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BASIC MICROBIOLOGY
MORPHOLOGY AND PHYSIOLOGY OF BACTERIA
Bacteria are prokaryotic microorganisms, which lack chlorophyll pigments
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The cell structure simpler than that of other
organisms as there is no nucleus or membrane bound organelles
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The morphological study of bacteria requires the use of microscopes
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Examination of wet films indicates the shape, arrangement, motility
and approximate size of the cells but the details cannot be cleared due to the lack of contrast
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Also the differences in refractive index between bacterial cells
and the surrounding medium make them clearly visible
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DARK FIELD MICROSCOPE
In this microscope reflected light is used instead of the transmitted light in the ordinary
microscope
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Due to the increased

BASIC MICROBIOLOGY
resolution of this microscope, very slender organisms like spirochetes can be clearly seen under
dark field microscope
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The resolving power of the electron microscope is about 0
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The scanning electron microscope is a useful innovation that permits the study of cell
surfaces with greater contrast and higher resolution
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These include,
 Simple staining
Dyes such as metylene blue or basic fuchsine are used, they provide same colour to all
bacteria
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 Impregnation methods
Where the structure to be visualized (spirochetes, flagella) is made thicker by deposition of
silver on the surface
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The two
most widely used differential stains are,
o The gram stain,
Gram Staining is the common, important, and most used differential staining
techniques in microbiology, which was introduced by Danish Bacteriologist Hans Christian
Gram in 1884
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When the bacteria is stained with primary stain Crystal Violet and fixed by the mordant,
some of the bacteria are able to retain the primary stain and some are decolorized by alcohol
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Decolorizing the cell causes this thick cell wall to
dehydrate and shrink which closes the pores in the cell wall and prevents the stain from
exiting the cell
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In case of gram negative bacteria, cell wall also takes up the CV-Iodine complex but due to
the thin layer of peptidoglycan and thick outer layer which is formed of lipids, CV-Iodine
complex gets washed off
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Then
when again stained with safranin, they take the stain and appear red in colour
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So this method is also called Ziehl-Neelsen staining techniques
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Thus Ziehl-Neelsen staining techniques were developed
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This method is used for those microorganisms which are not staining by simple or Gram
staining method, particularly the member of genus Mycobacterium, are resistant and can only
be visualized by acid-fast staining
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Then
after all cell appears red
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The nonacid fast organism lack the lipoid material in their cell wall due to which they are easily
decolorized, leaving the cells colorless
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Only decolorized cells absorb the counter stain and take its color and appear
blue while acid-fast cells retain the red colour
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In fact, structure of bacteria
has two aspects, arrangement and shape
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In shape they may principally be Rods
(bacilli), Spheres (cocci), and Spirals (spirillum)
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The limit of resolution with unaided eye is 200microns
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Bacteria of medical importance generally measure 0
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5micrometre in length
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The average diameter of spherical bacteria is 0
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0μm
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25-1
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 E
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1 to 1
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0 to 6
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• Spirochaetes occasionally reach 500 μm in length and the cyanobacterium Oscillatoria is
about 7 μm in diameter
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• One group of bacteria, called the Mycoplasmas,have individuals with size much smaller
than these dimensions
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25 μm and are the smallest cells known so far
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• Mycoplasma gallicepticum, with a size of approximately 200 to 300 nm are thought to be
the world smallest bacteria
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Usually it is 0
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3 mm
(100—300 μm) across, but bigger cells have been observed up to 0
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Thus
a few bacteria are much larger than the average eukaryotic cell (typical plant and animal
Cells are around 10 to 50μm in diameter)
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• Bacilli (or bacillus for a single cell) are rod-shaped bacteria
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Many spirilla are rigid and capable
of movement
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ARRANGEMENT OF COCCI
Cocci bacteria can exist singly, inpairs (as diplococci ), in groups of four (as tetrads ), in
chains (asstreptococci ), in clusters (as stapylococci ), or in cubes consisting of eight
cells(as sarcinae)
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Cocci may
remain attached after cell division
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1
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Examples: Streptococcus pneumoniae, Moraxella catarrhalis, Neisseria gonorrhoeae,
etc
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Streptococci
The cocci are arranged in chains, as the cells divide in one plane
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Tetrads
The cocci are arranged in packets of four cells, as the cells divide in two plains
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Sarcinae
The cocci are arranged in a cuboidal manner, as the cells are formed by regular cell
divisions in three planes
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Examples: Sarcina ventriculi, Sarcina ureae, etc
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Staphylococci
The cocci are arranged in grape-like clusters formed by irregular cell divisions in three
plains
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1
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Diplobacilli appear in pairs after division
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2
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Examples: Streptobacillus moniliformis
3
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They look like coccus and bacillus
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Palisades
The bacilli bend at the points of division following the cell divisions, resulting in a
palisade arrangement resembling a picket fence and angular patterns that look like
Chinese letters
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Many spirilla are rigid and capable of movement
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1
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Example: Vibrio cholerae

BASIC MICROBIOLOGY
2
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Spirillum with many turns can superficially resemble
spirochetes
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Example: Campylobacter jejuni, Helicobacter pylori, Spirillum winogradskyi, etc
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Spirochetes
Spirochetes have a helical shape and flexible bodies
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Examples: Leptospira species (Leptospira interrogans), Treponema pallidum,
Borrelia recurrentis, etc
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Filamentous Bacteria
They are very long thin filamentshaped bacteria
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Example: Candidatus Savagella
2
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Rectangular Bacteria
Examples: Haloarcula spp (H
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marismortui)
4
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They can change their shape
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Examples: Mycoplasma pneumoniae, M
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BACTERIAL ANATOMY
Bacteria, despite their simplicity, contain a well-developed cell structure which is
responsible for many of their unique biological structures
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Because of the
simplicity of bacteria relative to larger organisms and the ease with which they can be
manipulated experimentally, the cell structure of bacteria has been well studied,
revealing many biochemical principles that have been subsequently applied to other
organisms
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The
macromolecules are made up of primary subunits such as nucleotides, amino acids and
sugars
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Structurally, there are three architectural regions: appendages (attachments to the cell
surface) in the form of flagella and pili (or fimbriae); a cell envelope consisting of a
capsule, cell wall and plasma membrane; and a cytoplasmic region that contains the cell
chromosome (DNA) and ribosomes and various sorts of inclusions

BASIC MICROBIOLOGY
Summary of characteristics of typical bacterial cell structures

structure

flagella

function

Swimming movement

Predominant chemical
composition

Protein

Pili
Sex pilus

Stabilizes mating bacteria
during DNA transfer by
conjugation

Protein

Common pili or
fimbriae

Attachment to surfaces;
protection against
phagotrophic engulfment

Protein

Attachment to surfaces;
protection against
Capsules
phagocytic engulfment,
(includes "slime
occasionally killing or
layers" and
digestion; reserve of
glycocalyx)
nutrients or protection
against desiccation

Usually polysaccharide;
occasionally polypeptide

Cell wall
Gram-positive
bacteria

Prevents osmotic lysis of cell Peptidoglycan (murein)
protoplast and confers
complexed with teichoic
rigidity and shape on cells
acids

Gram-negative
bacteria

Peptidoglycan prevents
osmotic lysis and confers
rigidity and shape; outer
membrane is permeability
barrier; associated LPS and
proteins have various
functions

Plasma
membrane

Permeability barrier;
transport of solutes; energy
Phospholipid and protein
generation; location of
numerous enzyme systems

Ribosome’s

Sites of translation (protein
RNA and protein
synthesis)

Inclusions

Often reserves of nutrients; Highly variable;
additional specialized
carbohydrate, lipid,
functions
protein or inorganic

Peptidoglycan (murein)
surrounded by
phospholipid proteinlipopolysaccharide "outer
membrane"

BASIC MICROBIOLOGY
Chromosome

Genetic material of cell

DNA

Plasmid

Extra chromosomal genetic
DNA
material

GROWTH AND MULTIPLICATION OF BACTERIA
Bacteria divide by binary fission
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During this
process there is an orderly increase in cellular structures and components, replication and
segregation of the bacterial DNA and formation of a septum or cross wall which divides the cell
into two progeny cells
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The DNA molecule is believed to be attached to a point on the
membrane where it is replicated
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This draws the DNA molecules in opposite directions while new cell wall and membrane are
laid down as a septum between the two chromosomal compartments
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The time interval required for a bacterial cell
to divide or for a population of bacterial cells to double is called the generation time
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Fig: Some Methods used to measure bacterial growth
Method

Application

Direct microscopic count

Enumeration of bacteria in milk Cannot distinguish living from
or cellular vaccines
nonliving cells

Viable cell count (colony counts)

Enumeration of bacteria in
milk, foods, soil, water,
laboratory cultures, etc
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g
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Microbiological assays

Comments

Very sensitive if plating conditions
are optimal

Requires a fixed standard to relate
chemical activity to cell mass
and/or cell numbers

Measurement of dry weight or wet
Measurement of total cell yield probably more sensitive than total
weight of cells or volume of cells
in cultures
N or total protein measurements
after centrifugation

BASIC MICROBIOLOGY

BACTERIAL GROWTH CURVE
When a fresh medium is inoculated with a given number of cells, and the population
growth is monitored over a period of time, plotting the data will yield a typical bacterial
growth curve
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When bacteria are grown in a closed system
(also called a batch culture), like a test tube, the population of cells almost always
exhibits these growth dynamics: cells initially adjust to the new medium (lag phase) until
they can start dividing regularly by the process of binary fission (exponential
phase)
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Note the parameters of the x
and y axes
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Generation times are calculated during the exponential phase of growth
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