Bacterial Cell Structure

Cell Walls

• Cell wall is a structure that completely surrounds the cell protoplast.

• (Almost) all bacteria have a cell wall.

Cell Walls

• They are essential structures in bacteria.• They are made of chemical components

present only in bacteria.• They may cause symptoms of disease.

• They are the site of action of some important antibiotics.

Profile of the bacterial cell envelope

• Gram-positive cell wall is thick homogeneous monolayer

• Gram-negative cell wall is thin heterogeneous multilayer

Function of the bacterial cell wall

• Prevent of osmotic lysis

Chemical nature of bacterial cell walls

• Bacterial cell walls always contain murein, which is a type of peptidoglycan

• Chemical nature of murein accounts for the function of the cell wall

• Murein is only found in the cell walls of bacteria

E. coli peptidoglycan

Chemical nature of bacterial cell walls

Peptidoglycan is made up of• 2 amino sugars

N-acetyl-glucosamine = G N- acetylmuramic acid = M

• 4 amino acidsL-alanine = L-alaD-glutamic acid = D-gludiaminopimelic acid = DAPD-alanine = D-ala

Chemical nature of bacterial cell walls

Gram-negative murein. Murein is a polymer of the peptidoglycan subunit. The sugars form the glycan backbone (G-M-G-M-etc.) and the amino acids comprise the peptide side chains of the molecule.

Chemical nature of bacterial cell walls

Gram-negative murein showing the sites of action of the antibiotic penicillin and the enzyme lysozyme

Penicillin preventsformation of this Interpeptide bond

Lysozyme breaks this glycoside bond between M and G

Chemical nature of bacterial cell walls

Gram-positive murein has a thicker glycan a backbone and there are interpeptide bridges that join amino acid side chains together.

Chemical nature of bacterial cell walls

Gram-positive murein showing the sites of action of the antibiotic penicillin and the enzyme lysozyme

Penicillin blocks theInsertion of the inter-peptide bridge

Lysozyme breaks the glycoside bond between M and G

Other characteristics of bacterial cell walls

Gram-positive cell walls contain teichoic acids

Teichoic acids are thought to stabilize the Gram positive cell wall and may be used in adherence.

Other characteristics of bacterial cell wallsGram-negative cell walls include an outer membrane

Other characteristics of bacterial cell wallsOuter membrane of Gram-negatives has two important

properties1. It protects the cells from permeability by many

substances including penicillin and lysozyme.2. It is the location of lipopolysaccharide (endotoxin) which

is toxic for animals.

Table: Correlation of the Gram stain with properties of bacterial cell walls

Property Gram-positive Gram-negative

Thickness of wall thick (20-80 nm) thin (10 nm)

Number of layers 1 2-3

Peptidoglycan (murein) content

>50% 10-20%

Teichoic acids in wall present absent

Protein/lipoprotein content

0-3% >50%

Lipopolysaccharide content

0 13

Sensitivity to penicillin sensitive resistant

Sensitivity to lysozyme sensitive resistant

Cell (cytoplasmic) membrane

• Completely encloses the bacterial cell protoplast

• Composed of 60% protein and 40% phospholipid

• Arranged as a bilayer

Section of a cytoplasmic membrane

Membrane structure and assembly

• The membrane bilayer is formed by phospholipidmolecules made up of glycerol and fatty acids

Membrane structure and assembly

• Phospholipids arrange themselves spontaneously in water: lipid “tails” inward; glycerol “heads” outward

Membrane structure and assembly

The proteins associate with both sides of the membrane, or may imbed in the membrane, or pass through the membrane.

The fluid mosaic model of a membrane

Membrane structure and assembly

Proteins in the cytoplasmic membrane have a variety of functions including transport and energy transformations

The cytoplasmic membrane of E. coli

Functions of the cytoplasmic membrane

• Osmotic or permeability barrier: the membrane is impermeable to molecules that are charged or greater than molecular weight of 100

Functions of the cytoplasmic membrane

• Location of transport systems to import all the needed molecules that are charged or greater than molecular weight 100

Transport systems in bacteria

Functions of the cytoplasmic membrane

• Energy generation: location of the electron transport system (ETS) and the ATP synthsizing enzyme ATPase

Functions of the cytoplasmic membrane

• Specialized functions involving cell wall synthesis, cell division and DNA replication.

Cytoplasmic Constituents of Bacterial Cells

• Cytoplasm

• Genetic material: chromosome and Plasmids (DNA)

• Ribosomes

• Inclusions

ribosomes DNA (chromosome)

Cytoplasm of bacterial cells is gel-like and contains the chromosome, ribosomes,various macromolecules and small molecules in water solution.

Small molecules present in a growing bacterial cell

Molecules Approximate number of kinds

Amino acids, their precursors and derivatives 120

Nucleotides, their precursors and derivatives 100

Fatty acids and their precursors 50

Sugars, carbohydrates and their precursors or derivatives 250

quinones, porphyrins, vitamins, coenzymes and prosthetic groups and their precursors 300

Ions (PO4, NH3, SO4, etc.) 20

The Bacterial Chromosome or “Nucleoid”

Bacterial DNA released from a “gently lysed” E. coli cell

DNA (deoxyribonucleic acid) is the genetic material of the cell. It can be replicated in a semiconservative fashion and passed on to progeny cells.

The procaryotic ribosome (L) is 70S in size, being composed of a 50S (large) subunit and a and 30S (small) subunit. The eucaryotic ribosome (R) is 80S in size and is composed of a 60S and a 40S subunit.

Ribosome Structure and Composition

Ribosomes are made of two subunits, a large subunit and a small subunit. Each subunit is made up of RNA and various proteins.

Ribosome Structure and Composition

Ribosomes function in protein synthesis. Amino acids are assembled into proteins according to the genetic code on the surfaces of ribosomes during the process of translation.

Ribosome Function

Inclusion Composition

Glycogen poly-glucose Reserve carbon and energy source

Poly-betahydroxybutyric acid (PHB)

lipid Reserve carbon and energy source

Poly-phosphates polymers of PO4 Reserve phosphate, possibly high-energy PO4

Sulfur globules elemental S Reserve energy and or electrons

Magnetosomes magnetite (iron oxide) Provide orientation in magnetic field

Gas vesicles protein shells inflated with gases

Provide buoyancy in aquatic environments

Parasporal crystals protein Produced by endospore-forming Bacilli - toxic to insects

Function

Some inclusions in Bacterial Cells

Some inclusions in Bacterial Cells

Bacterial Inclusions. A. PHB granules; b. a parasporal BT crystal in the sporangium of Bacillus thuringiensis; c. carboxysomes in Anabaena viriabilis, showing their polyhedral shape; d. sulfur globules in the cytoplasm of Beggiatoa.

Endospores are produced as intracellular structures within the cytoplasm of certain bacteria, most notably Bacillus and Clostridium species.

Endospore forming bacteria left to right: Clostridium botulinum, Bacillus brevis, Bacillus thuringiensis

Endospore formation is NOT a mechanism of reproduction. Rather it is a mechanism for survival in deleterious environments. During the process of spore formation, one vegetative cell develops into one endospore.

The sequential steps of endospore formation in a Bacillus species. The process of endospore formation takes about six hours. Eventually the mature endospore is released from its “mother cell” as a free spore

Free endospore

Vegetative cell

Endospore within mother cell

Under favorable nutritional and environmental conditions, an endospore germinates into a vegetative cell.

Medically-important Endospore-forming Bacteria

• Bacillus anthracis causes anthrax• Bacillus cereus causes food poisoning• Clostridium tetani causes tetanus• Clostridium botulinum causes botulism• Clostridium perfringens causes food

poisoning and gas gangrene• Clostridium difficile causes antibiotic-induced

diarrhea and pseudomembranous colitis

Properties of Endospores

• Resting (dormant) cells - “cryptobiotic” i.e., show no signs of life…..primarily due to lack of water in the spore

Properties of Endospores

• Several unique surface layers not found in vegetative cells: exosporium, spore coat, cortex, and core wall

Properties of Endospores

• Highly resistant to heat (boiling), acids, bases, dyes ( don’t stain) irradiation, disinfectants, antibiotics, etc.

Properties of Endospores

• Spores and parasporal crystals produced by some bacteria are toxic to insects

Parasporal crystalEndospore