I. Bacterial Cell Division 5.1Cell Growth and Binary Fission 5.2Fts Proteins and Cell Division...

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

© 2012 Pearson Education, Inc.

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


Figure 5.1

Septum

Cellelongation

Septumformation

Completionof septum;formation ofwalls; cellseparation

On

e g

en

era

tio

n


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


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


Figure 5.2

Cytoplasmic membrane

Outer membrane

Cytoplasmic membrane

Divisomecomplex

Peptidoglycan

FtsZ ring


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© 2012 Pearson Education, Inc.


• 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


Figure 5.4a

Cell wall

Cytoplasmicmembrane

MreB

Sites of cellwall synthesis

FtsZ


Figure 5.4b



• Most archaeal genomes contain FtsZ and MreB-like proteins, thus cell morphology is similar to that seen in Bacteria


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



• 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


Figure 5.5FtsZ ring

Wall bands Growth zone

Septum


Figure 5.4a

Cell wall

Cytoplasmicmembrane

MreB

Sites of cellwall synthesis

FtsZ



• 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



• Glycolases: enzymes that interact with bactoprenol (Figure 5.7a)– Insert cell wall precursors into growing points

of cell wall

– Catalyze glycosidic bond formation


Figure 5.7a

Transglycosylaseactivity

Growing pointof cell wall

Cytoplasmicmembrane

Autolysinactivity

Pentapeptide

Bactoprenol

Peptidoglycan

Out

In



• 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


Figure 5.7b

Transpeptidation


II. Population Growth

• 5.5 The Concept of Exponential Growth• 5.6 The Mathematics of Exponential

Growth• 5.7 The Microbial Growth Cycle


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


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)


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 CurveAnimation: Bacterial Growth Curve

Figure 5.10

Time

Lo

g10

via

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org

an

ism

s/m

l

Op

tic

al

de

ns

ity

(O

D)10

9

8

7

6

1.0

0.75

0.50

0.25

0.1

Viable count

Turbidity(optical density)

Lag Exponential Stationary Death

Growth phases



• 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



• Death Phase– If incubation continues after cells reach

stationary phase, the cells will eventually die


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


Figure 5.18

Enzymatic reactions occurringat maximal possible rate

Enzymatic reactions occurringat increasingly rapid rates

Membrane gelling; transportprocesses so slow that growthcannot occur

Protein denaturation; collapseof the cytoplasmic membrane;thermal lysis

Temperature

Gro

wth

ra

te

Minimum

Optimum

Maximum


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


Figure 5.19

Temperature (°C)

Gro

wth

rat

e

Psychrophile

Mesophile

Thermophile

Hyperthermophile Hyperthermophile

0 10 20 30 40 50 60 70 80 90 100 110 120

4°

39°

60°88° 106°

Example:Polaromonas vacuolata

Example:Escherichia coli

Example:Geobacillusstearothermophilus Example:

Thermococcus celerExample:Pyrolobus fumarii


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


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



• 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



• Molecular Adaptations to Psychrophily (cont’d)– Transport processes function optimally at low

temperatures• Modified cytoplasmic membranes

– High unsaturated fatty acid content


Figure 5.22


Figure 5.23


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



• 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



• 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



• Hyperthermophiles produce enzymes widely used in industrial microbiology– Example: Taq polymerase, used to automate

the repetitive steps in the polymerase chain reaction (PCR) technique


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


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)


Figure 5.24

Volcanic soils, watersGastric fluidsLemon juiceAcid mine drainageVinegarRhubarbPeaches

Acid soilTomatoesAmerican cheeseCabbagePeasCorn, salmon, shrimp

Pure water

Seawater

Very alkaline natural soilAlkaline lakesSoap solutionsHousehold ammoniaExtremely alkaline soda lakesLime (saturated solution)

Neutrality

Increasingacidity

Increasingalkalinity

107 107

Aci

do

ph

iles

Alk

alip

hile

s

pH Example Moles per liter of:H OH

101

102

103

104

105

106

108

109

1010

1011

1012

1013

1014

1014

1013

1012

1011

1010

109

108

106

105

104

103

102

101

1

1



• 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



• 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



• Microbial culture media typically contain buffers to maintain constant pH


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



• 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


Example:Staphylococcusaureus

Example:Aliivibrio fischeri Example:

Halobacteriumsalinarum

Example:Escherichia coli

NaCl (%)

Gro

wth

ra

te

0 5 10 15 20

Nonhalophile

Halotolerant Halophile Extremehalophile

Figure 5.25



• Osmophiles: organisms that live in environments high in sugar as solute

• Xerophiles: organisms able to grow in very dry environments



• 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


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



• 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


Figure 5.26

Oxic zone

Anoxic zone



• 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)


Figure 5.27


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I. Bacterial Cell Division 5.1Cell Growth and Binary Fission 5.2Fts Proteins and Cell Division...

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