This document provides an introduction to microbiology and common microorganisms. It discusses that microbiology is the study of microorganisms like bacteria, viruses, protozoa, and fungi. Most microorganisms are too small to be seen without a microscope. The document classifies microorganisms and describes some of the most common types like bacteria, viruses, protozoa, and fungi. It provides details on the structure and characteristics of different microorganisms and how they are identified.

1. INTRODUCTION TO
MICROBIOLOGY AND
COMMON
MICROORGANISMS
AND EPIDEMIOLOGY

2. MICRO-ORGANISMS
Microbiology is a branch of medical science which
deals with the study of shape, size, structure,
reproduction, physiology and classification of micro-
organisms (small forms of living organisms).
Mostly micro-organisms are smaller than 1
micrometer in size and are not visible to the naked
eye. They can be seen only through microscope.
Some of the pathogenic micro- organisms eg.
Viruses are too small to be seen through an ordinary
microscope. Viruses can be seen only with an
electron microscope. Viruses vary in dimater from 20
nanometers to 250-400 nmonly a few number of
micro-organisms are pathogenic (disease producing)
to humans. But a large number of micro-organisms
are not harmful to humans and are known as non-
pathogenic organisms.

3. CLASSIFICATION OF MICRO-
ORGANISMS
 Micro-organisms are neither plants nor animals so they are
included in the kingdom named as Protista. Disease
producing micro-organisms are classified as below: 1.
Bacteria, 2. Rickettsia, 3. Mycoplasma, 4. Algae, 5. Fungi,
moulds, yeast, 6. Viruses, 7. Protozoa.

4. CLASSIFICATION OF MICRO-
ORGANISMS
 Depending upon the cellular structure, the micro-organisms
may be classified under thefollowing two broad groups:(i)
Prokaryotic: They have relatively primitive cells e.g. Bacteria,
rickettsia, mycoplasma. (ii) Eukaryotic: They have more
advanced cells eg, algae, fungi, moulds, protozoa.

5. BACTERIA
 Bacteria are extremely minute one celled organisms. They occur very
widely in nature and in very large numbers. They are found in animals,
plants, soil, water and the atmosphere, Bacteria generally vary in size
from 0.2 to 1.5 micrmeter (micron) in diameter and 3 to 14 micron in
length. They reproduce by an asexual process under favourable
conditions. The majority do not produce diseases but serve as a useful
scavengers in the breakdown of organic matter. Many of them are useful
in the preparation of curd, wine, antibiotics and synthesis of vitamins in
the body.

6.  Classification: Depending upon the shape, bacteria can be
classified as: (a) Cocci: They are spherical bodies with a
smooth outline and diameter in the range of 0.75 micron to
3.0 micron.Depending upon the characteristic arrangement
the cocci are classified asi) Coccus : The cells are arranged
singly.(ii) Diplococci : The cells are arranged in pairs e.g.
Pneumococci, Meningococci, Gonococci.

7.  (iii) Streptococci : The cells are arranged in chains of different
length e.g. Streptococci pyogenes.(iv) Tetra cocci: The cells
are arranged in groups of four.(v) Staphylococci : The cells
are arranged in clusters like a bunch of grapes e.g.
Staphylococcus aureus.(b) Bacilli (Rods) : Bacilli are almost
cylindrical in shape and vary greatly in length and diameter
e.g. Length 0.75 to 10 micron and diameter 0.75 micron to 3
micron. Bacilli are also arranged in chains or groups of two
(Diplobacilli).

8.  (c) Vibrios: These are comma shaped.(d) Sprilla: These are in
the form of partial rigid spirals.
 (e) Spirochaete: These have a number of turns and are
flexuous.

9. ACTINOMYCETES
Actinomycetes: They form true branches and
remain adherent in chains. Thus they
resemble fungal mycelia. Hence they are
sometimes known as ray fungi.
Clear space between the bacterium and the
surrounding film. Chemically it consists of
polysaccharide (usually) or polypeptide. When
it is a thick and prominent layer, it is called a
capsule. It protects the organism against
changes of environment. (ii) Cell wall: In most
bacteria, the cell wall is a rigid structure but in
some species of soil bacteria the cell wall is
flexible. It consists of protein and polypeptide
with mucia, polysaccharide or phospholipid
components.

10.  (iii) Flagella and Fimbria: These are out-growths of the protoplasm protruding throughthe cell wall. Flagella
are uncommon in spherical bacteria but occur in about 50% of rods(bacilli). They are responsible for
movement in most types of motile bacteria. Fimbria areshorter than flagella and they help the bacterium in
attachment and in some organisms theyare involved in the transfer of genetic material from the donor
(male) cell to the recipient(female) cell.(iv) Cytoplasmic membrane: It is a closely packed lipoprotein layer,
two to four molecules thick beneath the cell wall. It is a semi-permeable membrane and the site of
considerable physiological adivity.(D) Cytoplasmic constituents:(a) Nuclear material: The bacterial cell
doesn’t have a defined nucleus bounded by a nuclear membrane but it does contain genetic material in
the form of one or twodeeply staining bodies possibly presenting chromosomes.(b) Ribosomes: Ribosomes
are the sites of protein synthesis.(c) Vacuoles: Their presence in cells grown on glucose agar is used to
differentiate species of the genus bacillus. In addition to above inclusions – oil and wax droplets and other
crystal-line materials are also present.

11. SPORES
 Under conditions unfavourable for growth of the cell, certain
bacteria can produce a spore (resting body) within the cell
wall. Spores are 1 to 3 micron in diameter and ovoid in shape.
The outer coat is water repellant and the whole structure
contains much less water than the vegetative bacterium from
which it is founded. Spores are resistant to a wide range of
chemical and physical agents. Spores germinate into bacterial
cell when they come across the favourable conditions

12. ALGAE
 Algae are a group of chlorophyll bearing plants. Some of the
algae are unicellular whereas other are multicellular which may
form filaments. They are predominantly aquatic, living either in
fresh or salt water. Many species are terrestrial and are found
growing in or on moist soil, on rocks and snow. Due to the
presence of chloroplyll the algae are autotrophic in their mode
of nutrition. i.e. (They can produce their food by
photosynthesis).The various species exhibit a great variety in
colour ranging from green, yellow-green and blue green to
red, yellow, orange, olive and brown as the green colour is
marked by other pigments. In algae, a definite nucleus is
present inside the cell and they may reproduce sexually or
asexually.Several species of algae are used as human food in
different parts of the world. Because they are rich in
carbohydrates and vitamins. They form an important source of
food for fishes and other animals.

13. FUNGI, MOULDS AND
YEASTS
 Fungi are eukaryotic protists and constitute the largest sub
group of thallophyta. They cause a variety of diseases in
humans. They are achlorophyllous: so they can’t manufa their
own food. Fungi have rigid cell wall containing polysaccharide
cellulose or chitin or (fungus cellulose). The cytoplasmic
membrane of fungi contains sterols. They are unicellul or
multicellular. They reproduce sexually, asexually or by both
mechanisms The vegetative body is a simple thallus and in the
majority of fungi the thallus consists ofa large number of
slender, branched or unbranched tubular filaments known as
hyphae. Thehyphae are loosely interwoven to form
mycelium.Types of fungi: Depending on the cell morphology
the fungi are divided into the following four groups (0) Yeasts:
They are generally unicellular fungi. Their cells are spherical or
ellipsoidal They are found in the air, soil, intestine of animals
and on fruits. They reproduce by buddingThe only pathogenic
yeast known is Cryptococcus neoformans Yeast like fungi: They
partly grow as yeast and partly as hyphae called
pseudomyceline eg. Candida albicans. It produces thrush in
humans.

14. Moulds: The small multicellular,
multinuclear, filamentous fungi are called
mouldsSeveral fungal moulds provide very
important chemicals (Gallic acid, Diastase)
and antibiona (Penicillin). A few of them
produce diseases in plants, animals and
human beings. (iv) Dimorphic fungi: These
can occur as filaments or as yeasts. Most of
systemic fungalinfections are caused by
dimorphic fungi.
RICKETTSIAThese are very small, unicellular,
gram negative obligate intracellular
parasites. They are pleomorphic and may
be rod shaped or spherical in shape. They
have typical bacterial cell walls and they
multiply by fission. They are an
intermediate form of life between bacteria
and virus. Typhus fever, Q fever etc. Are
caused by rickettsia

15.  MYCOPLASMAThese are prokaryotic, very small cellular organisms. They don’t possess true cell wall The
cell size ranges from 0.15 m to 1 m in diameter. Most mycoplasma are parasites on man or animals. Primary
atypical pneumonia and genital infections are examples of Mycoplasmic infections.VIRUSESThe term virus
literally means poison. These are non-cellular, ultramicroscopic disease producing particles of organic
matter which can grow and multiply within the host cells only. They contain only one type of nucleic acid
either RNA or DNA.Viruses can cause many infectious diseases in human beings ranging from common
cold to highly fatal diseases such as AIDS, cancer, rabies, meningitis or yellow fever.

16. SIZE, SHAPE AND
STRUCTURE
 Size: The virus particles are very small so they are measured in
milli microns. Their size vary from 10 milli micron to 450 m
microns.Shape: When examined under electron microscope,
they are observed to be spherical, oval, rod like or tadpole
shaped.Structure: A virus particle capable of infecting a specific
host is termed as virion. A typical virion consists of a core of
genetic material or nucleic acid (RNA or DNA, never both),
partly or wholly surrounded by a sheath of non-genetic
protein coat. The former is known. As nucleoid and the latter
as capsid. The nucleic acid or the nucleoid in plant viruses
always consists of RNA whereas in animals and bacterial
viruses (bacteriophages), with a few exceptions, it is made up
of DNA.

17. PROTOZOA
 These are the lowest and simplest forms of all animal life. These are unicellular organisms
with protoplasm clearly differentiated into a nucleus and cytoplasm. The unicellular
protozoa contain all the structures, necessary to carry out metabolism and reproduction.
Under unfavourable conditions some species of protozoa form round, thick walled cysts
which are important for their survival and spread of the organism. Protozoa multiply
asexually by binary fission or multiple fission. Certain species may multiply sexually. The
important human protozoal infections include amoebiasis, malaria, giardiasis,
trypanosomiasis, balantidiasis.IDENTIFICATION OF BACTERIABecause bacteria are very
small it is often difficult to differentiate between the closely related species. Generally a
large number of tests are necessary to establish identity.(i) Sensory characters: The
shapes and colours of the colonies, their consistency and the odour of metabolic
products may help towards, at least a partial conclusion.(ii) Microscopic examination: The
morphological study of the isolated causative micr organism is carried out by using a
compound microscope. Micro-organisms may be examined in stained or unstained
conditions. Since bacterial protoplasm has almost the same refractive index as the
medium in which it grows, therefore the micro-organisms do not show much structural
details. So, they are stained prior to their microscopic examination. Staining helps the
bacteria to be seen completely than in direct preparation. Various types of staining
solutions commonly used in microbiology include crystal violet, methylene blue, carbol-
fuchsin and aqueous nigrosin. The use of phase contrast illumination avoids the artefacts
introduced by using stainsVery delicate bacteria such as syphilis spirochaete are more
easily seen under darkillumination.Preparation of Smear or Film: The preparation of a film
is the first step in staining procedure. Film is usually prepared on 3” x 1” glass slide or
sometimes on cover slip A loopful of fluid specimen is taken and spread as thin film on a
slide. The film is dried in air and then heat fixed by passing gently through the Bunsen’s
flame.

18. STAINING METHOD
 (i) Simple staining (Monochrome staining): In this method only one staining
solutionis used to study the morphological characteristics and motility of
the micro-organisms. Dyesused in this method are crystal violet, methylene
blue, fuchsin and safranin. These dyesimpart the same colour to all
bacteria.1. Staining Schedule:(I) Apply the stain to heat fixed film and allow
to react for 30 secs to 3 minutes (depending upon the type of stain used).(iI)
Wash the smear with water and dry.(iii) Examine the slide under oil
immersion lens of the microscope. 2. Negative staining: In this method,
nigrosin or India ink solution is used, which provide a uniformly coloured
background. The unstained bacteria can be seen against the coloured
background. This method is useful for observing bacterial capsules which
don’? Take simple stains.3. Differential staining: Simple staining with a single
dye affords information about themorphology of the bacteria, but
differential staining with more than one dye distinguishesdifferent types of
bacteria. The various staining methods for differential staining are: (a)Gram
staining method, (b) Acid fast staining method.(a) Gram staining method:
This type of staining was first used by a Danish bacteriologist Christian
Gram is 1884 and hence known as Gram staining Staining Schedule(i)
Primary staining: Stain bacterial smear for one minute with crystal violet (or
methy! Violet or gentian violet) and then wash off the stain with water. (ii)
Application of iodine: Immerse for 1 minute in iodine solution prepared by
adding 1g iodine and 2g of potassium iodide to 300 ml of distilled water.
Wash off the slide with water and blot dry.

19.  (ii) Decolouration : Decolourize for 30 seconds
with gentle agitation in 95% alcohol and blot
dry.(iv) Counter staining: Counter stain for 10
seconds with alcoholic safranin solution and
wash with water. Dry and examine the smears
under microscope.

20. RESULT
 Gram +ve bacteria: Gram +ve bacteria resist decolouration and retain the primary stain
and appear as blue (or violet).Gram-ve bacteria: These are decolourized by organic
solvents and take up counterstain and appear red in colour.Gram staining method is
routinely used to detect Mycobacterium, Staphylococci, Streptococci, Gonococci, E.coli
etc. But this method cannot be used to detect capsules, spores, flagella, fungi and
protozoa(b) Acid Fast Staining: Acid fast staining method was first developed by Paul
Ehrlich in 1882 for differential staining of micro-organisms. By using this method,
bacteria can be categorised as Acid fast and other bacteria. Acid fast cells retain the
primary stain and resist decolouration by both acids and alcohols. While non-acid fast
bacteria lose the stain and get decolourized with the treatment of acid and alcohol.
 Staining Schedule(1) Primary Staining: Prepare the smears and fix them on a flat surface
over boiling water. Stain for 3 to 5 minutes with Ziehl’s carbol fuchsin, apply heat to
permit gentlesteaming and rinse in tap water. (ii) Decolouration : Decolourize in 95%
alcohol containing 3% by volume of concentratedHCI until only a faintly pink smear
appears. Wash with the water. (iii) Counter Staining: Counterstain with saturated aqueous
methylene blue or Loeffler’s methylene blue. Wash with water.Dry and examine the
smears under the microscope. Results: Acid fast bacteria are stained pink or bright red
while the other bacteria are stained blue or green. This method is used to detect
Mycobacterium species (ie M. Tuberculosis and M. Leprae).4. Fluoroscence method: When
bacteria are stained with a fluoroscent dye (eg. Auramine O, Acridine orange etc.) and are
examined under a microscope with U.V. Light, they become luminous and are seen as
golden yellow objects against a dark background. This method is used to identify scanty
acid fast organisms especially Mycobacterium tuberculosis and Mycobacterium leprae.