bacterial morphology and classification

Bacteria – Morphology & Classification

Dr. Ashish JawarkarConsultant Pathologist

Parul Sevashram Hospital

Dr. Ashish Jawarkar 2

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


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




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


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


Anatomy of a Bacterial Cell


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


CELL WALL Outermost layer, encloses cytoplasm

1.Confers shape and rigidity

2.10 - 25 nm thick

3.Composed of peptidoglycan


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


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


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.


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



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



Nucleus

No nucleolus No nuclear membraneGenome –

single, circular double stranded DNA.


Additional Organelles

1. Plasmid – Extranuclear genetic elements consisting of

DNA Transmitted to daughter cells Confer certain properties e.g. drug

resistance, toxicity


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


Additional Organelles3. Flagella –

Long (3 to 12 µm), filamentous surface appendages

Organs of locomotion


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


Additional Organelles4. Fimbriae/ Pili –

Thin, hairlike appendages on the surface of many Gram-negative bacteria

10-20µ long, acts as organs of adhesion


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




Cytoplasm Mitochondria Absent Present

Lysosomes Absent Present

Golgi apparatus

Absent Present

Endoplasmic reticulum

Absent Present

Vacuoles Absent Present

Ribosomes Present Present




Cell Wall Present Absent

Except Fungi

Locomotor organelles

Flagella Flagella/ Cilia



GROWTH AND MULTIPLICATION

07.09.08 Dr. Ashish Jawarkar

Binary Fission

DNA replicationPlasma membrane invaginateCell wall deposited in invaginated spaceCross wall completedCells separate


Binary Fission

Light micrograph


Binary Fission


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)


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


Calculation of Generation Time

1 5 10Time (hours)

Log phase

Double # cells

Generation time

Log Number of Bacteria


GENERATION TIME / population doubling time

E-coli – 20 minMTb - 20 hoursMleprae - 20 days


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


Phototrophs – derive energy from sunlightChemotrophs – from chemical reactionsAutotrophs – synthesize organic

compoundsHeteretrophs – cannot synthesize, depend

on others


Oxygen requirement

Aerobic – V. choleraAnaerobic - Clostridia


Carbon dioxide

All requireSome like Brucella, req 5-10% k/a

capnophilic


Temperature

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

thermal death point


Other factors

MoisturepH

Date post:	07-May-2015
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bacterial morphology and classification

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