BACTERIA
Background
Antony van Leeuwenhoek in 1676 discovered the microbial world with his simple microscope
and was believed to have exclaimed: “Dear God, what marvels there are in so small a creature.” It
was, however only after the invention of compound microscope by Robert Hooke in 1820, that
bacteria came to be known better. Women who saw a drop of curd or vinegar under the
compound microscope, with teeming millions of small creatures, vowed never to eat curd.
These minute creatures first designated as “small microscopic species” later came to be known as
“microbe”, a term coined in 1878 by Se’dillot, a retired French surgeon of the French army and
immediately adopted by Louis Pasteur. The importance of microbes was known only after the
discovery of their role in fermentation, disease causation, and degradation of dead organic
matter. Pasteur said: “But for the work of microbes, death itself would be incomplete’’. During
the same period people believed in the theory of spontaneous generation i.e. microbes like
bacteria came into being just spontaneously (by itself). But Pasteur went on to disprove this
theory. He asserted that sterilised fruit juice remained free from bacterial contamination. He dealt
a death-blow to the then accepted theory of spontaneous generation. Robert Koch further
discovered that bacteria are the causes of diseases such as anthrax, tuberculosis and Asiatic
Cholera. Pasteur and Koch thus built a new branch of biology, called bacteriology; it has not only
advanced by rapid strides in recent years but also revolutionised the medical science.
What are bacteria?
Bacteria are unicellular organisms that are visible only under high power microscopes. Bacteria
live all around us and within us. Bacteria live in the soil, in our food, and on plants and
animals. Even our bodies are home to many different kinds of bacteria. Bacteria are often
maligned as the causes of human and animal disease but certain bacteria, the
actinomycetes, produce antibiotics such as streptomycin and nocardicin; others live
symbiotically in the guts of animals (including humans) or elsewhere in their bodies, or
on the roots of certain plants, converting nitrogen into a usable form. Bacteria play
important roles in the environment by decomposing organic wastes. In industry, they are
useful for production of antibiotics, gum, and dairy products etc.
On the other hand, bacteria are harmful to plants, animals and humans. Bacteria that are harmful
to plants are known as plant pathogenic bacteria. Much of our experience with bacteria
involves disease in human beings. Bacteria cause many cases of gastroenteritis but the
most common bacterial disease is tooth decay. Dental plaque, the sticky film on our teeth,
consists primarily of masses of bacteria. These bacteria ferment (break down) the sugar
we eat to produce acids, which over time can dissolve the enamel of the teeth and create
cavities (holes) in the teeth.
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Morphology
The bacterial cell:
Bacteria are one-celled organisms visible only through a high power microscope. Bacterial cells
are so small that scientists measure them in units called micrometers (µm) or microns. The
ordinary bacterial cell is about 1.0 to 1.5 microns long and 0.5 microns wide. One micron or m
= 1/1000 mm. One micrometer equals to a millionth of a metre or one thousandth of a millimetre
(0.000001 m or 0.001mm) and an average bacterium is about one micrometer long. It would take
about 1000 bacteria, 1 µm in length, placed end-to-end to equal one millimetre which is about the
width of a pencil line. In other words, hundreds of thousands of bacteria would fit in a rounded
dot made by a pencil. Bacteria lack a true nucleus, a feature that distinguishes them from
plant and animal cells. In plants and animals the nucleus carries genetic material in the
form of deoxyribonucleic acid (DNA). Bacteria also have DNA but it floats within the cell,
usually in a loop or coil. A tough but resilient protective shell surrounds the bacterial cell.
Biologists classify all life forms as either prokaryotes or eukaryotes. Prokaryotes are
simple, single-celled organisms like bacteria. They lack a defined nucleus of the sort
found in plant and animal cells. More complex organisms, including all plants and
animals, whose cells have a nucleus, belong to the group called eukaryotes. The word
prokaryote comes from Greek words meaning “before nucleus”; eukaryote comes from
Greek words for “true nucleus.”
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A typical bacterium, shown here, is comparatively much simpler than a typical eukaryotic
cell.
Bacteria lack the membrane-bound nuclei of eukaryotes; their DNA forms a tangle
known as a nucleoid, but there is no membrane around the nucleoid, and the DNA is not
bound to proteins and organized into linear pieces of chromosomes like in the eukaryotes.
Bacterial DNA forms loops, called plasmids, that can be transmitted from one cell to
another, either in the course of sex or by viruses. This ability to exchange genes makes
bacteria amazingly adaptable; beneficial genes (for the bacteria), like those for antibiotic
resistance, may be spread very rapidly through bacterial populations. It also makes
bacteria favourites of molecular biologists and genetic engineers because new genes can
be inserted into bacteria with ease.
Bacteria do not contain membrane-bound organelles such as mitochondria or
chloroplasts, as eukaryotes do. The cell membrane is surrounded by a cell wall in all
bacteria except one group, the mollicutes, which includes pathogens such as the
mycoplasmas. The composition of the cell wall varies among species and is an important
character for identifying and classifying bacteria. In this diagram, the bacterium has a
fairly thick cell wall made of peptidoglycan (carbohydrate polymers cross-linked by
proteins); such bacteria retain a purple color when stained with a dye known as crystal
violet, and are known as Gram-positive (after the Danish bacteriologist who developed
this staining procedure). Other bacteria have double cell walls, with a thin inner wall of
peptidoglycan and an outer wall of carbohydrates, proteins, and lipids. Such bacteria do
not stain purple with crystal violet and are known as Gram-negative.
The cell walls of bacteria of most species are enveloped by a viscous, gummy material,
which, if thin is known as a slime layer, but if thick, forming a definitive mass around the
cell, is called a capsule. Most plant pathogenic bacteria have delicate, threadlike flagella,
the number of which can vary according to species.
Morphological types of bacteria
Individual cells may occur singly, in pairs or in groups and have shapes that are rod-shaped,
spherical, curved or spirals. Bacterial cells are fundamentally of three shapes:
1) Rods - Rod-shaped bacteria (known as bacillus - plural, bacilli) are cylindrical rods with
the ends more or less rounded. Curved rods are known as vibrio
2) Spheres- spherical cells are typically round cells. They are known as cocci (singular –
coccus).
3) Spirals- in spiral forms (known as spirillum) the elongated cells have one or more bends.
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Arrangement of bacterial cells
a) Bacillus (plu. bacilli): These are small rods which may be flagellated or non-flagellated. The
flagella may be present at one end, on both ends or all over the body. This number may also vary
from one to many.
b) Coccus (plu. cocci): These are spherical cells which exist singly or in groups of two or more
cells. When it occurs singly the individual cells are known as micrococci. Two cocci formed by
simple division of a coccus remain attached with each other, to form diplococcus. Continued
division of a diplococcus in the same plane forms a chain of spheres resembling a strand of beads.
This arrangement is known as streptococcus. When there are four cells in a cube, it results in a
tetrad. If the tetrad divides to form four spheres back to back with four other spheres, this packet
of eight cocci is a sarcina. Haphazard multiplication in all directions resulting in masses of cocci
in grape-like clusters give us an arrangement called a staphylococcus. These arrangements of
cocci help in the identification of genera and species of the bacterium.
c) Vibrio (plu.vibrios): These are short, curved, comma-shaped, flagellated and monotrichous
bacteria.
d) Spirillum (plu. spirilla): These are spirally twisted and non-flexible, may be flagellated or nonflagellated. When a spirillum has several bends which are twisted in a ‘cork-screw’ fashion, they
may give the appearance as if a number of cells are attached with each other.
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e) Stalked bacteria: These are single-celled bacteria with a stalk. A tuft of flagella is present on
one side. The base of the stalk is knoblike and sticky. Several stalked cells are thus joined by their
knobs to form a rosette-like structure.
f) Budding bacteria: The cells are like a football having a swollen part and a thinner tube. The
tube gradually increases in length and swells to form a new globular cell. Ultimately a network of
cells is formed.
g) Actinomyces: For a long time actinomyces were included under fungi, but after the study of
their prokaryotic cell structure they were shifted under bacteria. The morphological similarities
with the fungi are, however, remarkable. Compared to fungi, the filaments of actinomyces are
very thin (1-5 microns). These form asexual reproductive bodies, conidia and sporangiospores,
and also multiply by fragmentation. Actinomyces are present in soil and are the most important
source of antibiotics. Examples of some genera belonging to this group are: Mycobacterium,
Nocardia, Actinomyces, Streptomyces, Micromonospora, Actinoplanes etc.
h) Myxobacteria(=Gliding bacteria)
These bacteria lack a rigid cell wall and also the flagella. They show gliding movement. The
individual cells are cigar-shaped and divide by binary fission. The cells live in colonies in a
common slimy mass. The gliding bacteria live mostly in soil, dung and water. The aquatic forms
are important cellulose decomposers. Some cause most destructive diseases of fishes.
i) Spirochaetes: These are relatively large, helical or spiral bacteria which lack a rigid cell wall.
Flagella are absent. The locomotion is not by flagellar movement or gliding but by flexions of the
body, caused by contraction of an axial filament called crista, which is wound around the cell and
anchored in the cytoplasm at each end.
A typical bacterial cell
The bacterial cell is composed of protoplasm which includes cytoplasm, chromatin, vacuoles, and
globules. The whole mass is surrounded by a well-defined membrane, the cell-wall. The cell wall
functions principally as a protective device for the underlying protoplasm and gives the cell its
shape. The cell wall consists of three distinct layers: cytoplasmic membrane, the cell wall proper,
and the slime layer. Motile bacteria have flagella whose number and position varies. Minute hairlike superficial appendages called pili covers the surface of most of the gram -ve bacteria that
help in holding the cells to water surface and during conjugation. Bacterial cell show very little
structure and lacks an organised nucleus and other organelles.
The structure of bacteria
Structurally the bacterial cell can be divided into 5 regions:
I.
Surface appendages – flagella, pilli
II.
Surface adherents – capsules and slime layers
III.
Cell wall
IV.
Cytoplasm and organelles
V.
Special structures – endospores, stalks
I. Surface appendages
a) Flagella (sing. flagellum). Flagella are the organs of locomotion in
motile forms and are many times longer than the bacterial cell. The
flagellum consists of three morphological parts: a basal body, the hook
and the filament.
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b) Pili (=Fimbriae). These are superficial appendages, which are much
smaller and narrower than the flagella, and are present in great
numbers. The pili help in adhesion of bacterial cells during conjugation
and to the water surface for better aeration. Pili, like flagella, are made
entirely of proteins.
II.
Surface adherents
Some bacteria have a gelatinous covering around them. If the covering is a loose mass, it is called
slime. When it is relatively narrow and well-defined, it is called a capsule. Chemically capsule
and slime are same – a polysaccharide of glucose. Depending upon the thickness, the capsule is
designated as a macrocapsule or a microcapsule. The capsules provide protection to bacteria and
also serve as a food reserve. Certain pathogenic bacteria (e.g. Pseudomonas solanacearum,
causing wilt disease) owe their virulence to the capsule or slime.
III.
The cell wall
The cell wall functions principally as a protective device for the underlying protoplasm and gives
the cell its shape. The cell wall of bacteria (and also of blue-green algae) differ from rest of the
plants in being made up of mucopeptide and not cellulose. This difference provides a site where
bacterial pathogens can be attacked by antibiotics without damaging the diseased eukaryotic plant
or animal.
Christian Gram, a Danish physician, in 1884 divided bacteria by a staining method, into two
major groups, the gram+ve and Gram-ve. Those that retain the Gram’s stain after alcohol
treatment are called Gram +ve, while those that lose the stain are designated as Gram –ve.
The cell walls of Gram+ve and Gram-ve bacteria differ in their chemical composition. The wall
of Gram +ve bacteria is homogenous containing 85% or more of mucopeptide and simple
polysaccharide. The cell wall of Gram-ve bacteria, contain only 3-12 % mucopeptide, the rest
being lipo-protein and lipopolysaccharides. The wall of Gram –ve bacteria therefore appear 3
layered.
IV.
Cytoplasm and organelles
The protoplasm of any cell of living organism has been defined as a system of chemical
compounds held together in a colloidal suspension and containing carbon, hydrogen, nitrogen,
phosphorus, sulphur and other elements in the form of proteins, carbohydrates and fats.
The term nucleoplasm or karyoplasm is applied to the material within the nucleus and cytoplasm
to the material surrounding the nucleus. The cytoplasm itself is a homogenous aqueous solution
of soluble proteins, enzymes, cell solutes, inorganic ions and metabolites of small molecular
weights.
Nucleus: Bacteria lacks a well organised nucleus. And moreover the genetic material (DNA) is
not bound with proteins to form chromosomes. Instead it has a circular ring of DNA molecules
that represents the genome.
Lamellae and chromatophores: Photosynthetic bacteria have lamellae (thylakoid) or vesicles, also
called chromatophores instead of chloroplasts. They contain pigments together with enzymes and
are involved in the photosynthetic process.
Ribosomes: These are small particles which along with the reserve materials, give a granular
appearance to the otherwise homogenous cytoplasm. They are mainly involved in the protein
synthesis.
Mesosomes: These are extensions of the plasma-membrane are actively involved in the cell
division.
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Reserve materials (granules): There are mainly two types of granules- volutin granules glycogen.
The volutin granules act as reserves of phospates, while glycogen are reserves of polysaccharides
(sugars).
Reproduction and life-cycle of bacteria
The spherical cells divide by binary fission into two daughter cells each of which assume the size
of the mother cell and becomes separated. The usual method of reproduction is by binary fission.
Cells are divided by a simple division into two halves. Each half becomes an independent
individual. The process may be repeated once in every 20 or 30 minutes. Within this time the cell
usually attains full growth, matures, and is ready to multiply again.
Nutrition of bacteria
Like every other living organism bacteria also must eat to provide energy and to allow metabolic
processes to take place. Being a unicellular organism, they employ an ingenious method to ingest
the desired type of food. The entire cell is capable of absorbing dissolved nutrients by diffusion
though the cell membrane. This is known as holophytic feeding. Since the cell membrane is
selectively permeable it partially accounts for selectivity of food shown by different bacterial
species. The nutrients are absorbed by a process called osmosis.
In general, bacteria have a great deal of diversity in their food requirements. Certain species
(autotrophic bacteria) cannot utilize organic matter and may even be harmed by its presence.
These species synthesize complex compounds from simple inorganic salts. Carbon is taken from
carbon dioxide and energy from oxidation of certain inorganic compounds or even elements.
Thus such bacteria are independent of vegetable or animal life for their existence. For other
species (obligate or strict heterotrophic bacteria) organic matter is the sole source of food which
they decompose and then ingest. The facultative heterotrophic bacteria show characteristics
between the autrotrophs and obligate heterotrophs, being capable of utilizing organic as well as
inorganic materials. The majority of bacteria fall in this group.
Symptoms caused by bacteria
Plant pathogenic bacteria cause leaf spots, blights, soft rots of fruits, roots and storage organs,
wilts, overgrowths, scabs, and cankers.
Reference:
1) Agrios, G.N. (1988). Plant Pathology.
2) Dube, H.C. (1978). A Textbook of Fungi, Bacteria & Viruses. Vani Educational Books.
3) Pelczar, M.J., Chan, E.C.S. & Krieg, N.R. (1986). Microbiology. Tata McGraw-hill
Publishing Co. Ltd. N.Delhi.
4) Singh, R.S. (1983). Plant Disease. Oxford & IBH Publishiong Co. N. Delhi.
5) Agrios, G.N. (1997). Plant Pathology. Academic Press.
Websites:
http://www.umsl.edu/~microbes/pdf/introductiontobacteria.pdf
http://encarta.msn.com/encyclopedia_761574409_1/Bacteria.html
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