1. I. Bacterial Cell Division
5.1 Binary Fission
5.2 Fts Proteins and Cell Division
5.3 MreB and Cell Morphology
5.4 Peptidoglycan Biosynthesis

2. 5.1 Binary Fission Growth: increase in the number of cells
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

3. Figure 5.1 Cell elongation Septum Septum formation
One generation
Septum
Completion of septum; formation of
walls; cell separation
Figure 5.1 Binary fission in a rod-shaped prokaryote.
Figure 5.1 3

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; tubulin homolog
FtsK protein mediates separation of chromosomes to daughter cells
FtsA: helps connect FtsZ ring to membrane and also recruits other divisome proteins
Homologous to actin

5. Figure 5.2 ATP GTP Figure 5.2 The FtsZ ring and cell division. 5
Outer membrane
ZipA
FtsI
Peptidoglycan
FtsA
Cytoplasmic
membrane
FtsK
ATP
GTP
GDP + Pi
ADP + Pi
FtsZ
ring
Divisome
complex
FtsZ ring
Cytoplasmic membrane
Figure 5.2 The FtsZ ring and cell division.
Figure 5.2 5

6. 5.3 MreB and 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 5.5a and b)
Not found in coccus-shaped bacteria

7. 5.3 MreB and 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
Thought to control rod vs coccus morphology-hence direction or polarity of cell

8. Figure 5.5a FtsZ Cell wall Cytoplasmic membrane MreB Sites of cell
wall synthesis
Figure 5.5a MreB and crescentin as determinants of cell morphology.
Figure 5.5a 8

9. 5.3 MreB and Cell Morphology
Crescentin: shape-determining protein produced by vibrio-shaped cells of Caulobacter crescentus
Crescentin protein organizes into filaments ~10 nm wide that localize on the concave face of the curved cells (Figure 5.5c)
Vibrio cholerae is the most famous of the vibrio organisms-an organism for the list
Causative agent of cholera-a serious
GI disorder

10. 5.3 MreB and Cell Morphology
Most archaeal genomes contain FtsZ and MreB-like proteins; thus, cell morphology is similar to that seen in Bacteria

11. A review on prokaryotic cell structure

12. 5.4 Peptidoglycan Biosynthesis
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

13. 5.4 Peptidoglycan Biosynthesis
Preexisting peptidoglycan needs to be severed to allow newly synthesized peptidoglycan to form (Figure 5.6)
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

14. Figure 5.6 FtsZ ring Wall bands Growth zone Septum
Figure 5.6 Cell wall synthesis in gram-positive Bacteria.
Figure 5.6 14

15. 5.4 Peptidoglycan Biosynthesis
Bactoprenol: carrier molecule that plays major role in insertion of peptidoglycan precursors
C55 alcohol (Figure 5.7)
Bonds to N-acetylglucosamine/N-acetylmuramic acid/pentapeptide peptidoglycan precursor

16. Hydrophobic portion Peptidoglycan precursor
Bactoprenol: carrier molecule that plays major role in insertion of peptidoglycan precursors
C55 alcohol (Figure 5.7)Bonds to N-acetylglucosamine/N-acetylmuramic acid/pentapeptide peptidoglycan precursor
Figure 5.7 Bactoprenol (undecaprenol diphosphate).
Peptidoglycan
precursor
Figure 5.7 16

17. Figure 5.8a Peptidoglycan Growing point of cell wall Cytoplasmic
Transglycosylase
activity
Growing point
of cell wall
Cytoplasmic
membrane
Autolysin
activity
Out In
Figure 5.8a Peptidoglycan synthesis.
Pentapeptide
Bactoprenol
Figure 5.8a 17

18. 5.4 Peptidoglycan Biosynthesis
Transpeptidation: final step in cell wall synthesis (Figure 5.8b)
Forms the peptide cross-links between muramic acid residues in adjacent glycan chains
Inhibited by the antibiotic penicillin

19. II. Population Growth 5.5 Quantitative Aspects of Microbial Growth
5.6 The Growth Cycle
5.7 Continuous Culture

20. 5.5 Quantitative Aspects of Microbial Growth
Most bacteria have shorter generation times than eukaryotic microbes
Generation time is dependent on growth medium and incubation conditions

21. 5.5 Quantitative Aspects of Microbial 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 5.9)

22. Figure 5.9 Logarithmic plot Arithmetic plot
Figure 5.9 The rate of growth of a microbial culture.
Figure 5.9 22

23. Disambiguation
Above is only an illustration of one phase of the growth of a bacterial culture-an illustration of exponential growth. In real life the full growth curve is more complicated. Dependent on factors such as availability of nutrients and accumulation of wastes.

24. 5.5 Quantitative Aspects of Microbial Growth
A relationship exists between the initial number of cells present in a culture and the number present after a period of exponential growth:
N = N02n
N is the final cell number
N0 is the initial cell number
n is the number of generations during the period of exponential growth

25. 5.5 Quantitative Aspects of Microbial Growth
Generation time (g) of the exponentially growing population is
g = t/n
t is the duration of exponential growth n is the number of generations during the period of exponential growth
Generation time (g) of the exponentially growing population is the time it takes for the population to double

26. 5.5 Quantitative Aspects of Microbial Growth
Specific growth rate (k) is calculated as
k = 0.301/g
Division rate (v) is calculated as
v = 1/g

27. 5.6 The Growth Cycle
Batch culture: a closed-system microbial culture of fixed volume-most common system
Typical complete growth curve for population of cells grown in a closed system is characterized by four phases (Figure 5.11):
Lag phase
Exponential phase
Stationary phase
Death phase

29. 5.6 The Growth Cycle Lag phase Exponential phase Stationary phase
Interval between inoculation of a culture and beginning of growth
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

30. 5.6 The Growth Cycle Death phase
If incubation continues after cells reach stationary phase, the cells will eventually die

31. IV. Effect of Temperature on Microbial Growth
5.11 Temperature Classes of Microorganisms
5.12 Microbial Life in the Cold
5.13 Microbial Life at High Temperatures

32. 5.11 Temperature Classes of Microorganisms
Temperature is a major environmental factor controlling microbial growth
Temperature optimum = temperature at which growth is fastest
Cardinal temperatures: the minimum, optimum, and maximum temperatures at which an organism grows (Figure 5.19)

33. 5.11 Temperature Classes of Microorganisms
Microorganisms can be classified into groups by their growth temperature optima (Figure 5.20)
Psychrophile: low temperature
Mesophile: midrange temperature
Thermophile: high temperature
Hyperthermophile: very high temperature

34. Figure 5.20 Temperature and growth response in different temperature classes of microorganisms.

35. 5.12 Microbial Life in the Cold
Other definitions
Extremophiles
Organisms that grow under very hot or very cold conditions
Psychrotolerant
Organisms that can grow at 0ºC but have optima of 20ºC to 40ºC
More widely distributed in nature than psychrophiles

36. 5.12 Microbial Life in the Cold
Molecular adaptations that support psychrophily
Production of enzymes that function optimally in the cold; features that may provide more flexibility
More α-helices than β-sheets
More polar and less hydrophobic amino acids
Fewer weak bonds
Decreased interactions between protein domains

37. 5.12 Microbial Life in the Cold
Molecular adaptations that support psychrophily (cont'd)
Transport processes function optimally at low temperatures
Modified cytoplasmic membranes
High unsaturated fatty acid content (needs lower temperature to solidify)

38. 5.13 Microbial Life at High Temperatures
Above ~65ºC, only prokaryotic life forms exist
Hyperthermophiles in hot springs
Chemoorganotrophic and chemolithotrophic species are present (Figure 5.23)
High prokaryotic diversity (both Archaea and Bacteria are represented)

39. Figure 5.24 Growth of thermophilic cyanobacteria in a hot spring in Yellowstone National Park.

40. 5.13 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

41. 5.13 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 resists unfolding in the aqueous cytoplasm

42. 5.13 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

43. 5.13 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

44. V. Other Environmental Effects on Microbial Growth
5.14 Effects of pH on Microbial Growth
5.15 Osmolarity and Microbial Growth
5.16 Oxygen and Microbial Growth

45. 5.14 Effects of pH on Microbial Growth
The pH of an environment greatly affects microbial growth (Figure 5.25)
Some organisms have evolved to grow best at low or high pH, but most organisms grow best between pH 6 and 8 (neutrophiles)

46. 5.14 Effects of pH on Microbial Growth
Acidophiles: organisms that grow best at low pH (<6)
Some are obligate acidophiles; membranes are destroyed at neutral pH
Stability of cytoplasmic membrane is critical
Alkaliphiles: organisms that grow best at high pH (>9)
Some have sodium motive force rather than proton motive force

47. 5.14 Effects of pH on Microbial Growth
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 acidophiles and alkaliphiles, respectively

48. 5.15 Osmolarity and Microbial Growth
Water activity (aw): water availability; expressed in physical terms
Defined as ratio of vapor pressure of air in equilibrium with a substance or solution to the vapor pressure of pure water

49. 5.15 Osmolarity and 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

50. 5.15 Osmolarity and Microbial Growth
Halophiles: organisms that grow best at reduced water potential; have a specific requirement for NaCl (Figure 5.26)
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

51. Figure 5.26 Halotolerant Halophile Extreme halophile Nonhalophile
Example:
Staphylococcus
aureus
Example:
Aliivibrio fischeri
Example:
Halobacterium
salinarum
Figure 5.26 Effect of NaCl concentration on growth of microorganisms of different salt tolerances or requirements.
Nonhalophile
Example:
Escherichia
coli
Figure 5.26

52. 5.15 Osmolarity and Microbial Growth
Osmophiles: organisms that live in environments high in sugar as solute
Xerophiles: organisms able to grow in very dry environments

53. 5.15 Osmolarity and 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
Synthesizing or concentrating organic solutes
Compatible solutes: compounds used by cell to counteract low water activity in surrounding environment

54. 5.16 Oxygen and Microbial Growth
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

55. 5.16 Oxygen and Microbial Growth
Several toxic forms of oxygen can be formed in the cell (Figure 5.29):
Single oxygen
Superoxide anion
Hydrogen peroxide
Hydroxyl radical

56. Figure 5.29 Four-electron reduction of O2 to H2O by stepwise addition of electrons.

57. 5.16 Oxygen and Microbial Growth
Enzymes are present to neutralize most of these toxic oxygen species (Figure 5.30)
Catalase (Figure 5.31)
Peroxidase
Superoxide dismutase
Superoxide reductase

58. Figure 5.30 Enzymes that destroy toxic oxygen species.

59. V. Control of Microbial Growth
5.17 General Principles and Growth Control by Heat
5.18 Other Physical Control Methods: Radiation and Filtration
5.19 Chemical Control of Microbial Growth

60. 5.17 General Principles and Growth Control by Heat
Sterilization
The killing or removal of all viable organisms within a growth medium
Inhibition
Effectively limiting microbial growth
Decontamination
The treatment of an object to make it safe to handle
Disinfection
Directly targets the removal of all pathogens, not necessarily all microorganisms

61. 5.17 General Principles and Growth Control by Heat
Heat sterilization is the most widely used method of controlling microbial growth (Figure 5.32a)
High temperatures denature macromolecules
Some bacteria produce resistant cells called endospores
Can survive heat that would rapidly kill vegetative cells

62. 5.18 Other Physical Control Methods: Radiation and Filtration
Microwaves, UV, X-rays, gamma rays, and electrons can reduce microbial growth
UV has sufficient energy to cause modifications and breaks in DNA
UV is useful for decontaminating surfaces (Figure 5.34)
Cannot penetrate solid, opaque, or light-absorbing surfaces

63. 5.18 Other Physical Control Methods: Radiation and Filtration
Ionizing radiation
Electromagnetic radiation that produces ions and other reactive molecules
Generates electrons, hydroxyl radicals, and hydride radicals
Some microorganisms are more resistant to radiation than others
Deinococcus duroradians or “Conan the Bacterium”

64. 5.18 Other Physical Control Methods: Radiation and Filtration
Sources of radiation include cathode ray tubes, X-rays, and radioactive nuclides
Radiation is used for sterilization in the medical field and food industry
Radiation is approved by the WHO and is used in the USA for decontaminating foods particularly susceptible to microbial contamination
Hamburger, chicken, spices may all be irradiated

65. 5.18 Other Physical Control Methods: Radiation and Filtration
Filtration avoids the use of heat on sensitive liquids and gases
Pores of filter are too small for organisms to pass through but not viruses
Pores allow liquid or gas to pass through
Medical materials

66. 5.19 Chemical Control of Microbial Growth
Antimicrobial agents can be classified as:
Bacteriostatic: stops growth
Bacteriocidal: “kills” microbe
Bacteriolytic : destruction of microbe with physical bursting or “lysis”
(Figure 5.39)