bacterial morphology and classification

This is a series of lectures on microbiology useful for undergraduate medical and paramedical students

1. Bacteria – Morphology &
Classification
Dr. Ashish Jawarkar
Consultant Pathologist
Parul Sevashram Hospital

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Learning Objectives
After completing this section you should be able to
perform the following objectives:
list the differences between prokaryotic and eukaryotic cell
Describe the structure of a bacterial cell and explain the
function of its components
Explain why cell wall forms the basis for classification of
bacteria
Explain the structural modifications (flagella) of the cell and
their functional importance

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Size of Bacteria
Unit of measurement in bacteriology is the
micron (micrometre, µm)
1 micrometre (10
-6
)= 1/1000 mm = 1/10000
cm = 1/100000 metre
1 nanometer (10
-9
)= 1/1000 micrometer =
1/100000000 meter
Bacteria of medical importance
0.2 – 1.5 µm in diameter
3 – 5 µm in length

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Introduction:
Based on the organization of their cellular
structures, all living cells can be divided into two
groups: eukaryotic and prokaryotic
Eukaryotic cell types - Animals, plants, fungi,
protozoans
Prokaryotic cell types - bacteria

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Prokaryotic Cells
prokaryotes are molecules surrounded by a
membrane and cell wall.
they lack a true nucleus and don’t have
membrane bound organelles like mitochondria,
etc.
large surface-to-volume ratio : nutrients can
easily and rapidly reach any part of the cells
interior

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Anatomy of a Bacterial Cell

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Anatomy of A Bacterial Cell
Outer layer – two components:
1.Rigid cell wall
2.Cytoplasmic (Cell/ Plasma) membrane – present
beneath cell wall
Cytoplasm – cytoplasmic inclusions,
ribosomes, mesosomes, genetic material
Additional structures –capsule, flagella,
fimbriae (pili), spores

10. Structure & Function of Cell
Components

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CELL WALL
Outermost layer, encloses cytoplasm
1.Confers shape and rigidity
2.10 - 25 nm thick
3.Composed of peptidoglycan

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Cell Wall
Cell wall –
4.Chemical nature of the cell wall helps to divide
bacteria into two broad groups – Gram positive &
Gram negative
5.Carries bacterial antigens – important in virulence &
immunity – gm –ve cell wall has lipopolysachhrides –
fever and necrosis
6.Several antibiotics may interfere with cell wall
synthesis e.g. Penicillin, Cephalosporins

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Gram positive cell wall
The Gram-positive cell wall is composed of a thick, multilayered
peptidoglycan sheath outside of the cytoplasmic membrane. Teichoic
acids are linked to and embedded in the peptidoglycan, and lipoteichoic
acids extend into the cytoplasmic membrane

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Gram negative cell wall
The Gram-negative cell wall is composed of an outer membrane linked to
thin, mainly single-layered peptidoglycan by lipoproteins.The outer
membrane includes porins, which allow the passage of small hydrophilic
molecules across the membrane, and lipopolysaccharide molecules that
extend into extracellular space.

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Cytoplasmic (Plasma) membrane
Thin layer 5-10 nm, separates cell wall from
cytoplasm
Acts as a semipermeable membrane: controls
the inflow and outflow of metabolites

Composed of lipoproteins with small amounts of
carbohydrates

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Other Cytoplasmic Components
Ribosomes – protein synthesis
Mesosomes –
1.Multilaminated structures formed as
invaginations of plasma membrane
2.Principal sites of respiratory enzymes
Intracytoplasmic inclusions – reserve of energy
& phosphate for cell metabolism e.g.
Metachromatic granules in diphtheria bacilli

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Nucleus
No nucleolus
No nuclear membrane
Genome –
single, circular double stranded DNA.

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Additional Organelles
1.Plasmid –
Extranuclear genetic elements consisting of
DNA
Transmitted to daughter cells
Confer certain properties e.g. drug
resistance, toxicity

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Additional Organelles
2.Capsule–
Viscous layer secreted around the cell
wall.
Polysaccharide / polypeptide in nature
Capsule – sharply defined structure,
antigenic in nature
•Protects bacteria
•Stained by negative staining using India
Ink

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Additional Organelles
3.Flagella –
Long (3 to 12 µm), filamentous surface appendages
Organs of locomotion

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Types of flagellar arrangement
Polar/ Monotrichous – single
flagellum at one pole
Lophotrichous – tuft of flagella at
one pole
Peritrichous – flagella all over
Amphitrichous – flagella at both
poles
Amphilophotrichous – tuft of flagella
at both ends

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Additional Organelles
4.Fimbriae/ Pili –
Thin, hairlike appendages on the surface of
many Gram-negative bacteria
10-20µ long, acts as organs of adhesion

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Additional Organelles
5.Spores –
Highly resistant resting
stages formed during adverse
environment (depletion of
nutrients)
Formed inside the parent cell,
hence called Endospores
Very resistant to heat,
radiation and drying and can
remain dormant for hundreds
of years.
Formed by bacteria like
Clostridia, bacillus

26. Differences between prokaryotic & eukaryotic cells
Character Prokaryotes Eukaryotes
Nucleus Nuclear
membrane
Absent Present
Nucleolus Absent Present
Chromosome One circular One or more
paired and linear
Cytoplasmi
c
membrane
Structure and
Composition
fluid phospholipid
bilayer, lacks
sterols
fluid phospholipid
bilayer containing
sterols

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Differences between prokaryotic & eukaryotic cells
Character Prokaryotes Eukaryotes
Cytoplasm Mitochondria Absent Present
Lysosomes Absent Present
Golgi
apparatus
Absent Present
Endoplasmic
reticulum
Absent Present
Vacuoles Absent Present
Ribosomes Present Present

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Differences between prokaryotic & eukaryotic cells
Character Prokaryotes Eukaryotes
Cell Wall Present Absent
Except Fungi
Locomotor
organelles
Flagella Flagella/ Cilia

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GROWTH AND MULTIPLICATION

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Binary Fission
DNA replication
Plasma membrane invaginate
Cell wall deposited in invaginated space
Cross wall completed
Cells separate

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Binary Fission
Light micrograph

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Binary Fission

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Consequences of Binary Fission
Very large number of cells very fast
Mathematical progressions
arithmetic (1>2>4>6>8>10>12>14>16)
geometric(1>2>4>8>16)
exponential expression (2
0
> 2
1
> 2
2
>2
3
>2
4
)
logarithmic expression(0 >log
21>log
22>log
23>log
24)

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Bacterial Growth Curve
1 5 10
Time (hours)
Lag phase
Log phase
Stationary phase
Death
phase

36. G: Generation time
Time in minutes or hours for a
population of bacteria to double in
number

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Calculation of Generation Time
1 5 10
Time (hours)
Log phase
Double
# cells
Generation time
Log Number
of Bacteria

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GENERATION TIME / population
doubling time
E-coli – 20 min
MTb - 20 hours
Mleprae - 20 days

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Minimum Growth requirements
Water as a source of carbon
Nitrogen source
Inorganic salts like phosphate, sulphate,
sodium, potassium, iron etc – need to be
supplied in culture media

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Phototrophs – derive energy from sunlight
Chemotrophs – from chemical reactions
Autotrophs – synthesize organic
compounds
Heteretrophs – cannot synthesize, depend
on others

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Oxygen requirement
Aerobic – V. cholera
Anaerobic - Clostridia

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Carbon dioxide
All require
Some like Brucella, req 5-10% k/a
capnophilic

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Temperature
Mesophilic – 25-40
Psychrophilc - <20
Thermophilic – 55-80
Except thermophilic, most die at 50-60, k/a
thermal death point

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Other factors
Moisture
pH