Bacterial Cell Structure and Function Part 2: cell envelope and cytoplasmic constituents

Cell Walls Cell wall is a structure that completely surrounds the cell protoplast. (Almost) all bacteria have a cell wall.

Cell Walls Why study bacterial cell walls? They are essential structures in bacteria. They are made of chemical components found nowhere else in nature. They may cause symptoms of disease in animals. They are the site of action of some of our most important antibiotics.

Profile of the bacterial cell envelope Gram-positive cell wall is thick homogeneous monolayer Gram-negative cell wall is thin heterogeneous multilayer

Primary function of the bacterial cell wall To prevent rupture or osmotic lysis of the cell protoplast Lysis of a pair of dividing E. coli cells

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 acids L-alanine = L-ala D-glutamic acid = D-glu diaminopimelic acid = DAP D-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 Penicillin prevents formation of this Interpeptide bond Lysozyme breaks this glycoside bond between M and G Gram-negative murein showing the sites of action of the antibiotic penicillin and the enzyme lysozyme

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 Penicillin blocks the Insertion of the inter- peptide bridge Lysozyme breaks the glycoside bond between M and G Gram-positive murein showing the sites of action of the antibiotic penicillin and the enzyme lysozyme

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 walls Gram-negative cell walls include an outer membrane

Other characteristics of bacterial cell walls Outer membrane of Gram-negatives has two important properties It protects the cells from permeability by many substances including penicillin and lysozyme. 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% Lipopolysaccharide content 13 Sensitivity to penicillin sensitive resistant Sensitivity to lysozyme

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 fluid mosaic model of a membrane The proteins associate with both sides of the membrane, or may imbed in the membrane, or pass through the 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 Genetic material: chromosome and Plasmids (DNA) Ribosomes Inclusions

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

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.

Ribosome Structure and Composition 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 Function 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.

Some inclusions in Bacterial Cells Composition Glycogen poly-glucose Reserve carbon and energy source Poly-betahydroxybutyric acid (PHB) lipid 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 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 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 Endospore within mother cell Vegetative 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 Parasporal crystal Endospore Spores and parasporal crystals produced by some bacteria are toxic to insects