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Microbial Growth

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Title: Chapter 6 Author: Pam Seshadri Last modified by: SCCC Created Date: 2/11/2010 2:40:33 PM Document presentation format: On-screen Show (4:3) Company – PowerPoint PPT presentation

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Title: Microbial Growth


1
Chapter 7
  • Microbial Growth

2
Growth
  • microbiologists usually study population growth
    ( cells in a bacterial population) rather than
    growth of individual cells
  • because cells are small in size

3
The Procaryotic Cell Cycle
  • cell cycle is sequence of events when a new cell
    divides into two.
  • most bacteria divide by binary fission
  • two pathways function during cycle
  • DNA replication and partition
  • cytokinesis

4
Figure 6.1
5
Chromosome Replication and Partitioning
  • most procaryotic chromosomes are circular
  • origin of replication site at which replication
    begins
  • terminus site at which replication is
    terminated
  • replisome (at the origin of replication) group
    of proteins needed for DNA synthesis parent DNA
    spools through the replisome as replication
    occurs
  • MreB an actin homolog plays role in
    determination of cell shape and chromosome
    movement. (opposite poles of the cell)

6
Figure 6.3
7
Cytoskeletal Proteins - Role in Cytokinesis
  • process not well understood
  • protein MreB
  • similar to eucaryotic actin
  • plays a role in determination of cell shape and
    movement of chromosomes to opposite cell poles
  • protein FtsZ,
  • similar to eucaryotic tubulin - microtubules
  • plays a role in Z ring formation which is
    essential for septation
  • Divides the cell in two by constriction

8
Figure 6.4
9
DNA Replication in Rapidly Growing Cells
  • cell cycle completed in 20 minutes
  • 40 minutes for DNA replication
  • 20 minutes for septum formation and cytokinesis
  • look at timing-how can this happen?
  • Second, third or fourth round of replication can
    begin before first round of replication is
    completed

10
The Growth Curve
  • observed when microorganisms are cultivated in
    batch culture
  • culture incubated in a closed vessel with a
    single batch of medium
  • usually has four distinct phases

11
Fig. 6.6
12
Lag Phase
  • cell synthesizing new components
  • e.g., to replenish spent materials
  • e.g., to adapt to new medium or other conditions
  • varies in length
  • in some cases can be very short or even absent

13
Exponential Phase
  • also called log phase
  • rate of growth is constant
  • population is most uniform in terms of chemical
    and physical properties during this phase
  • Culture is most sensitive to antibiotics
  • Metabolically most active

14
Stationary Phase
  • total number of viable cells remains constant
  • may occur because metabolically active cells stop
    reproducing
  • may occur because reproductive rate is balanced
    by death rate
  • cells dividing equals cells dying

15
Possible reasons for entry into stationary phase
  • nutrient limitation
  • limited oxygen availability
  • toxic waste accumulation - acids

16
Starvation responses
  • morphological changes
  • e.g., endospore formation
  • decrease in size, protoplast shrinkage, and
    nucleoid condensation
  • production of starvation proteins
  • increased virulence harder to kill
  • Strengthen peptidoglycan, chaperone protein
    prevent denaturation

17
Death Phase
  • More and more cells are dying
  • cells dying exceeds the new cells formed

18
Generation time
  • generation (doubling) time
  • time required for the population to double in
    size ((1cell to divide into two)
  • Varies depending on species of microorganism and
    environmental conditions
  • E.coli 20 minutes
  • M. leprae 14 days

19
The Influence of Environmental Factors on Growth
  • most organisms grow in fairly moderate
    environmental conditions
  • extremophiles
  • grow under harsh conditions that would kill most
    other organisms

20
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21
Adaptations of thermophiles
  • protein structure stabilized by a variety of
    means
  • e.g., more H bonds
  • e.g., more proline
  • e.g., chaperones
  • histone-like proteins stabilize DNA
  • membrane stabilized by variety of means
  • e.g., more saturated, more branched and higher
    molecular weight lipids

22
  • Microbial growth is also influenced by pH.
  • Optimum growth pH 6.5 to 7.5
  • Acidophiles like acidic pH
  • Sulfolobus temperature 70oC
  • pH 2 (thermophile)
  • Alkalophiles basic pH
  • Nitrosomonas pH 8 to 8.8
  • Nitrosomonas soil
  • Fungi pH 5 to 6

23
pH
  • most acidophiles and alkalophiles maintain an
    internal pH near neutrality
  • The plasma membrane is impermeable to protons
  • some synthesize proteins that provide protection
  • e.g., acid-shock proteins chaperone
  • Pump protons out

24
  • Osmosis movement of water molecules from an
    area of high concentration of water to an area of
    low concentration of water through a selectively
    permeable membrane.
  • hypotonic solute concentration lower outside
    the cell. Water moves into the cell.
  • Cell swells up and breaks down. Osmotic lysis.
  • Cell walls are damaged bacteria undergo osmotic
    lysis.

25
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27
  • Osmotic environment
  • Most bacteria cannot survive in hypertonic
    environment.
  • Plamolysis
  • High concentration of salt is used to preserve
    food products.
  • Halophiles need high concentration of salt for
    growth ( at least 30 salt)
  • Salt lakes, dead sea
  • Halobacterium

28
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29
  • Facultative halophiles can grow in an
    environment that has 2 to 15 salt.
  • Do not need high concentration of salt for
    growth.
  • Grow in the presence or absence of salt
  • Staphylococcus aureus

30
Osmotolerant organisms
  • grow over wide ranges of water activity
  • many use compatible solutes to increase their
    internal osmotic concentration
  • solutes that are compatible with metabolism and
    growth amino acids
  • some have proteins and membranes that require
    high solute concentrations for stability and
    activity

31
Oxygen Concentration
ignore oxygen
lt 2 10 oxygen
need oxygen
prefer oxygen
oxygen is toxic
Figure 6.22
thioglycolate
32
Basis of different oxygen sensitivities
  • oxygen easily reduced to toxic products
  • superoxide radical
  • hydrogen peroxide
  • aerobes produce protective enzymes
  • superoxide dismutase (SOD)
  • catalase

33
Oxygen requirement
  • Obligate aerobes need O2 for growth
  • Micrococcus
  • Facultative anaerobe can grow in the presence
    or absence of O2. it grows better in the presence
    of O2. E.coli
  • Obligate anaerobe only grow in the absence of
    O2. Clostridium

34
  • Cells use O2 during cellular respiration.
  • Small amount of toxic O2 super oxide free
    radical is made damage to the structures
  • Obligate aerobes and facultative anaerobes
    enzymes to neutralize the toxic O2
  • Super oxide dismutase (SOD)
  • Free radical hydrogen peroxide (H2O2)
  • Catalase H2O2 water and oxygen
  • Aerotolerant anaerobes tolerate O2 do not use
    O2. Lactobacillus

35
Figure 6.24
36
Responses to low nutrient levels (oligotrophic
environments)
  • organisms become more competitive in nutrient
    capture and use of available resources
  • morphological changes to increase surface area
    and ability to absorb nutrients
  • mechanisms to sequester certain nutrients
  • Group translocation
  • Caulobacter

37
Biofilms
  • ubiquitous in nature
  • complex, slime enclosed colonies attached to
    surfaces
  • Layers of slime on rocks or other objects in
    water
  • Different species live together in a cocoon made
    up of slime
  • when form on medical devices such as implants
    (hip and knee) often lead to illness
  • can be formed on any surface

38
Biofilm formation slime encased communities
  • Layers of slime on rocks
  • Surface conditioned with proteins
  • microbes reversibly attach to surface and release
    polysaccharides, proteins, and DNA matrix
    extracellular polymeric substances (EPS) stable
    attachment
  • additional polymers are produced as biofilm
    matures
  • interactions occur among the attached organisms

39
Figure 6.28
40
  • Waste product of one microbe can be the energy
    source of another microbe.
  • DNA in the EPS can be taken up by some of the
    microbes - changes the genetic make up

41
Biofilm Microorganisms
  • the EPS and change in attached organisms
    physiology protects microbes from harmful agents
  • UV light, antibiotics, antimicrobials
  • when formed on medical devices, such as implants,
    often lead to illness
  • sloughing off of organisms can result in
    contamination of water phase above the biofilm
    such as in a drinking water system. Inner wall of
    pipes in the water supply

42
Biofilms
  • a mature biofilm is a complex, dynamic community
    of microorganisms
  • heterogeneity is differences in metabolic
    activity and locations of microbes
  • interactions occur among the attached organisms
  • exchanges take place metabolically, DNA uptake
    and communication

43
Figure 7.34
Competent To pick up DNA Peptides activate
genes That code for Toxins to Kill cells that
are Not competent
44
Biofilm microorganisms
  • extracellular matrix and change in attached
    organisms physiology protects them from
    harmful agents such as UV light and antibiotics
  • sloughing off of organisms can result in
    contamination of water phase above the biofilm
    such as in a drinking water system

45
Cell to Cell Communication Within the Microbial
Populations
  • bacterial cells in biofilms communicate in a
    density-dependent manner called quorum sensing
  • produce small proteins that increase in
    concentration as microbes replicate (increase in
    number) and convert a microbe to a competent
    state. In this state
  • DNA uptake occurs, bacteriocins are released
    kill cells that are not competent

46
Quorum Sensing
  • Minimum number cells have to be present for the
    regulation of certain genes
  • regulate the expression of bunch genes in a cell.
  • For this process to take place, there has to be
    minimum number of cells in the area.

47
Quorum Sensing Systems
  • processes regulated by quorum sensing involve
    host-microbe interactions
  • symbiosis Vibrio fischeri and bioluminescence
    in squid first discovered
  • Free living no light
  • Whole bunch of cells in the same area
  • pathogenicity and increased virulence factor
    production
  • DNA uptake for antibiotic resistance genes
  • Light organ, once the cell density increases
  • Cells give out light (flash light)

48
Figure 7.36a
49
Figure 7.36b
Catch its prey Escape from Predators Flash
light
50
Quorum Sensing
  • acylhomoserine lactone (AHL) is an autoinducer
    molecule produced by many gram-negative organisms
    (activating certain genes)
  • Few cells AHL stays in the environment. As the
    cells reproduce, cell density incases. More AHL
    produced. diffuses across plasma membrane.once
    inside the cell it induces expression of target
    genes that regulate a variety of functions

51
  • Dental plaque formed on the surface of teeth is a
    biofilm
  • Different species of bacteria and such as S.
    mutans and candida albicans (yeast)
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