bacteria: morphology and classification

7/27/2019 bacteria: morphology and classification

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Bacteria –

Morphology &Classification

Dr. Ashish Jawarkar Consultant Pathologist

Parul Sevashram Hospital



Dr. Ashish Jawarkar 2

Learning Objectives

After completing this section you should be able toperform 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 themicron (micrometre, µm)

1 micrometre (10-6)= 1/1000 mm =

1/10000 cm = 1/100000 metre1 nanometer (10-9)= 1/1000 micrometer =

1/100000000 meter

Bacteria of medical importance0.2 – 1.5 µm in diameter

3 – 5 µm in length




Prokaryotic Cells

prokaryotes are molecules surrounded by amembrane and cell wall.

they lack a true nucleus and don’t have

membrane bound organelles like mitochondria,etc.

large surface-to-volume ratio : nutrients caneasily and rapidly reach any part of the cellsinterior




Anatomy of a Bacterial Cell



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 tothin, 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 fromcytoplasm

Acts as a semipermeable membrane: controlsthe 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 cellwall.

Polysaccharide / polypeptide in nature

Capsule – sharply defined structure,antigenic in nature

• Protects bacteria

• Stained by negative staining using IndiaInk




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 restingstages formed during adverseenvironment (depletion of nutrients)

Formed inside the parent cell,hence called Endospores

Very resistant to heat,radiation and drying and can

remain dormant for hundredsof years.

Formed by bacteria likeClostridia, 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

Cytoplasmi

c

membrane

Structure and

Composition

fluid phospholipid

bilayer, lacks

sterols

fluid phospholipid

bilayer containing

sterols





Character Prokaryotes EukaryotesCytoplasm Mitochondria Absent Present

Lysosomes Absent Present

Golgiapparatus

Absent Present

Endoplasmic

reticulum

Absent Present

Vacuoles Absent Present

Ribosomes Present Present





Character Prokaryotes Eukaryotes

Cell Wall Present Absent

Except Fungi

Locomotor

organelles

Flagella Flagella/ Cilia



Binary Fission

DNA replication

Plasma membrane invaginate

Cell wall deposited in invaginated space

Cross wall completed

Cells separate




Binary Fission



Bacterial Growth Curve



Bacterial Growth Curve

1 5 10

Time (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



Calculation of Generation Time

1 5 10

Time (hours)

Log phase

Double

# cells

Generation time

Log Number of Bacteria




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




Phototrophs – derive energy from sunlight

Chemotrophs – from chemical reactions

Autotrophs –

synthesize organiccompounds

Heteretrophs – cannot synthesize, depend

on others




Oxygen requirement

Aerobic – V. cholera

Anaerobic - Clostridia




Carbon dioxide

All require

Some like Brucella, req 5-10% k/a

capnophilic




Temperature

Mesophilic – 25-40

Psychrophilc - <20

Thermophilic –

55-80Except thermophilic, most die at 50-60, k/a

thermal death point




Other factors

Moisture

pH

bacteria: morphology and classification

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