Bacterial Cell Structures
Stijn van der Veen
How do I know what bacterium makes my
patient ill?

Bacterial species can be differentiated by:

Morphology (shape)

Composition (cell envelope and other structures)

Metabolism & growth characteristics

Genetics
Bacteria are tiny!!!!!
10-6

10-1
Comparing bacteria with a
football is comparing a
football with Mount Everest.
So, what do we need?
Microscopes!!!!!!!
104
meter
Optical Methods

The light microscope


Phase contrast microscope


Magnification: 1000x (100x obj. +
10x oc.)
Observe living cells.
Fluorescence microscope

Observe fluorescent dyes or proteins
Optical Methods

Confocal microscope


Provide three-dimensional images
in multiple layers (z-stack)
Electron microscopes

Transmission electron microscope
(TEM) can resolve particles with 1
nm in size

Scanning
electron
microscope
(SEM)
can
provide
threedimensional images
Differentiating bacterial species

Morphology (shape)

Composition (cell envelope and other
structures)

Metabolism & growth characteristics

Genetics
Shape (morphology) of bacteria

Spherical (coccus)

Rod (bacillus)

Twisted (spiral)
Morphology

Bacteria are unicellular…
but they can stick together!!
Spherical-shaped bacteria
Cocci may remain attached after cell division.
These group characteristics are often used to
help identify certain cocci.

Cocci that remain in pairs after dividing are called
diplococci.

Cocci that remain in chains after dividing are called
streptococci.

Cocci that divide in two planes and remain in groups
of four are called tetrads.

Cocci that divide in three planes and remain in
groups cube like groups of eight are called sarcinae.

Cocci that divide in multiple planes and form grape
like clusters or sheets are called staphylococci.
Rod-shaped bacteria
Bacilli only divide across their
short axis.

Most bacilli appear as single rods.

Diplobacilli appear in pairs after
division.

Streptobacilli appear in chains
after division.

Some bacilli are so short and fat
that they look like cocci and are
referred to as coccobacilli.
Twisted bacteria
Spiral bacteria have one or
more twists.

Vibrios look like curved rods.

Spirilla have a helical shape
and fairly rigid bodies.

Spirochetes have a helical
shape and flexible bodies.
Other shapes…

Some odd types…that you (as doctor) would
generally never encounter!!
Differentiating bacterial species

Morphology (shape)

Composition (cell envelope and other
structures)

Metabolism & growth characteristics

Genetics
Prokaryote vs. Eukaryote
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Staining methods

Methods to study bacterial morphology and
composition:


Common differential staining methods

Gram stain （Gram-positive vs. Gram-negative)

Acid-fast stain (Mycobacteria)
Special staining methods

The spore staining methods

The flagella staining methods

The capsule staining methods

DNA staining methods
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Cell envelope
Cell envelope

The cell envelope consists of the cell membrane, cell
wall, and associated structures

Bacterial cell envelopes fall into two major categories

Gram positive & Gram negative

This is based on Gram staining characteristics that reflect
major structural differences between the two bacterial
groups.
Gram stain
1884: Christian Gram: First publication for the Gram stain method
Gram-positive cocci
Gram-negative bacilli
Gramm staining procedure




Crystal violet (1 min)
Iodine (1 min)
Acetone / Alcohol (10–15 sec)
Safrinin (1 min)
=> rinse
=> rinse
=> rinse
=> rinse & dry
Cell envelope
Simplified diagram and electronic microscopy
pictures of the cell envelope of G+ and G- bacteria
(murein = peptidoglycan)
Gram-positive cell envelope
Gram-negative cell envelope
Peptidoglycan

The peptidoglycan is a single bag-shaped, highly
cross-linked macromolecule that surrounds the
bacterial cytoplasmic membrane and provides rigidity
(which decides the shape of a bacterium) .

It is huge (billions in molecular weight).

Peptidoglycan is found in all eubacteria except
Chlamydia and Mycoplasma.
Peptidoglycan structure

Glycan (polysaccharide) backbone
of alternating residues of N-acetyl
muramic acid and N-acetyl
glucosamine connected by -1,4
linkage.

Tetrapeptide side chains usually
containing D- and L- amino acid
residues, and in some instances
diaminopimelic acid (DAP)
residues.

The side chains are cross-linked by
peptide bridges. These peptide
bridges vary in structure among
bacterial species (gram-negative
bacteria have no peptide bridges).
Function of the cell wall

Maintain the bacterial characteristic shape.

Provide resistance to osmotic changes.

Provide anchoring for surface appendages such as
flagella and pili.

Assist in cell division
The effect of lysozyme on the cell wall

Lysozyme can cut the -1,4 linkage.

So lysozyme can kill G+ and G- bacteria by destroying their
glycan backbone .
Effect of penicillin on the cell wall

Penicillin can block the linkage of tetrapeptide side chains
and peptide bridges.

So penicillin can kill bacteria by inhibiting their
peptidoglycan synthesis.

But…only replicating/growing
bacteria are killed.
Characteristics of gram-positive cell wall

Gram-positive cell wall is typically 20-80 nm thick.

It contains 15-50 peptidoglycan layers.

It may contain additional components such as teichoic
acids and proteins

Teichoic acids are water-soluble
polymers of polyphosphates.

Wall teichoic acids are linked to the
peptidoglyacan.

Lipoteichoic acids are anchored in
the cytoplasmic membrane.
Characteristics of gram-negative cell wall

Gram-positive cell wall is thin: 10-15 nm.

It only contains 1-2 peptidoglycan layers

No teichoic acids.
Gram negative outer membrane

Outer membrane consists of lipopolysaccharide (LPS)
and phospholipids.

It contains lipoproteins such as porins.

Porins form channels to allow passage of small hydrophilic
nutrients (such as sugars, amino acids and certain ions) through
the outer membrane.
phospholipids
Lipopolysaccharide (LPS)

LPS is an endotoxin because it is poisonous to
mammalian cells.

LPS consists of 3 regions

O antigen: highly variable
polysaccharide region
composed of repeating units of
specific monosaccharides.
 Core polysaccharide:
conserved within a genus.
 Lipid A: contains β-hydroxy fatty
acids (bacteria specific), which
display endotoxin activity.
 Free lipid A may trigger fever, inflammation, and
septic shock
Summary
Property
Gram positive Gram negative
Peptidoglycan layers
15-50
1-2
Peptidoglycan content
>50%
10-20%
Teichoic acids
+
-
Outer membrane
-
+
lipopolysaccharide
-
+
Sensitive to penicillin
yes
Less sensitive
Digested by lysozyme
yes
weakly
Mycobacterial cell envelope
 The Mycobacterial cell
envelope is waxy.
 This enables
Mycobacteria to
survive exposure to:





acids
alkalis
detergents
oxidative bursts
lysis by immune
system
 many antibiotics
1. Outer lipids
2. Mycolic acid
3. Polysaccharides
4. Peptidoglycan
5. Molecules involved
5. Plasma membrane
in evading host
6. 6 &
immune cells &
function.
Acid-fast (Mycobacterial) staining
procedure



Ziehls carbol fuchsin (3 – 5 min heat) => rinse
Acid Alcohol (10 – 15 sec)
=> rinse
Crystal violet (1 min)
=> rinse & dry
Cell membrane

Separates the cell from its environment

Consists of a phospholipid bilayer

Semipermeable (important for osmosis)

Flexible
Cell membrane and osmosis

Osmosis is the diffusion of water across a semi-permeable
membrane.

Changes in the bacterial environment such as the amount of
dissolved molecules results in changes of the osmotic pressure.
 Water will move in or out of the cell.

Cells need water, which is important for many metabolic
reactions.

The cell wall will protect the bacteria from exploding when too
much water moves into the cell, but bacteria are sensitive to
conditions when to much water moves out of the cell
(dehydration/desiccation).
Cell membrane proteins

Transmembrane proteins, porins, membrane anchored
proteins, etc.

Important for many processes

Sensing the environment
 Provide active transport across the cell membrane

Proteins
 Solutes
 Lipids
 Cell wall polymers

Generation of energy
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Cytoplasm

Jelly intracellular environment composed
largely of water (80%), proteins, nucleic acids,
lipids, salts, sugars, and various low molecular
weight molecules.

The cytoplasm also harbors:

Chromosome
 Ribosomes
 Inclusions
 Plasmids
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Chromosome
Bacterial chromosome (nucleoid)

Freely floating double stranded DNA (not
covered by membrane)

Haploid

More efficient => grows quicker
 Mutations allow adaptation to environment

Circular
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Ribosome

Translates messenger RNA’s (mRNA) into proteins

Bacterial cell contains multiple copies (usually thousands).

Free floating or attached to
cell membrane.

Composed of ribosomal RNA
(rRNA) and proteins.

Typically composed of two
subunits (large and small)
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Plasmids

Small extra-chromosomal double stranded DNA.

Generally circular (in few instances linear).

Usually present in multiple copies.

Capable of self-replication.

Often encode antibiotic resistance
markers and virulence factor.

Generally not essential for
survival.
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Flagellum

Hair-like appendage on the bacterial surface that is responsible
for movement (motility).

Consist of various different proteins

The main protein of the filament (flagellin) can be used for
identification (H-antigen).

Flagellum-dependent motility is important for virulence and
chemotaxis (movement towards food and away from toxics).

Only visible with light microscopy after
specific flagellum staining
Flagellum structure
Flagellum arrangement

Monotrichous: single polar
flagellum

Lophotrichous: multiple
flagella at single pole

Amphitrichous: flagella at
both poles

Peritrichous: flagella
distributed all around
Flagellum-dependent movement
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Pilus (plural pili)

Hair-like appendage on the bacterial surface that is involved in
adhesion to host cells, surfaces, and other bacteria.

Composed of several different proteins and the structural
protein of the filament is pilin.

Important for virulence.

Two major types can be distinguished:
 Common pilus (frequently referred to as fimbria).

Shorter, thinner, and numerous present per bacterium.
 Major role in adhering to host cells.

Sex (F) pilus

Longer, broader, and only 1-4 per bacterium
 Important for bacterial conjugation (sex).
Conjugation
Donor
Recipient
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Capsule and slime layer (glycocalyx)

Layer surrounding the outside of the cell
envelope.

Usually composed of polysaccharides, and less frequently of
polypeptides, glycoproteins, or glycolipids

Not present in all bacteria and even variable within capsule
containing species.

Capsule contributes to virulence of pathogens and protects
against phagocytosis and antimicrobial compounds secreted by
host cells.
Capsule and slime layer

Helps in surface attachment and nutrient absorption, and
prevents dehydration.

Not essential for viability.

Capsule: firmly attached and structured layer surrounding cells.

Slime layer: loosely attached unorganized layer surrounding
cells
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Inclusion

Aggregates of storage molecules, found as small bodies in the
cytoplasm.

Consist of organic molecules (such as glycogen or PHB), or
inorganic molecules (such as sulfur or polyphosphate).

Inclusions accumulate in conditions of excess nutrients.
Bacterial cell structures overview

Basic structures






Cell membrane
Cell wall
Cytoplasm
Chromosome
Ribosome
Specific structures






Plasmid
Flagellum
Pilus
Capsule
Inclusion
Endospores (not shown)
Endospore

Highly specialized bacterial cell that is very resistant to extreme
conditions such as heat, cold, desiccation, radiation, starvation,
etc.

It is produced under unfavorable conditions and enables
survival of the species.

Dormant endospores can survive for many years.

Endospores are unable to replicate in this form.

Under favorable conditions, endospores germinate (change)
into vegetative (standard) cells again and are able to replicate.

Commonly found in the soil.
Sporulation (endospore formation)
Endospore structure
Serological identification

Serological detection and identification of bacterial cell
surface antigens.

Binding of specific antibodies to the cell surface
antigens.

Agglutination assays (clumping of bacteria due to
antibody binding.

Serotyping (capsule, O/H antigens)
 Lancefield grouping (Streptococcus)
 Etc.
Next lecture

Bacterial Metabolism &
Growth Characteristics