bacterial morphology and classification
DESCRIPTION
This is a series of lectures on microbiology useful for undergraduate medical and paramedical studentsTRANSCRIPT
Bacteria – Morphology & Classification
Dr. Ashish JawarkarConsultant 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
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 membrane2. 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 membraneGenome –
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 Organelles2. 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 Organelles3. Flagella –
Long (3 to 12 µm), filamentous surface appendages
Organs of locomotion
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Types of flagellar arrangementPolar/ 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 Organelles4. 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 Organelles5. 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
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
Cytoplasmic 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 replicationPlasma membrane invaginateCell wall deposited in invaginated spaceCross wall completedCells 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 fastMathematical progressions
arithmetic (1>2>4>6>8>10>12>14>16) geometric(1>2>4>8>16)
exponential expression (20 > 21 > 22 >23>24) logarithmic expression(0 >log21>log22>log23>log24)
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Bacterial Growth Curve
1 5 10Time (hours)
Lag phase
Log phase
Stationary phase
Death phase
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 10Time (hours)
Log phase
Double # cells
Generation time
Log Number of Bacteria
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GENERATION TIME / population doubling time
E-coli – 20 minMTb - 20 hoursMleprae - 20 days
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Minimum Growth requirements
Water as a source of carbonNitrogen sourceInorganic salts like phosphate, sulphate,
sodium, potassium, iron etc – need to be supplied in culture media
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Phototrophs – derive energy from sunlightChemotrophs – from chemical reactionsAutotrophs – synthesize organic
compoundsHeteretrophs – cannot synthesize, depend
on others
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Oxygen requirement
Aerobic – V. choleraAnaerobic - Clostridia
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Carbon dioxide
All requireSome like Brucella, req 5-10% k/a
capnophilic
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Temperature
Mesophilic – 25-40Psychrophilc - <20Thermophilic – 55-80Except thermophilic, most die at 50-60, k/a
thermal death point
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Other factors
MoisturepH