chains (Streptobacillus), or in palisade arrangement.
Bacillus anthracis, B. subtilis, Lactobacillus and Clostridium are the common
examples of bacilli bacteria.
Characteristic groupings of cocci; A. micrococcus; B. diplococcus; C.
streptococcus; D. tetracoccus
E. sarcina; F. staphylococcus.
Vibrio
When the bacilli bacteria are so curved that they look like a comma, they are
called vibrios. They seldom exceed 10 J.1 in length and 1.5 to 1.7 J.1 in
diameter, e.g., Vibrio comma.
Spirillum
When the bacilli bacteria are coiled like a cork-screw through 1-5 complete
turns, they are referred to as Spirilla. They range from 10-50 μ in
diameter. e.g. Spirillum undulum and S. volutans.
Stalked Bacteria
These are unicellular bacteria having well
defined stalks. In some cases, the stalk is a part of the cell (Caulobacter),
in others it is formed as a result of secretion from the cell (Gallionella).
Usually these bacteria have sticky, knob-like base that join each other forming
a rosette-like structure.
Budding Bacteria
These unicellular bacteria are globose having a small,
thin tube-like structure. The whole call looks like a foot-ball. The tubular
structure elongates and swells forming a new cell. This process results in a
network of globular cells, e.g., Rhodomicrobium.
Filamentous Bacteria
Some bacteria have filamentous and
branched mycelial body and for a long time they were considered to be fungi. It
was the prokaryotic nature of their cells that enabled the microbiologist to
put them under bacteria. These filamentous bacteria are called 'Actinomycetes'
and vary from 1-5μ in diameter. Some actinomycetes are pathogenic to man, while
others cause important plant diseases. Mostly these are present in soil.
Actinomycetes are very important bacteria as they are one of the most important
sources of antibiotics, e.g., Mycobacterium, Actinomyces, Streptomyces,
Actinoplanes etc.
Morphological types of bacteria, A. cocci; B. bacilli;
C. vibrios; D. spirilla; E. stalked bacteria [(i) rosette-like; (ii) single
bacterial cell)]; F. budding bacteria; G. filamentous bacteria; H. myxobacteria
[(i) fruiting bodies bearing cysts; (ii) germinating cysts releasing
cigar-shaped cells)]; I. and J. spirochaetes (helical and spiral) .
Myxobacteria
The
myxobacteria love mostly soil though they are also present in dung and water.
The cells of myxobacteria are cigar-shaped and usually form a colony in a
common slimy mass. They are peculiar for their 'gliding movement' as they lack
flagella and hence also called gliding bacteria. Though some of the gliding
bacteria do not form any fruiting body, others form. At the time of
fruetification the cells come closer and heap-up. The mucilaginous covering
around them becomes hard and the whole structure looks a tree with the branches
hearing brightly coloured oval or spherical cysts. In each cyst hundreds of
bacterial cells are present which glide out when the cyst-wall ruptures.
Examples of myxobacteria are, Beggiatoa, Chondromyces.
Spirochaete
Spirochaetes
are the bacteria having spiral-shaped body but lacking a rigid cell wall. They
measure 3-495μ in length; are flexuous and motile, lacking flagella and the
movement is spinning or whirling brought about by flexions of the body. The
flexion is caused by contraction of fibrils called crista. Each end of crista
is anchored in the cytoplasm. Spirochaetes are found in fresh, sea and polluted
waters. They divide by binary fission and do not produce resting spores.
Examples of spirochaetes are, Cristispira, Treponema, Spirochaeta etc.
Nanobacteria
Nanobacteria are very minute bacteria existing in nature. The size of putative
nanobacteria is considered to be of 0.1 mm diameter for coccus-shaped
structures. Opponents of the existence of nanobacteria claim that these are
simply the artifacts of chemical or geochemical reactions of non-living
materials and that even the smallest bacterial cells are significantly larger
than reported nanobacteria. Opponents also argue that if one considers the
space needed to store all of the essential biomolecules of life, it is highly
unlikely that these could arrange themselves within the volume available to a
structure of 0.1 μm or less. Therefore, the very existence of nanobacteria is
controversial. If such very small cells do exist, they would represent the
smallest known living structures.
Size of Bacteria and the Significance of Their Being Small
Bacteria vary in size from cells as small
as 0.1-0.2 μm in diameter to those more than 50 μm in diameter. The dimensions
of an average rod-shaped bacterium, Escherichia coli, for example, are about 1
x 3 μm. For comparison, typical eukaryotic cells may be 2 µm to more than 200
μm in diameter. Bacteria are thus extremely small in comparison to eukaryotes.
Surface area and volume relationships in speres
Small size of bacteria (and almost all
other prokaryotes) affects a number of their biological properties. For
convenience, the rate at which the nutrients are taken in and wastes area
passed out of a cell is in general inversely proportional to cell size. This is
because transport rates are to some degree a function of the membrane surface
area available; small cells have more surface area available than do large
cells. As a cell increases in size, its surface area-to-volume (SN) ratio
decreases. The surface area-to-volume (SN) ratio of a sphere (cell) is
expressed as 3/r.A small cell having a smaller r (radius) value has a higher SN
ratio than a larger cell and, therefore, can enjoy more efficient exchange of
nutrients with its surroundings than can a large cell, show more rapid growth
rates and the formation of larger cell populations. The parameters of rapid
growth and larger cell populations greatly affect microbial ecology. It is so
because high numbers of rapidly metabolizing cells can cause major
physio-chemical changes in an ecosystem even over very short periods of time.
The Largest Known Bacterium
Heidi Schulz (1997) has discovered the
largest of all known bacteria, Thiomargarita namibiensis from the ocean
sediment off the cost of Namibia.
This bacterium is spherical often forming chains of cells and ranging between
100-750 mm in diameter. It is considered to be over 100 times larger in volume
than Epulopiscium fishelsoni (a bacterium related to. the gram positive genus
Clostridium about a million times larger in volume than Escherichia coli and
discovered by Fishelsion et al. in 1985). T. namibensis possess a vacuole that
occupies 98% of the cell volume and is surrounded by a 0.5-2.0 mm layer of
cytoplasm filled with sulphur granules
Flagellation
The pattern of flagellar arrangement (flagellation) is a good identification
mark in bacteria. Flagella are either confined to the pole or poles or it may
be present alround the body of the bacterium. However, bacteria can be grouped
as under on the basis of flagellation:
Bacterial Flagellation
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A. Atrichous
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B.Monotrichous
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C. Flagellation
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D. Amphitrichous
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E.Lophotrichous
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F.Peritrichous
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Atrichous - Bacteria that lack flagella. E.g. Staphylococcus
aureus
Monotrichous - Single flagellum on either of the poles of the
bacterial cell. E.g. Pseudomonas aeruginosa
Amphitrichous - One flagellum or more on each pole of the bacterial
cell. E.g. Spirillium volutans
Lophotrichous - Flagella in groups present on one pole of the
bacterium. E.g. Pseudomonas florescens
Peritrichous - Flagella present all around the body of the
bacterial cell. E.g. Salmonalla typhi
Flagellar Movement
Bacterial (prokaryotic) flagella operate
in different manner when compared to eukaryotic flagella. Each individual
flagellum is a semi-rigid helix and moves by rotation like propellers on a boat
(Fig. 3.4). The rotatory motion of the flagellum is imparted from the motor at
the base. A rod or shaft extends from the hook and ends in the M-ring which can
rotate freely in the plasma-membrane. Though the exact mechanism of flagellar
rotation still is not clear, it is believed that the S-ring is attached to the
cell wall in gram-positive bacterial cell and does not rotate. The P and
L-rings of gram-negative bacteria act as bearings for the rotating rod.
The energy required for the rotation of
the flagellum comes from the proton motive force. (PMF), not
directly by ATP as is the case with eukaryotic flagella. The plasma membrane
becomes energetically charged and during this state protons (H+) are separated
from hydroxyl ions (OH-)
across its surface. This charge separation is a form of energy. This energized
state of plasma membrane is called proton motive force.
Calculations have shown that about 1000 protons must be translocated per single
rotation of the flagellum.
Monotrichous and lophotrichous polar
flagella rotate counter-clockwise and make the bacterial cell spring around and
run forward from place to place with the flagellum trailing behind. These
bacteria stop and tumble randomly by reversing the direction of flagellar
rotation. The flagella of peritrichous bacteria rotate counter clockwise, like
montrichous and lophotrichous ones, to move forward. The flagella bend at their
hooks to form a rotating bundle that propels them forward. Clockwise rotation
of the flagella disrupts the bundle and the cell tumbles
A bacterial cell can run in water from 20
to almost 90 μm/second which is equivalent to travelling from 2 to over 100
cell (body) length/second. In contrast, an exceptionally fast 6 ft man can run
around 5 body length/second and a cheetah (the fastest animal) can run about 25
body lengths/second. Thus, when size is accounted for, bacterial cells run
actually much faster than larger organisms:
Flagella (Sing. Flagellum)
These are long filamentous organs of
locomotion that arise from the cytoplasmic membrane and pass out through the
cell wall. A flagellum of bacteria cell consists of three distinct parts-the
basal body, the hook, and the filament. The basal body
constitutes the extreme basal part of the flagellum attached with the plasma
membrane; the hook represents a somewhat broader and thicker
basal region of a flagellum and passes out through the cell wall; and the filament
is the thinner, elongated, terminal part. However, the structure of the
bacterial flagellum allows it to spin like a propeller and thereby helps moving
the bacterial cell
Ultrastructure of a typical bacterial cell
(diagrammatic)
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1.
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Flagellum
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9.
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RNA
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2.
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Pilus
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10.
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Nucleoid
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3.
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Slime Layer
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11.
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Gas Vacuole
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4.
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Cell Wall
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12.
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Poly-β-Hydroxy-Butyric Acid
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5.
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Cyoplasmic Membrane
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13.
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Thylakoids (lamellae)
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6.
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Chromatophores (vesicles)
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14.
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Polyribosome
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7.
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RiBosomes
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15.
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Plasmid
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8.
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Mesosome
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16.
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Metachromatin Granules (Volutin Granules)
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A. Motion
of Monotrichous Polar Bacterium
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B. Motion
of Peritrichous Bacterium
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Figure: Flageller motility. A.
motion of monotrichous polar bacterium, B. motion of peritrichous bacterium
Figure: Bacterial Flagellum. A. morphological views,
longitudinal view of flagellin molecule chains arranged in 8- rows. C.
cross-section of the flageller filament
Filament
The filament is a fine, cylindrical, helical hollow structure, about 120-200Ǻ
in diameter. It is composed of a fibrin protein called 'flagellin' structurally
similar to keratin and myosin proteins and ranging in mol. wt. from 30,000 to
60,000 dalton. A cross section of the filament reveals that there are eight
flagellin molecules surrounding a central hollow cylinder. Actually, the 8
flagellin molecules seen in the cross section are the parts of flagellin
molecule-chain eight in number and running longitudinally around the central
hollow cylinder. Each chain contains approximately 1,000 spherical, smaller
flagellin molecules each of 40 Ǻ diameter. In this way, the bacterial flagellum
fundamentally differs from the flagellum of an eukaryotic cell, which has 9 + 2
type of arrangement in its filament.
Hook
As said earlier, hook is a somewhat broader and thicker basal region of a
flagellum and passes out through the cell wall. It is made up of a single type
of protein and functions to connect the filament to the basal body of the
flagellum
Basal Body (The Motor Portion of the Flagellum)
The
basal body is the most complex part of a flagellum. In gram-negative bacteria,
the basal body has four rings connected to a central rod. The outer L and
P rings remain embedded in the lipopolysaccharide and the
peptidoglycan layers respectively. The inner Sand M rings are
located within the cytoplasmic membrane. In gram-positive bacteria, which lack
the outer lipopolysaccharide layer, only the inner pair of rings is present. There
are a pair of proteins called Mot A and Mot B that
surround the inner ring and are associated to the cytoplasmic membrane. In
addition to Mot proteins, there is a final set of other proteins called Fli
proteins.
Actually,
the portion of the basal body that rotates (the motor) is consisted of the rod,
the M-ring, and the Mot and Ph proteins. The Mot proteins actually drive the
motor causing a torque that rotates the filament whereas the Fliproteins
function as the motor switch reversing rotation of the flagella in response to
signals send by the bacterial cell.
Details of the flagellum of gram-negative bacterium
showing mechanism of flageller movement
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1.
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Filament
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6.
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Peptidoglycan Layer
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11.
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Mot A
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2.
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Hook
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7.
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Periplasmic Space
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12.
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Fli Proteins
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3.
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L-Ring
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8.
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Plasma Membrane
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13.
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Mot Proteins
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4.
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P-Ring
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9.
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S-Ring
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14.
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M - Ring
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5.
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Outer Membrane
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10.
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Mot B
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15.
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H
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Pili, Fimbriae and
Spinae
Some
bacteria possess fine hair-like projections on their surface. These fine
projections are called pili or fimbriae and spinae,
and originate from the cell membrane. The term pili was introduced by Brinton
(1950) and fimbriae by Duguid et al. (1955). Both these are structurally
similar to flagella, but are not involved in motility. They have been observed
mostly in gram-negative bacteria, and measure 3-25 nm in diameter and 0.5-20
(mu) m in length. Both are made up of protein (pilin)
molecules with a molecular weight of about 17,000
Pili and fimbriae
Fimbriae are
considerably shorter than flagella and are more numerous. The functions of
fimbrae are not known with certainty, some evidences suggest that they enable
microbes to stick to surfaces of host in the case of pathogenic bacteria, or to
form pellicles or biofilms on their surfaces.
Pili are generally longer than fimbriae and only one or a
few pili are present on the surface. They genetically determined and their
number and type vary in different strains. Some are used for attachment;
pathogenic bacteria, for example, use them to identify and to attach to their
host cells.
Others,
the F or sex pili, are
involved in bacterial conjugation and are found exclusively on the cells that
donate DNA during this process. If these pili are absent or if the bridge
established by sex-pili between the donor and recipient cell is interrupted,
the conjugation process is not completed. Pili, however, also act as receptor
sites and provide site for attachment for some bacteriophages on the bacterial
cells.
In some gram-positive bacteria, spinae
have been reported. These are tubular, pericellular, non-prosthecate rigid
hairy-appendages said to help adjust bacterial cells to some environment
conditions such as pH, salinity, temperature etc. Spinae are made up of a
single protein, namely, spinin.
The Glycocalyx :
Slime Layers And Capsules
At
the time of their active growth, many bacteria produce
polysaccharide-containing substances of high molecular weight. These substances
collect on the surface of the cells and form a gelatinous covering around them.
This covering is called glycocalyx. When the glycocalyx does
not form a persistent layer, but is present more diffusedly forming a loose
mass around the bacterial cell, it is called slime layer. The
slime layer can very easily be removed by washing the bacterial cells. When the
gelatinous covering forms a well-defined persistent layer, it is called capsule.
Capsules are composed generally of polysaccharides
(e,g., Streptococcus mutans, S. salivarious. Xanthomonas. Corynebacteria) that
contain, apart from glucose, aminosugars, rhamnose, uronic acids of various
sugars, 2-keto-3-deoxygalactonic acid, and organic acids such as pyruvic acid
and acetic acid. However, the capsules of some bacilli bacteria (e.g., Bacillus
subtilis, B. anthracis) consist of polypeptides, mainly poly glutamic acid.
Capsules can be seen by negative staining with dyes that do not penetrate the
capsular material, such as India ink, Chinese ink, nigrosin or Congo red. In
negative staining, the capsule appears light against a dark background. Slime
layers and capsules may protect the bacterial cell against dehydration and a
loss of nutrients. In addition, the capsule is especially important in
protecting bacterial cells against phagocytosis by various protozoa and white
blood cells and, therefore, capsulated bacteria usually prove to be virulent
pathogens in comparison to non capsulated ones which are easily subjected to
phagocytosis by WBCs.
