1. I. Bacterial Cell Division
5.1 Cell Growth and Binary Fission
5.2 Fts Proteins and Cell Division
5.3 MreB and Determinants of Cell Morphology
5.4 Peptidoglycan Synthesis and Cell Division
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2. 5.1 Cell Growth and Binary Fission
Binary fission: cell division following enlargement of a cell to twice its minimum size (Figure 5.1)
Generation time: time required for microbial cells to double in number
During cell division, each daughter cell receives a chromosome and sufficient copies of all other cell constituents to exist as an independent cell
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3. One generation Cell elongation Septum formation Septum
Figure 5.1
Cell elongation
One generation
Septum formation
Septum
Figure 5.1 Binary fission in a rod-shaped prokaryote.
Completion of septum; formation of walls; cell separation
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4. 5.2 Fts Proteins and Cell Division
Fts (filamentous temperature-sensitive) Proteins (Figure 5.2)
Essential for cell division in all prokaryotes
Interact to form the divisome (cell division apparatus)
FtsZ: forms ring around center of cell; related to tubulin
ZipA: anchor that connects FtsZ ring to cytoplasmic membrane
FtsA: helps connect FtsZ ring to membrane and also recruits other divisome proteins
Related to actin
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5. 5.2 Fts Proteins and Cell Division
DNA replicates before the FtsZ ring forms (Figure 5.3)
Location of FtsZ ring is facilitated by Min proteins
MinC, MinD, MinE
FtsK protein mediates separation of chromosomes to daughter cells
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6. Outer membrane Peptidoglycan Cytoplasmic membrane Divisome complex
Figure 5.2
Outer membrane
Peptidoglycan
Cytoplasmic membrane
Divisome complex
FtsZ ring
Cytoplasmic membrane
Figure 5.2 The FtsZ ring and cell division.
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7. 5.3 MreB and Determinants of Cell Morphology
Prokaryotes contain a cell cytoskeleton that is dynamic and multifaceted
MreB: major shape-determining factor in prokaryotes
Forms simple cytoskeleton in Bacteria and probably Archaea
Forms spiral-shaped bands around the inside of the cell, underneath the cytoplasmic membrane (Figure 5.4a and b)
Not found in coccus-shaped bacteria
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8. 5.3 MreB and Determinants of Cell Morphology
MreB (cont’d)
Localizes synthesis of new peptidoglycan and other cell wall components to specific locations along the cylinder of a rod-shaped cell during growth
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9. Sites of cell wall synthesis
Figure 5.4a
FtsZ
Cell wall
Cytoplasmic membrane
MreB
Sites of cell wall synthesis
Figure 5.4 MreB and crescentin as determinants of cell morphology.
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10. Figure 5.4b
Figure 5.4 MreB and crescentin as determinants of cell morphology.
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11. 5.3 MreB and Determinants of Cell Morphology
Most archaeal genomes contain FtsZ and MreB-like proteins, thus cell morphology is similar to that seen in Bacteria
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12. 5.4 Peptidoglycan Synthesis and Cell Division
Production of new cell wall material is a major feature of cell division
In cocci, cell walls grow in opposite directions outward from the FtsZ ring
In rod-shaped cells, growth occurs at several points along length of the cell
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13. 5.4 Peptidoglycan Synthesis and Cell Division
Preexisting peptidoglycan needs to be severed to allow newly synthesized peptidoglycan to form
Beginning at the FtsZ ring, small openings in the wall are created by autolysins
New cell wall material is added across the openings
Wall band: junction between new and old peptidoglycan
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14. Septum FtsZ ring Wall bands Growth zone Figure 5.5
Figure 5.5 Cell wall synthesis in gram-positive Bacteria.
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15. Sites of cell wall synthesis
Figure 5.4a
FtsZ
Cell wall
Cytoplasmic membrane
MreB
Sites of cell wall synthesis
Figure 5.4 MreB and crescentin as determinants of cell morphology.
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16. 5.4 Peptidoglycan Synthesis and Cell Division
Bactoprenol: carrier molecule that plays major role in insertion of peptidoglycan precursors
C55 alcohol (Figure 5.6)
Bonds to N-acetylglucosamine/ N-acetylmuramic acid/pentapeptide peptidoglycan precursor
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17. 5.4 Peptidoglycan Synthesis and Cell Division
Glycolases: enzymes that interact with bactoprenol (Figure 5.7a)
Insert cell wall precursors into growing points of cell wall
Catalyze glycosidic bond formation
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18. Growing point of cell wall
Figure 5.7a
Peptidoglycan
Growing point of cell wall
Transglycosylase activity
Cytoplasmic membrane
Autolysin activity
Out
Figure 5.7 Peptidoglycan synthesis. In
Pentapeptide
Bactoprenol
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19. 5.4 Peptidoglycan Synthesis and Cell Division
Transpeptidation: final step in cell wall synthesis (Figure 5.7b)
Forms the peptide cross-links between muramic acid residues in adjacent glycan chains
Inhibited by the antibiotic penicillin
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20. Transpeptidation Figure 5.7b Figure 5.7 Peptidoglycan synthesis.
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21. II. Population Growth 5.5 The Concept of Exponential Growth
5.6 The Mathematics of Exponential Growth
5.7 The Microbial Growth Cycle
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22. 5.5 The Concept of Exponential Growth
Most bacteria have shorter generation times than eukaryotic microbes
Generation time is dependent on growth medium and incubation conditions
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23. 5.5 The Concept of Exponential Growth
Exponential growth: growth of a microbial population in which cell numbers double within a specific time interval
During exponential growth, the increase in cell number is initially slow but increases at a faster rate (Figure 5.8)
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24. 5.7 The Microbial Growth Cycle
Batch culture: a closed-system microbial culture of fixed volume
Typical growth curve for population of cells grown in a closed system is characterized by four phases (Figure 5.10):
Lag phase
Exponential phase
Stationary phase
Death phase
Animation: Bacterial Growth Curve
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25. Growth phases Figure 5.10 Lag Exponential Stationary Death 1.0 10 0.759
Turbidity (optical density)
Optical density (OD)
Log10 viable organisms/ml
0.50
Viable count 8
0.25 7
Figure 5.10 Typical growth curve for a bacterial population. 6
0.1
Time
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26. 5.7 The Microbial Growth Cycle
Lag phase
Interval between when a culture is inoculated and when growth begins
Exponential phase
Cells in this phase are typically in the healthiest state
Stationary phase
Growth rate of population is zero
Either an essential nutrient is used up or waste product of the organism accumulates in the medium
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27. 5.7 The Microbial Growth Cycle
Death Phase
If incubation continues after cells reach stationary phase, the cells will eventually die
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28. IV. Temperature and Microbial Growth
5.12 Effect of Temperature on Growth
5.13 Microbial Life in the Cold
5.14 Microbial Life at High Temperatures
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29. Optimum Minimum Maximum
Figure 5.18
Enzymatic reactions occurring at maximal possible rate
Optimum
Enzymatic reactions occurring at increasingly rapid rates
Growth rate
Minimum
Maximum
Figure 5.18 The cardinal temperatures: minimum, optimum, and maximum.
Temperature
Membrane gelling; transport processes so slow that growth cannot occur
Protein denaturation; collapse of the cytoplasmic membrane; thermal lysis
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30. 5.12 Effect of Temperature on Growth
Microorganisms can be classified into groups by their growth temperature optima (Figure 5.19)
Psychrophile: low temperature
Mesophile: midrange temperature
Thermophile: high temperature
Hyperthermophile: very high temperature
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31. Thermophile Hyperthermophile Mesophile Psychrophile Hyperthermophile
Figure 5.19
Thermophile
Example: Geobacillus stearothermophilus
Hyperthermophile
Hyperthermophile
Example: Pyrolobus fumarii
Mesophile
Example: Thermococcus celer
Example: Escherichia coli
60°
88°
106°
Growth rate
Psychrophile
39°
Example: Polaromonas vacuolata 4°
Figure 5.19 Temperature and growth response in different temperature classes of microorganisms. 10 20 30 40 50 60 70 80 90
100
110
120
Temperature (°C)
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32. 5.12 Effect of Temperature on Growth
Mesophiles: organisms that have midrange temperature optima; found in
Warm-blooded animals
Terrestrial and aquatic environments
Temperate and tropical latitudes
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33. 5.13 Microbial Life in the Cold
Extremophiles
Organisms that grow under very hot or very cold conditions
Psychrophiles
Organisms with cold temperature optima
Inhabit permanently cold environments (Figure 5.20)
Psychrotolerant
Organisms that can grow at 0ºC but have optima of 20ºC to 40ºC
More widely distributed in nature than psychrophiles
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34. 5.13 Microbial Life in the Cold
Molecular Adaptations to Psychrophily
Production of enzymes that function optimally in the cold; features that may provide more flexibility
More -helices than -sheets
More polar and less hydrophobic amino acids
Fewer weak bonds
Decreased interactions between protein domains
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35. 5.13 Microbial Life in the Cold
Molecular Adaptations to Psychrophily (cont’d)
Transport processes function optimally at low temperatures
Modified cytoplasmic membranes
High unsaturated fatty acid content
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36. Figure 5.22 Figure 5.22 Growth of hyperthermophiles in boiling water.
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37. Figure 5.23
Figure 5.23 Growth of thermophilic cyanobacteria in a hot spring in Yellowstone National Park.
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38. 5.14 Microbial Life at High Temperatures
Studies of thermal habitats have revealed
Prokaryotes are able to grow at higher temperatures than eukaryotes
Organisms with the highest temperature optima are Archaea
Nonphototrophic organisms can grow at higher temperatures than phototrophic organisms
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39. 5.14 Microbial Life at High Temperatures
Molecular Adaptations to Thermophily
Enzyme and proteins function optimally at high temperatures; features that provide thermal stability
Critical amino acid substitutions in a few locations provide more heat-tolerant folds
An increased number of ionic bonds between basic and acidic amino acids resist unfolding in the aqueous cytoplasm
Production of solutes (e.g., di-inositol phophate, diglycerol phosphate) help stabilize proteins
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40. 5.14 Microbial Life at High Temperatures
Molecular Adaptations to Thermophily (cont’d)
Modifications in cytoplasmic membranes to ensure heat stability
Bacteria have lipids rich in saturated fatty acids
Archaea have lipid monolayer rather than bilayer
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41. 5.14 Microbial Life at High Temperatures
Hyperthermophiles produce enzymes widely used in industrial microbiology
Example: Taq polymerase, used to automate the repetitive steps in the polymerase chain reaction (PCR) technique
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42. V. Other Environmental Factors Affecting Growth
5.15 Acidity and Alkalinity
5.16 Osmotic Effects on Microbial Growth
5.17 Oxygen and Microorganisms
5.18 Toxic Forms of Oxygen
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43. 5.15 Acidity and Alkalinity
The pH of an environment greatly affects microbial growth (Figure 5.24)
Some organisms have evolved to grow best at low or high pH, but most organisms grow best between pH 6 and 8 (neutrophiles)
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44. Increasing alkalinity
Figure 5.24
Moles per liter of:
pH Example H
OH 1
1014
Volcanic soils, waters Gastric fluids Lemon juice
101
1013
102
1012
Acid mine drainage Vinegar
Increasing acidity
Rhubarb Peaches
103
1011
Acidophiles
Acid soil Tomatoes
104
1010
American cheese Cabbage
105
109
Peas Corn, salmon, shrimp
106
108
Neutrality
107
107
Pure water
Seawater
108
106
Very alkaline natural soil
109
105
Figure 5.24 The pH scale.
Alkaline lakes
1010
104
Increasing alkalinity
Soap solutions
Alkaliphiles
Household ammonia Extremely alkaline soda lakes
1011
103
1012
102
Lime (saturated solution)
1013
101
1014 1
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45. 5.15 Acidity and Alkalinity
Acidophiles: organisms that grow best at low pH (<6)
Some are obligate acidophiles; membranes destroyed at neutral pH
Stability of cytoplasmic membrane critical
Alkaliphiles: organisms that grow best at high pH (>9)
Some have sodium motive force rather than proton motive force
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46. 5.15 Acidity and Alkalinity
The internal pH of a cell must stay relatively close to neutral even though the external pH is highly acidic or basic
Internal pH has been found to be as low as 4.6 and as high as 9.5 in extreme acido- and alkaliphiles, respectively
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47. 5.15 Acidity and Alkalinity
Microbial culture media typically contain buffers to maintain constant pH
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48. 5.16 Osmotic Effects on Microbial Growth
Typically, the cytoplasm has a higher solute concentration than the surrounding environment, thus the tendency is for water to move into the cell (positive water balance)
When a cell is in an environment with a higher external solute concentration, water will flow out unless the cell has a mechanism to prevent this
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49. 5.16 Osmotic Effects on Microbial Growth
Halophiles: organisms that grow best at reduced water potential; have a specific requirement for NaCl (Figure 5.25)
Extreme halophiles: organisms that require high levels (15–30%) of NaCl for growth
Halotolerant: organisms that can tolerate some reduction in water activity of environment but generally grow best in the absence of the added solute
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50. Halotolerant Halophile Extreme halophile Nonhalophile
Figure 5.25
Halotolerant
Halophile
Extreme halophile
Example: Staphylococcus aureus
Example: Aliivibrio fischeri
Example: Halobacterium salinarum
Growth rate
Figure 5.25 Effect of sodium chloride (NaCl) concentration on growth of microorganisms of different salt tolerances or requirements.
Nonhalophile
Example: Escherichia coli 5 10 15 20
NaCl (%)
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51. 5.16 Osmotic Effects on Microbial Growth
Osmophiles: organisms that live in environments high in sugar as solute
Xerophiles: organisms able to grow in very dry environments
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52. 5.16 Osmotic Effects on Microbial Growth
Mechanisms for combating low water activity in surrounding environment involve increasing the internal solute concentration by
Pumping inorganic ions from environment into cell
Synthesis or concentration of organic solutes
compatible solutes: compounds used by cell to counteract low water activity in surrounding environment
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53. 5.17 Oxygen and Microorganisms
Aerobes: require oxygen to live
Anaerobes: do not require oxygen and may even be killed by exposure
Facultative organisms: can live with or without oxygen
Aerotolerant anaerobes: can tolerate oxygen and grow in its presence even though they cannot use it
Microaerophiles: can use oxygen only when it is present at levels reduced from that in air
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54. 5.17 Oxygen and Microorganisms
Thioglycolate broth (Figure 5.26)
Complex medium that separates microbes based on oxygen requirements
Reacts with oxygen so oxygen can only penetrate the top of the tube
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55. Oxic zone Anoxic zone Figure 5.26
Figure 5.26 Growth versus oxygen (O2) concentration.
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56. 5.17 Oxygen and Microorganisms
Special techniques are needed to grow aerobic and anaerobic microorganisms (Figure 5.27)
Reducing agents: chemicals that may be added to culture media to reduce oxygen (e.g., thioglycolate)
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57. Figure 5.27 Figure 5.27 Incubation under anoxic conditions.
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