Structure & Classification of
Microbes
- Khyati Chaudhari

MICROORGANISM……?
 Micro means small, very small,
can’t see by naked eyes.
 Which can be seen by using
electron microscope..

Classification of Microorganism
 All living organisms are classified into the five kingdoms of life :
1. Monera
2. Protista
3. Fungi
4. Plantae
5. Animalia

Kingdom Monera
 Monera is non-nucleated unicellular organisms.
 They are prokaryotes.
 They have a cell wall. They have no membrane-bound organelles
such as mitochondria, Golgi complex.
 They lack a true nucleus. Instead, they have nucleoid, genetic
material without a nuclear membrane.
 Examples include Bacteria, cyanophyceae (Blue-Green algae),
Nitrogen-fixing organisms etc.

Kingdom Monera
Some examples include:
 Helicobacter pylori.
 E. coli.
 Hay bacillus.
 Salmonella.
 Staphylococcus aureus.

Kingdom Protista
 Protista are simple eukaryotic organisms that are neither animals,
plants nor fungi.
 Protista are unicellular in nature, or they can be found as a colony
of cells.
 Most Protista live in water, damp terrestrial environments, or even
as parasites.
 The term ‘Protista’ is derived from the Greek word “protistos”,
meaning “the very first“.
 The cell of these organisms contain a nucleus which is bound to the
organelles. Some of them even possess structures that aid
locomotion like flagella or cilia.

Kingdom Protista
 Examples of protists include algae, amoebas, euglena,
plasmodium, and slime molds.

Kingdom Monera

Kingdom Fungi
 A fungus is any member of the group of eukaryotic organisms that
includes microorganisms such as yeasts and molds, as well as the
more familiar mushrooms. These organisms are classified as a
kingdom, fungi.
 Fungi are eukaryotic, non-vascular, non-motile and heterotrophic
organisms.
 They may be unicellular or filamentous.
 They reproduce by means of spores.
 Fungi exhibit the phenomenon of alternation of generation.

Kingdom Fungi

Kingdom Plantae
 Plants: Kingdom Plantae.
 Kingdom Plantae includes all the plants on the earth.
 They include familiar organisms such as trees, herbs, bushes,
grasses, vines, ferns, mosses, and green algae.
 They are multicellular, eukaryotes and consist of a rigid structure
that surrounds the cell membrane called the cell wall.
 Plants also have a green colored pigment called chlorophyll that is
quite important for photosynthesis.

Kingdom Plantae

Kingdom Animalia
 All animals are members of the Kingdom Animalia, also called
Metazoa.
 This Kingdom does not contain prokaryotes.
 There are over 9 million species of animals found on Earth.
 They range from tiny organisms made up of only a few cells, to the
polar bear and the giant blue whale.
 All of the organisms in this kingdom are multicellular and
heterotrophs - that means they rely on other organisms for food.

Kingdom Animalia

Classification of Microbes
 Microorganisms are a varied group of several distinct classes of
living beings classified under the Kingdom Protista.
 Based on differences in cellular organization & biochemistry,
Protista has been divided into two groups :
Prokaryotes &
Eukaryotes.

Cont.…
 Bacteria & blue-green algae are prokaryotes & while fungi, slime
molds & protozoa are eukaryotes.

Character Prokaryotes Eukaryotes
Nucleus
Nuclear membrane Absent Present
Nucleolus Absent Present
Chromosome Circular (1) Linear (>1)
Cytoplasm
Mitochondria Absent Present
Lysosomes Absent Present
Golgi apparatus Absent Present
Endoplasmic reticulum Absent Present
Chemical composition
Sterols Absent Present
Muramic acid Present Absent
 Some differences between prokaryotes & eukaryotes

Prokaryotic cell

Size of bacteria
 The unit of measurement used in bacteriology is the micron
(micrometer, µm).
 The limit of resolution with the unaided eye is about 200 microns.
 Bacteria, being much smaller, can be visualized only under
magnification.
 Bacteria of medical importance generally measure 0.2-1.5 µm in
diameter & about 3-3 µm in length.

 Morphology is a branch of biology that deals with the form of living
organisms & with relationships between their structures. Particular
form, shape or structure.
Morphological types of bacteria

Morphological types of bacteria
 Bacteria are classified
according to their shape.
1. Cocci
– from kokkos meaning
berry. They are spherical or oval
cells.

Morphological types of bacteria
2. Bacilli
-From baculus meaning rod.
-They are rod shaped cells.

Morphological types of bacteria
3. Vibrio
- They are comma-shaped
curve rods & derive their name
from their characteristic vibratory
motility.

Morphological types of bacteria
4. Spirilla
- They are rigid spiral forms.

Morphological types of bacteria
5. Spirochetes
- word came from speira
means coil & chaite means hair.
-they are flexuous spiral
forms.

Morphological types of bacteria
6. Actinomycetes
- This word came from Actis
means ray & Mykes means
fungus.
- They are branching
filamentous bacteria, so called
because of their resemblance to
the radiating rays of the sun,
when seen in tissue lesions.

Morphological types of bacteria
7. Mycoplasma
- are bacteria that do not have a
cell wall & hence do not possesses a
fixed shape. They occur as round or
oval bodies & as interlacing
filaments.
- Mycoplasma are bacteria that
have no cell wall and therefore have
no definite shape.

Arrangement of cocci
 Bacteria sometimes show characteristics cellular arrangement or
grouping .
 Thus, cocci may be arranged in pairs, chains, group of four, group
of eight, or grape like clusters.

Arrangement of cocci
In pair- d=Diplococci In chain- Streptococci

Arrangement of cocci
Group of 4- Tetrads Group of 8- Sarcina

Arrangement of cocci
 In grape like clusters-
staphylococci

Arrangement of bacteria
 Some bacilli are arranged at
angles to each other,
presenting a Chinese latter
pattern. They called corny
bacterium.

Bacterial cell structure & functions

Cell wall
 It is outer covering of most cells that protects the bacterial cell and
gives it shape.
 Bacterial cell walls are made of peptidoglycan (polysaccharides +n
protein) AKA murein.
 Mycoplasma are bacteria that have no cell wall and therefore have
no definite shape.
 The rigid structure of peptidoglycan gives the bacterial cell shape,
surrounds the plasma membrane and provides prokaryotes with
protection from the environment.

Cell wall
 Going further out, the bacterial world divides into two major
classes: Gram-positive and Gram-negative .
 Amount and location of peptidoglycan in the cell wall determines
whether a bacterium is G+ve or G-ve.

Gram-positive
 G+ve bacteria have a simpler chemical nature.
 G+ve bacteria possess thick cell wall containing many layers of
peptidoglycan and teichoic acids.
 In G+ ve cells, peptidoglycan is the outermost structure and makes
up as much as 90% of the thick compact cell wall.
 The cell wall caries bacterial antigens that are important in their
ability to cause disease & protect against disease.

Gram-negative
 G-ve bacteria have relatively thin cell wall consisting of few layers
of peptidoglycan surrounded by a second lipid membrane
containing lipopolysaccharides and lipoproteins
 The LPS present on the cell walls of G-ve bacteria account for their
endotoxic activity & O antigen specificity.
 Peptidoglycan makes up only 5 – 20% of the cell wall and is not
outermost layer, but lies between the plasma membrane and an
outer membrane.
 The endotoxins are responsible for inducing fever, tissue necrosis
&death.

Gram-negative
 The outermost layer of the G-ve bacterial cell wall is called the
outer membrane.
 It is similar to the plasma membrane, but is less permeable .
 It contains various proteins called outer membrane proteins
(OMP).

Cell wall characteristics Gram-positive Gram-negative
Thickness Thicker Thinner
Variety of amino acids Few Several
Aromatic & Sulphur
containing amino acids
Absent Present
Lipids Absent or scanty Present
Teichoic acid present Absent

Cell wall
 Antibiotics such as penicillin inhibit the formation of peptidoglycan
cross-links in the bacterial cell wall.
 The enzyme lysozyme, found in human tears, also digests the cell
wall of bacteria and is the body's main defense against eye
infections.

Cytoplasmic membrane
 The cytoplasmic membrane or plasma membrane is a thin layer
lining the inner surface of the cell wall.
 Which separating it from the cytoplasm.
 It works as semipermeable membrane by regulating the flow of
substances in and out of the cell.
 It consists of both lipids and proteins.
 It protects the cell from its surroundings.

Periplasmic space
 Gram-nagative bacteria :
-space between the cytoplasmic membrane and the cell wall and
space found between cell wall and the outer membrane.
 Gram-positive bacteria :
-space between the cytoplasmic membrane and the cell wall.
 The periplasm is filled with water and proteins.

Periplasmic cell
 However periplasm contains proteins and other molecules distinct
from those in the cytoplasm because the membrane prevents the
free exchange between these two compartments.
 Periplasmic proteins have various functions in cellular processes
including: transport, degradation and motility.
 Periplasm controls molecular traffic entering and leaving the cell.

Cytoplasm
 Cytoplasm is portion of the cell that lies within the plasma
membrane.
 substances within the plasma membrane, excluding the genetic
material.
 It is gel-like matrix composed of mostly water(4/5 th ), enzymes,
nutrients, wastes, and gases.
 It contains cell structures - ribosomes, chromosome and plasmids ,
as well as the components necessary for bacterial metabolism.
 It carries out very important functions for the cell - growth,
metabolism, and replication .

Constituents of cytoplasm are…
 Proteins including enzymes
 Vitamins
 Ions
 Nucleic acids and their precursors
 Amino acids and their precursors
 Sugars, carbohydrates and their derivatives
 Fatty acids and their derivatives

Ribosomes- protein synthesis machinery
 It consists of RNA and protein.
 Smaller than the ribosomes in eukaryotic cells-but have a similar
function.
 They are centers of protein synthesis.

Mesosomes
 Mesosomes are seen as
vesicular folds within the
plasma membrane, protruding
into the cytoplasm.
 They are more prominent in
Gram-positive bacteria.
 They are the principal sites of
the respiratory enzymes in
bacteria & are like the
mitochondria of eukaryotes in
function.

Mesosomes
 Mesosomes also coordinate nuclear & cytoplasmic division during
binary fission due to their position near the nuclear body.

Intracytoplasmic inclusions
 Inclusion bodies: Bacteria can have within their cytoplasm a variety
of small bodies collectively referred to as inclusion bodies.
 Some are called granules and other are called vesicles.
 Inclusions are considered to be nonliving components of the cell
that do not possess metabolic activity and are not bounded by
membranes.
 The most common inclusions are volutin, glycogen, lipid droplets,
droplets, crystals and pigments.

Intracytoplasmic inclusions
 Volutin granules need special staining techniques such as Albert’s
or Ponder’s stain to demonstrate the granules more clearly.
 Volutin granules are characteristically present in corynebacterium
diphtheria & are believed to store energy for cell metabolism.
 Polysaccharides granules & lipid granules are storage product.
 Vacuoles are fluid containing cavities separated from the cytoplasm
by a membrane.
 Their function & significance are uncertain.

Nucleus
 Bacterial nuclei may be seen
by electron microscopy.
 They appear as oval or
elongated bodies, generally
one per cell.
 The bacterial chromosome is
haploid & replicates by simple
binary fission instead of
mitosis as in other cells.

Nucleus
 Bacteria may possess extra-nuclear genetic elements consisting of DNA, called
plasmids, which carry genetic information.
 They can be transmitted to daughter cells during binary fission & also transferred
from one bacterium to another, either through conjugation or by bacteriophages.
 They confer properties such as toxigenicity & drug resistance on the cell.

Slime layer & capsule
 Many bacteria secrete a sticky material around the cell surface.
 When this is organized into a sharply defined structure, as in
streptococcus pneumonia, it is known as the capsule.
 Capsules may be polysaccharide or polypeptide.
 Large capsules may be readily demonstrated by negative staining
with India ink, when they are seen as clear halos around organism,
against a black background.

Slime layer & capsule
 Capsules protect bacteria from lytic enzymes found in nature & also
contribute to the virulence of pathogenic bacteria by inhibiting
phagocytosis.

Flagella
 Made up of protein subunits called flagellin.
 Each flagellum is attached to cell membrane with the help of
proteins other than flagellin.
 Flagella are the organ of the locomotion.
 The basal region has a hook like structure and a complex basal
body. The basal body consists of a central rod or shaft surrounded
by a set of rings.

Flagellar Arrangement
 Bacterial species differ in the number and arrangement of flagella
on their surface.
 Bacteria may have one, a few, or many flagella in different positions
on the cell.

Flagellar Arrangement
 Atrichous – no flagella
 Monotrichous - single flagellum
 Amphitrichous a flagellum at each end
 Lophotrichous - clusters of flagella at the poles of the cell
 Peritrichous - flagella distributed over the entire surface of the cell.

Fimbriae
 Hollow, hair like structures made of protein is called fimbrie or pili.
 They are shorter & thinner than flagella (about 0.5 µm long & less
than 10 nm thick) & project from the cell surface as straight
filaments.
 They arise from the cell membrane.

Fimbriae
 Fimbriae can be seen only under the electron microscope.
 They function as organs of attachment, helping the cell adhere
firmly to particles of various kinds.

Spores
 Some bacteria, particularly
members of the genera
Bacillus & Clostridium have
the ability to from highly
resistant resting stages called
spores.

Spores
 Sporulation (formation of spores) helps bacterial survival for long
periods under unfavorable conditions.
 Each bacterium forms one spore, which on germination forms a
method of reproduction.
 As bacterial spores are formed inside the parent cell, they are called
endospores.

Spore
 The fully developed spore has
at its core the nuclear body,
surrounded by the spore wall.
 Outside this spore cortex,
which is enclosed by multi
layered tough spore coat.
 Some spore have an
additional outer covering
called exosporium, which may
have distinctive ridges &
grooves.
 E.g. B. anthracis.

Shape & position

Resistance
 They are extremely resistant to drying & relatively resistant to
chemicals & heat.
 Though some spores may resist boiling for prolonged periods,
spores of all medically important species are destroyed by
autoclaving at 120 °C for 15 minutes.
 Methods of sterilization & disinfection should ensure that spores
are destroyed in addition to vegetative cells.
 Spores germinate in optimal conditions.

Resistance
 The spore wall is shed & the germ cell appears by rupturing the
spore coat & elongates to form the vegetative bacterium.

Pleomorphism
 Pleomorphism is the ability of some microorganisms to alter their
morphology( shape & size), biological functions or reproductive
modes in response to environmental conditions.