1. Bacterial Cell Structure
Assist. Prof. Emrah Ruh
NEU Faculty of Medicine
Department of Medical Microbiology

2. Bacterial cell structure
Cytoplasmic structures
Nucleoid
Ribosome
Cytoplasmic membrane
Cell wall
External structures

3. Bacterial cell structure Nucleoid
Prokaryotes:
No true nucleus; no nuclear membrane, no nucleolus
Bacterial chromosome:
Single, double- stranded circle found in the nucleoid

4. Bacterial cell structure Ribosome
Prokaryotic cell: 70S ribosome (30S + 50S subunits)
Eukaryotic cell: 80S ribosome (40S + 60S subunits)

5. Bacterial cell structure Cell membrane
Lipid bilayer structure
Does not usually contain sterols

6. Bacterial cell structure Cell membrane – Functions
Selective permeability and transport of solutes
Electron transport and oxidative phosphorylation
Excretion of hydrolytic exoenzymes
Functioning in DNA and cell wall synthesis
Bearing the receptors of the chemotactic and other sensory transduction systems

7. Bacterial cell structure Cell membrane
Mesosome:
Folded invaginations in the cytoplasmic membrane
Bind and pull apart daughter chromosomes during cell division

8. Bacterial cell structure Cell membrane – Transport of substances
Passive transport
Simple diffusion, faciliated diffusion, channel proteins
Active transport
Ion-coupled transport, ATP-binding cassette (ABC) transport
Group translocation
Special transport process

9. Bacterial cell structure Cell membrane – Transport of substances
Passive transport: (Diffusion; no energy)
Simple diffusion: Not selective; (eg, dissolved O2, CO2, and H2O)
Faciliated diffusion: Selective
Channel proteins: Rare in prokaryotes; selective channels  passage of specific molecules (eg, glycerol)

10. Bacterial cell structure Cell membrane – Transport of substances
Passive transport: (Diffusion; no energy)

11. Bacterial cell structure Cell membrane – Transport of substances
Active transport:
Ion-coupled transport:
Uniport: single transport of a solute
Symport: cotransport of a solute and H+ in same direction
Antiport: transport of two similar solutes in opposite directions

12. Bacterial cell structure Cell membrane – Transport of substances
Active transport:
Ion-coupled transport

13. Bacterial cell structure Cell membrane – Transport of substances
Active transport:
ATP-binding cassette (ABC) transport:
Uses ATP
Binding proteins
Gram-negative bacteria  periplasmic space
Gram-positive bacteria  outer surface of the cell membrane

14. Bacterial cell structure Cell membrane – Transport of substances
Active transport:
ATP-binding cassette (ABC) transport:
Bound substrate is transferred to a membrane- bound protein complex
Hydrolysis of ATP
Energy  membrane pore open  movement of substrate into the cell

15. Bacterial cell structure Cell membrane – Transport of substances
Active transport:
ATP-binding cassette (ABC) transport

16. Bacterial cell structure Cell membrane – Transport of substances
Group translocation:
Uptake of certain sugars (glucose, mannose…)
Not active transport
Phosphotransferase system:
Membrane carrier protein  phosphorylated (phosphoenolpyruvate)  binds the free sugar  transport into the cytoplasm: sugar phosphate

17. Bacterial cell structure Cell membrane – Transport of substances
Group translocation:

18. Bacterial cell structure Cell membrane – Transport of substances
Special transport process:
Iron (Fe): essential nutrient for bacteria
Siderophores  transport Fe into the cell

19. Bacterial cell structure Cell membrane – Transport of substances
Special transport process:

20. Bacterial cell structure
Cytoplasmic structures
Nucleoid
Ribosome
Cytoplasmic membrane
Cell wall
External structures

21. Bacterial cell structure Cell wall
Distinguish Gram-positive from Gram-negative bacteria
Most prokaryotes  Peptidoglycan (murein) layer
Rigidity, and the shape of the bacterial cell

22. Bacterial cell structure Cell wall

23. Bacterial cell structure Cell wall
Gram-positive bacteria
Peptidoglycan
Teichoic acid
Lipoteichoic acid
Gram-negative bacteria
Peptidoglycan
Periplasmic space
Outer membrane
Proteins
Lipopolysaccharide

24. Bacterial cell structure Cell wall – Peptidoglycan
Gram-positive bacteria:
Thick peptidoglycan; ~40 sheets (50% of the cell wall)
Gram-negative bacteria:
Thin peptidoglycan; or 2 sheets (5-10% of the cell wall)

25. Bacterial cell structure Cell wall – Peptidoglycan
Peptidoglycan layer;
Provides the strength to the bacterial cell wall
Provides the osmotic stability to the bacterial cell
Synonyms:
Murein, mucopeptide

26. Bacterial cell structure Cell wall – Peptidoglycan
Lysozyme (enzyme in tears, saliva and nasal secretions) degrades the peptidoglycan
Hypotonic media  Osmotic pressure differences  water flows into the cell  cell lysis

27. Bacterial cell structure Cell wall – Peptidoglycan
Isotonic media:
Gram-positive bacteria  Lysozyme  Protoplasts
Gram-negative bacteria  EDTA-lysozyme  Spheroplasts
If protoplasts/spheroplasts are able to grow and divide, they are called L-forms.

28. Bacterial cell structure Cell wall – Peptidoglycan
Lysozyme treatment – protoplasts

29. Bacterial cell structure Cell wall – Peptidoglycan synthesis
Glycan portion:
N-acetylglucosamine (GlcNAc, NAG)
N-acetylmuramic acid (MurNAc, NAM)
Peptide portion:
Tetrapeptide side chains
Peptide cross-bridges

30. Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transglycosylation:
NAM-NAG are linked together (b-1,4 glycosidic bond)
Enzyme: Transglycosylase

31. Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transpeptidation:
Peptide chains attach to N-acetylmuramic acid
Enzyme: Transpeptidase

32. Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transpeptidation:
Pentapeptide:
L-Alanine (L-Ala)
D-Glutamate (D-Glu)
L-Lysine (L-Lys) / Diaminopimelic acid (DAP)
D-Alanine (D-Ala)

33. Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transpeptidation:
Pentapeptide:
The terminal D-Alanine (D-Ala) from pentapeptide is removed by transpeptidase and carboxypeptidase enzymes
In mature peptidoglycan, peptide chains are tetrapeptides

34. Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transglycosylase, transpeptidase and carboxypeptidase enzymes are called “Penicillin-binding proteins (PBPs)”

35. Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transpeptidation:
Gram-negative bacteria:
Diaminopimelic acid of one tetrapeptide and terminal D-alanine of second tetrapeptide are directly linked

36. Bacterial cell structure Cell wall – Peptidoglycan synthesis
Transpeptidation:
Most Gram-positive bacteria:
L-lysine of one tetrapeptide and D-alanine of second tetrapeptide are linked by pentaglycine (Gly5) cross-bridge

37. Bacterial cell structure Cell wall – Peptidoglycan synthesis
L-Ala
D-Glu
DAP / L-Lys
D-Ala
Gly5

38. Bacterial cell structure Cell wall – Teichoic acid / Lipoteichoic acid
Gram-positive bacteria
Common surface antigens (distinguish bacterial serotypes)
Attachment (adherence) to the host cell
Important factors in virulence
Teichoic acids  peptidoglycan
Lipoteichoic acids  cytoplasmic membrane

39. Bacterial cell structure Cell wall – Teichoic acid / Lipoteichoic acid

40. Bacterial cell structure Cell wall – Periplasmic space
Gram-negative bacteria
The space between the inner and outer membranes
Contains the peptidoglycan layer
Possess hydrolytic enzymes (proteases, lipases,...) (breakdown of large molecules for metabolism)

41. Bacterial cell structure Cell wall – Periplasmic space

42. Bacterial cell structure Cell wall – Outer membrane
Gram-negative bacteria
Bilayered structure:
Inner leaflet  similar composition with the cell membrane
Outer leaflet  contains lipopolysaccharide (LPS)

43. Bacterial cell structure Cell wall – Outer membrane

44. Bacterial cell structure Cell wall – Outer membrane
Exclude hydrophobic molecules (unusual feature among biological membranes!)
Protect the bacterium (eg, digestive system)
Connected to both the peptidoglycan layer and the cytoplasmic membrane
Lipoprotein: connects peptidoglycan with outer membrane

45. Bacterial cell structure Cell wall – Outer membrane
Possess special channels (porins): passive diffusion of low-molecular-weight hydrophilic compounds (sugars, amino acids and ions)
Large antibiotic molecules penetrate slowly: antibiotic resistance!

46. Bacterial cell structure Cell wall – Outer membrane

47. Bacterial cell structure Cell wall – Lipopolysaccharide
Lipopolysaccharide (LPS)  Endotoxin of Gram-negative bacteria

48. Bacterial cell structure Cell wall – Lipopolysaccharide
O-specific polysaccharide:
Induces specific immunity (O-antigen)
Common core polysaccharide:
Same in all Gram- negative bacteria
Lipid A:
Responsible for primary toxicity

49. Bacterial cell structure
Cytoplasmic structures
Nucleoid
Ribosome
Cytoplasmic membrane
Cell wall
External structures

50. Bacterial cell structure External structures
Capsule/slime layer
Flagella
Fimbriae (pili)

51. Bacterial cell structure External structures – Capsule/slime layer
Distinct bacterial species
Polysaccharide
Capsule of Bacillus anthracis  Polypeptide (poly-D-glutamic acid)

52. Bacterial cell structure External structures – Capsule/slime layer
Capsule  condensed layer; closely surrounds the bacterium
Slime layer  loosely adherent; nonuniform in density and thickness
Capsule/slime layer: also called glycocalyx

53. Bacterial cell structure External structures – Capsule/slime layer
Protects bacteria against phagocytosis

54. Bacterial cell structure External structures – Capsule/slime layer
Plays a role in adherence (biofilm formation)
Artificial valves, catheters,…

55. Bacterial cell structure External structures
Capsule/slime layer
Flagella
Fimbriae (pili)

56. Bacterial cell structure External structures – Flagella

57. Bacterial cell structure External structures – Flagella
Thread-like appendages
Composed of protein subunits called flagellin
Subunits aggregate and form a helical structure
Highly antigenic (H antigens)
If removed by mechanical agitating; new flagella are rapidly formed and motility is rapidly restored

58. Bacterial cell structure External structures – Flagella
Types of arrangements of flagella:
Monotrichous single polar flagellum
Amphitrichous single polar flagella – opposite sites
Lophotrichous multiple polar flagella – opposite sites
Peritrichous flagella distributed over the entire cell

59. Bacterial cell structure External structures – Flagella

60. Bacterial cell structure External structures – Flagella
Hook:
Short curved structure
Joint between the basal body and the flagellum
Basal body:
Set of rings  1 pair (M-S ring) in Gram- positive bacteria; 2 pairs (L-P, M-S) in Gram- negative bacteria

61. Bacterial cell structure External structures – Flagella
Gram-negative
Gram-positive

62. Bacterial cell structure External structures – Flagella

63. Bacterial cell structure External structures – Flagella
Electric field and proton gradient across the membrane: Proton motive force
Flow of protons through the motor;
From the periplasmic space, to the cell membrane and the cytoplasm

64. Bacterial cell structure External structures – Flagella
Chemotaxis:
Moving toward (run, swim) food and away (tumble) poisons
Rotation of the flagellum:
Counterclockwise (run); clockwise (tumble)

65. Bacterial cell structure External structures – Flagella

66. Bacterial cell structure External structures
Capsule/slime layer
Flagella
Fimbriae (pili)

67. Bacterial cell structure External structures – Fimbriae (pili)
Gram-negative bacteria
Shorter and finer than flagella
Composed of protein subunits called pilins
Adhesins: minor proteins at the tips of pili; responsible for attachment

68. Bacterial cell structure External structures – Fimbriae (pili)
Ordinary pili:
Adherence of bacteria to host cells
Sex pili:
Attachment of donor and recipient cells in bacterial conjugation

69. Bacterial cell structure Endospores (spores)
Distinct bacterial genera; the most commons:
Bacillus (Gram-positive aerobic rod)
Clostridium (Gram- positive anaerobic rod)
Response to environmental conditions (depletion of nutrients)

70. Bacterial cell structure Endospores (spores)
Cannot be stained with Gram (seen colourless)
Bacillus
Clostridium
Spore

71. Bacterial cell structure Endospores (spores)
Special staining techniques (eg, malachite green stain)
Bacillus
Spore

72. Bacterial cell structure Endospores (spores)
Sporulation  formation of spore
Liberated when the mother cell undergoes autolysis
Resting cell; highly resistant to desiccation, heat, and chemical agents
Germination  favorable nutritional conditions; activation of spore to produce a single vegetative cell

73. Bacterial cell structure Endospores (spores) – Sporulation

74. Bacterial cell structure Endospores (spores)
Parts of spores (from in to outwards):
Core
Spore wall
Cortex
Coat
Exosporium
Core

75. Bacterial cell structure Endospores (spores) – Core
The core: is the spore protoplast
Contains chromosome
Does not contain ATP (energy of germination is stored as 3-phosphoglycerate)
Heat resistance of spores;
Dehydrated state
Calcium dipicolinate found in the core

76. Bacterial cell structure Endospores (spores) – Spore wall
Contains normal peptidoglycan
Becomes the cell wall of the germinating vegetative cell

77. Bacterial cell structure Endospores (spores) – Cortex
The thickest layer of the spore envelope
Contains an unusual type of peptidoglycan
Fewer cross-links
Extremely sensitive to lysozyme
Autolysis plays a role in spore germination

78. Bacterial cell structure Endospores (spores) – Coat
Composed of a keratin-like protein
Impermeable  resistance of spores against antibacterial chemical agents

79. Bacterial cell structure Endospores (spores) – Exosporium
The outermost layer of spore envelope
Composed of proteins, lipids, and carbohydrates
Consists of a paracrystalline basal layer and a hairlike outer region

80. Bacterial cell structure Classification
Gram-staining feature
Gram-positive
Gram-negative
Morphology
Coccus, bacillus, coccobacillus,…

81. Bacterial cell structure Classification – Gram staining
Introduced by Hans Christian Joachim Gram
Based on the differences of Gram-positive and Gram-negative cell wall

82. Bacterial cell structure Classification – Gram staining
Basic principle:
Crystal violet gets trapped in the thick and cross-linked peptidoglycan of Gram-positive bacteria
Gram-negative bacteria (thin peptidoglycan) are easily decolorized by alcohol and do not retain crystal violet

83. Bacterial cell structure Gram staining
Crystal violet
Lugol (iodine)
Alcohol
Safranin (fuchsin)

84. Bacterial cell structure Classification – Gram staining

85. Bacterial cell structure Classification – Gram staining
Gram-positive
Gram-negative

86. Bacterial cell structure Classification – Morphology (basic)
Coccus spherical
Bacillus rod-shaped
Coccobacillus transition form ……

87. Bacterial cell structure Classification – Morphology
Gram-positive cocci
Gram-negative cocci

88. Bacterial cell structure Classification – Morphology
Gram-positive bacilli
Gram-negative bacilli

89. Bacterial cell structure Classification – Morphology
Gram-negative coccobacilli