Bacterial Cell Structure and Classification

OPTOMETRY · SEMESTER 2

Bacterial Cell Structure and Classification

Microbiology and Parasitology

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Bacteria Cell Structure and Classification

Learning Objectives

By the end of this session, students are expected to be able to:
Define the terms bacteriology, prokaryotic cell, eukaryotic cell and bacteria
Describe the characteristics of a prokaryotic cell
Describe structure and functions of each part of a bacterial cell
Compare prokaryotic cells and eukaryotic cells
Classify bacteria according to various criteria

Definition of terms

Bacteriology: Is the Brach of microbiology which deals with the study of bacteria.
Prokaryotic Cells
Definition: Cells that lack a true nucleus and membrane-bound organelles.
Structure:
DNA floats freely in the cytoplasm (not enclosed in a nucleus).
Have a cell membrane, cytoplasm, and ribosomes.

Examples: Bacteria

Definition of terms cont…..

Eukaryotic Cells

Definition: Cells that have a true nucleus and membrane-bound organelles.
Structure:
DNA is enclosed within a nucleus.
Contains organelles like mitochondria, ER, Golgi apparatus, and lysosomes.
More complex internal organization.
Examples: Animals, plants, fungi, and protists.

Size and Complexity: Larger and more complex.

Definition of terms cont…..

What Are Bacteria?

Definition: Bacteria are microscopic, single-celled organisms with a prokaryotic cell structure, meaning they lack a true nucleus and membrane-bound organelles.

Diversity: There are millions of different types of bacteria found all over the world—including in soil, water, air, and inside living organisms

Characteristics of Prokaryotic Cells

Prokaryotic nature/no true nucleus: Bacteria lack a true nucleus and membrane-bound organelles. Their DNA floats freely in the cytoplasm.

Unicellular/single celled organisms: Each bacterium is a single celled, though some form colonies.

Cell Wall made up of peptidoglycan: Most bacteria have a rigid cell wall made of peptidoglycan, which provides shape and protection.

Characteristics of Prokaryotic Cells cont..

Posses different shapes: Common shapes include:
Coccus (spherical)
Bacillus (rod-shaped)

Spirillum (spiral-shaped)

Reproduction is by asexual reproduction: They reproduce asexually through binary fission, a process where one cell splits into two identical cells

Characteristics of Prokaryotic Cells cont..

Metabolism: Bacteria can be autotrophic (making their own food) or heterotrophic (consuming organic material) and chemotrophs Motility: Some bacteria move using flagella, while others glide or twitch.

Adaptability: Bacteria can survive extreme environments—hot springs, acidic lakes, deep-sea and vents.

Ribosomes: They contain 70S ribosomes, which are smaller than those in eukaryotic cells.

Pathogenicity: Some bacteria cause diseases in humans, animals, and plants, while others are beneficial (e.g., gut flora, nitrogen-fixing bacteria).

Bacteria parts and their functions

Structure
Function

Cell Wall

Provides shape and protection; made of peptidoglycan in most bacteria.

Cell Membrane

Controls movement of substances in and out of the cell.
Cytoplasm
Jelly-like substance where cellular processes occur.
Nucleoid

Region containing the bacterial DNA (not enclosed in a nucleus).

Bacteria parts and their functions cont.

Nucleoid
Region containing the bacterial DNA (not enclosed in a nucleus).

Plasmid

Small circular DNA molecules; often carry antibiotic resistance genes, uv resistance, and virulence

Ribosomes (70S)

Sites of protein synthesis.
Flagella
Tail-like structures used for movement.

Pili (Fimbriae)

Hair-like structures for attachment to surfaces or other cells.

Bacteria parts and their functions cont.

Capsule
Slimy outer layer that protects against desiccation and immune attack.

Inclusion Bodies

Storage sites for nutrients or other substances.
Endospore

Dormant, tough structure formed by some bacteria to survive harsh conditions.

Bacteria cell

Diagram of Bacterial cell

Functions of Each Part of the Bacterial Cell

Cell
component
Description
Functions
Cell wall
Is the outer most supporting
layer which protects the
internal structure of a cell
Protection of internal structures
(supporting layer)
Gives shape to the cell
Confers stability to osmotic pressure
(mucopeptide toughens the cell wall)
Role in division of bacterial
Offers resistance to harmful effect of

environment

Functions of Each Part of the Bacterial Cell cont..

Capsule
Is gelatinous secretion of
bacteria which gets organised
as a thick coat around cell wall
Protection against deleterious agents
(antibacterial agents) example lytic
enzymes
Contributes to the virulence of
Flagella
Are long contractile
filamentous appendages

They are organs of movements

Functions of Each Part of the Bacterial Cell cont..

Pilli
Are (Fimbriae) hair like
filaments that extend from the
cell surface. They are shorter
and straighter than flagella
They are organs of adhesion
Facilitate transfer of genetic materials
from one bacteria to another
Cytoplasmic/
Plasma
membrane
Is a thin and elastic layer
surrounding a cytoplasm. It is
made of lipoproteins and
phospholipids
.Controls the movement of water, ions,
nutrients and excretory substances in
and out of the cell.
Secrets extracellular hydrolytic enzymes

Functions of Each Part of the Bacterial Cell cont..

Cytoplasm
Is a viscous watery solution or
soft gell containing a variety
of organic and inorganic
solutes
Supports cellular structures like
ribosome, nutrient granules, metabolites
plasmids and nucleoid materials
Ribosomes
Are small granules and pack
the whole cytoplasm.

They are the sites for protein synthesis

Functions of Each Part of the Bacterial Cell cont..

Nucleoid (site
for DNA and
RNA
Is the area of the cytoplasm in
which DNA and RNA are
located
To store genetic information
Responsible for mutation of a bacteria

Eukaryotic cell

Eukaryotic Cells

Definition: Cells that have a true nucleus and membrane-bound organelles.
Structure:
DNA is enclosed within a nucleus.
Contains organelles like mitochondria, ER, Golgi apparatus, and lysosomes.
More complex internal organization.
Examples: Animals, plants, fungi, and protists.

Size and Complexity: Larger and more complex

Characteristics of eukaryotic cell

Nucleus: Contains the cell’s DNA enclosed within a nuclear membrane, allowing for organized gene expression and regulation.

Membrane-Bound Organelles: Includes mitochondria (energy production), endoplasmic reticulum (protein and lipid synthesis), Golgi apparatus (packaging and transport), lysosomes (digestion), and chloroplasts (photosynthesis in plants).

Characteristics of eukaryotic cell cont..

Cytoskeleton: A network of protein filaments (microtubules, microfilaments, intermediate filaments) that maintains cell shape, enables movement, and organizes organelles Multicellularity: Found in multicellular organisms with specialized cell types.

Characteristics of eukaryotic cell cont..

Plasma membrane: Phospholipid bilayer that regulates entry and exit of substances.

Compartmentalization: Organelles allow distinct environments for specialized biochemical processes.

Intracellular transport: Vesicles shuttle materials between organelles and to the membrane.

Larger size: Typically, 10–100 µm, much larger than prokaryotic cells

Main Distinguishing Features of Prokaryotic and Eukaryotic Cells

Features
Prokaryotes
Eukaryotes
Nucleus
No enclosing membrane
Enclosed by a membrane
Nucleolus
Absent
Present
Cell wall composition
Usually contain
peptidoglycan
Usually contain cellulose or
chitin
Mitochondria
Absent
May be present
Location of
chromosome
In the cytoplasm, usually
attached to the cell membrane
Within a true nucleus separated
from the cytoplasm by a

nuclear membrane

Main Distinguishing Features of Prokaryotic and Eukaryotic Cells cont..

Reproduction
Normally asexual
Asexual or sexual
Motility structures
Present

Absent

Eukaryotic cell structure

Major Similarities between Prokaryotes and Eukaryotes

Cell Membrane: Both types of cells are enclosed by a lipid bilayer that regulates the movement of substances in and out of the cell Cytoplasm: Each cell contains cytoplasm, a gel-like substance where cellular processes occur.

DNA as Genetic Material: Both use DNA to store genetic information, although it's organized differently (nucleus vs. nucleoid)

Major Similarities between Prokaryotes and Eukaryotes

Ribosomes for Protein Synthesis: Both have ribosomes, which are essential for translating genetic instructions into proteins Basic Metabolic Functions: They carry out essential life processes like energy production, waste elimination, and reproduction.

Cell Division: Both reproduce by cell division—prokaryotes typically by binary fission, and eukaryotes by mitosis or meiosis

Bacteria Classification

Bacteria are classified depending on different factors , bellow are the factors which can be considered when classifying bacteria 1.Shape(morphology) – coccus, bacillus, spirillum, Vibrio 2. Classification of Bacteria according to Staining Characteristics.

They have cell wall made up of peptidoglycan.
The composition and amount of peptidoglycan is the basis for Gram stain classification
3.Ability to grow in presence or absence of air

Bacteria Classification

4. Ability to form spores
5.Biochemical reactions
6.Type of flagella
7.pathogenic featues

8. Nutrition requirement

1. Classification of Bacteria According to Morphology/Shape

Cocci:
These are round or oval shaped bacteria.
When multiplying cocci may form:
Pairs (called diplococci for example meningococci and gonococci)
Chains (called streptococci example Streptococcus pyogenes)

Irregular groups or clusters (called staphylococci, example is staphylococcus aureus)

1. Classification of Bacteria According to Morphology/Shape cont.…

Bacilli:

These are rod shaped bacteria.

When multiplying bacterial rods do not usually remain attached to one another, but separate.

Occasionally however, they may form
Chains, example are Streptobacilli
Branching chains, example are Lactobacilli
Mass together; example is Mycobacterium leprae

Remains attached at various angles example Corynebacterium diphtheria.

1. Classification of Bacteria According to Morphology/Shape cont.…

Comma shaped:
These are small slightly curved rods (comma shaped). Example Vibrio cholera
Spirochaetes and spirilla
These are flexible, coiled, motile organisms they progress by rapid body movement.
Example,
treponema species,
borrelia species,

leptospires and spirillum minus

Bacteria shapes

Bacteria

Diplococci (Paired cocci)

GRAM NEGATIVE COCCI INPAIRS(DIPLOCOCCI)

Cocci in chains

Cocci in clusters

GRAM POSITIVE COCCI IN CLUSTERS

Bacilli in chains

GRAM NEGATIVE ROD(BACILLI)

Bacilli bacteria in mass together

(shown by arrows)

SPIRACHATES

Spiral shaped bacteria

COMMA SHAPE(VIBRIO CHOLERA)

Curved rods (comma shaped) bacteria

2. Classification of Bacteria basing on gram staining

Gram positive bacteria:

When stained by Gram stain, appears blue reflecting the colour of primary stain (either gentian violet, crystal violet or methyl violet). Examples, Staphylococci species, Streptococci species. Clostridium tetani.

Gram negative bacteria:

are bacteria which appears red, after losing the colour of primary stain and adapting the colour of a counter stain (dilute carbolfuchsin or safranin, or neutral red) when stained by gram stain o Examples, Neisseria species, salmonella species, haemophilus species

3. Classification of Bacteria basing on spore forming

Spores are highly resistant dormant state of bacteria with dense thick walls able to withstand dehydration heat, cold and action of disinfectants.
In this kind of classification there are
o Spore forming bacteria for examples
bacillus species
clostridium species
Non spore forming bacteria for examples like corynebactrium diphtheria
Listeria spp
Streptococcus

Escherichia

4. Classification of Bacteria basing on oxygen (air) requirement

some bacteria grow and reproduce in the presence of oxygen (aerobic bacteria)
Example bacillus anthracis and corynebacteria
And others grow and reproduce in absence of oxygen (anaerobic bacteria)

example clostridium species.

Basing biochemical reactions

1.Catalase test
Presence of catalase enzyme which breaks down hydrogen peroxide into water & oxygen.
Catalase positive bacteria produce bubbles (O2)

eg staphylococcus

According to biochemical reactions cont.….

2.Coagulase test

A coagulase test is a biochemical test that is used to differentiate Staphylococcus aureus from other Staphylococci species like S. epidermidis and S. saprophyticus on the basis of the ability to produce the coagulase enzyme.

There is production of coagulase enzyme which causes blood clot(converts fibrinogen into fibrin)

According to biochemical reactions cont….

6.Classification basing on flagella possession

Flagella are long thin projections that extend from the cell, arising from one or more locations on the surface of the cell wall and cell membrane, some bacteria posses these motility structures while others do not .

6.Classification basing on flagella possession

The flagella can be classified as follows
Atrichious –absence of flagella e.g. Lactobacilli
Monotrichious -presence of a single flagella at one end. e.g. Vibrio cholera
Amphitrichious – presence of a single flagella at both ends. e.g. Spirillum
Peritrichious – presence of flagella all over the cell surface. e.g.E.coli,Salmonella.
Lophotrichous –bacteria with tuft or cluster of flagella arising from one side

Example Helicobacter pylori, Pseudomonas spp

Basing on the flagella

7.Basing on nutritional requirements

Autotrophs
Use carbon dioxide (CO₂) as their carbon source to obtain energy
Can synthesize organic compounds from inorganic substances.

Examples: Nitrosomonas, Nitrobacter

7.Basing on nutritional requirements

Heterotrophs
Require organic compounds as their carbon source.
Depend on other organisms for nutrition.
Examples: Escherichia coli, Bacillus subtili
Phototrophs
Derive energy from sunlight.

Include photoautotrophs (e.g., cyanobacteria)

Classification of bacteria basing on pathogenicity

Pathogenic Bacteria: Cause disease in humans, animals, or plants. Examples: Mycobacterium tuberculosis, Streptococcus pyogenes.

Opportunistic Pathogens: Normally harmless but can cause disease in immunocompromised hosts. Example: Pseudomonas aeruginosa.

Non-Pathogenic Bacteria: Do not cause disease and may be beneficial. Example: Lactobacillus species.

Questions

What are the structural features of bacterial cell ?
What are the differences between prokaryotic cell and eukaryotic cell?
What are the 5 criteria for classification of bacteria and give examples?