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

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

3. 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.

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

5. 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

6. 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

7. 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

8. 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.

9. 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

10. 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.

11. 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

12. 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.

13. Other characteristics of bacterial cell walls
Gram-negative cell walls include an outer membrane

14. 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.

15. 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

16. 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

17. Membrane structure and assembly
The membrane bilayer is formed by phospholipidmolecules made up of glycerol and fatty acids

18. Membrane structure and assembly
Phospholipids arrange themselves spontaneously in water: lipid “tails” inward; glycerol “heads” outward

19. 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.

20. 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

21. 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

22. Functions of the cytoplasmic membrane
Location of transport systems to import all the needed molecules that are charged or greater than molecular weight 100

23. Transport systems in bacteria

24. Functions of the cytoplasmic membrane
Energy generation: location of the electron transport system (ETS) and the ATP synthsizing enzyme ATPase

25. Functions of the cytoplasmic membrane
Specialized functions involving cell wall synthesis, cell division and DNA replication.

26. Cytoplasmic Constituents of Bacterial Cells
Genetic material: chromosome and Plasmids (DNA)
Ribosomes
Inclusions

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

28. 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
Sugars, carbohydrates and their precursors or derivatives 250
quinones, porphyrins, vitamins, coenzymes and
prosthetic groups and their precursors
Ions (PO4, NH3, SO4, etc.)

29. The Bacterial Chromosome or “Nucleoid”
Bacterial DNA released from
a “gently lysed” E. coli cell

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

31. 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.

32. 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.

33. 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.

34. 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

35. 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.

36. 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

37. 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

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

39. 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

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

41. Properties of Endospores
Several unique surface layers not found in vegetative cells: exosporium, spore coat, cortex, and core wall

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

43. Properties of Endospores
Parasporal crystal
Endospore
Spores and parasporal crystals produced by some bacteria are toxic to insects