Transcript Microbiology - Chapter 5

Microbiology
An Evolving Science
Second Edition
Joan Slonczewski and John Foster
5
Environmental
Influences and
Control of
Microbial Growth
PowerPoint® Lecture Outlines
Prepared by Johnny El-Rady, University of South Florida
Copyright © 2010 W. W. Norton & Company, Inc. Permission required for reproduction or display
Chapter Overview
How the environment limits growth
● The microbial response to temperature
● How microbes cope with pressure
● The microbial response to changes in: water
activity, salt concentrations, pH, and oxygen
● Hungry microbes
● The control of microbes:
- Physical, chemical, and biological
●
2
Introduction
Microbes have both the fastest and the
slowest growth rates of known organisms
- Some hot-springs bacteria can double in
as little as 10 minutes, whereas deep-seas
sediment microbes may take as long as
100 years
These differences are determined by
nutrition and niche-specific physical
parameters like temperature and pH
3
Environmental Limits of
Microbial Growth
“Normal” growth conditions
- Sea-level; temperature 20-40o C; neutral
pH; 0.9% salt, and ample nutrients
Any ecological niche outside this window is
called “extreme”, and organisms
inhabiting them extremophiles
Figure 1.1
4
The environmental habitat (such as high salt
or low pH) that a species inhabits is based
on one main criterion
- The tolerance of that organism’s proteins
and other macromolecular structures to
the physical conditions within that niche
Note that multiple extremes in the
environment can be met simultaneously
Figure 1.1
5
Global approaches used to study gene
expression allow us to view how
organisms respond to changes in their
environment
- DNA microarrays assess which RNAs
are made in a given organism at a given
time or under a given condition
- Two-dimensional protein gels achieve
separation of proteins based on
differences in each protein’s isoelectric
point (first dimension) and molecular
weight (second dimension)
6
Figure 5.1
7
Table 5.1
8
Changes in Temperature
A bacterial cell’s temperature matches that of
its immediate environment
Changes in temperature impact every aspect
of microbial physiology
Each organism has an “optimum” temperature,
as well as minimum and maximum
temperatures that define its growth limits
Microbes that grow at higher temperatures can
typically achieve higher rates of growth
9
Changes in Temperature
Microorganisms can be classified by their
growth temperature
- Psychrophiles ~ 0-20o C
- Mesophiles ~ 15-45o C
- Thermophiles ~ 40-80o C
- Hyperthermophiles ~ 65-121o C
All of these organisms have membranes and
proteins best suited for their temperatures
10
Figure 5.2
11
Figure 5.3
Figure 5.4
12
Heat-Shock Response
Rapid temperature changes experienced
during growth activates batches of stress
response genes
- Resulting in the heat-shock response
The protein products include chaperones that
maintain protein shape and enzymes that
change membrane lipid composition
This type of response has been documented
in all living organisms examined so far
13
Variations in Pressure
Barophiles or piezophiles are organisms
adapted to grow at very high pressures
- Up to 1,000 atm (101 MPa, or 14,000 psi)
Barotolerant organisms grow well over the
range of 1-50 MPa, but their growth falls off
thereafter
Note that many barophiles are also
psychrophiles because the average
temperature at the ocean floor is 2o C
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Figure 5.5
Figure 5.6
15
Changes in Water Activity
Water activity (aw) is a measure of how much
water is available for use
Osmolarity is a measure of the number of
solute molecules in a solution, and is
inversely related to aw
Aquaporins are membrane-channel proteins
that allow water to traverse the membrane
much faster than by diffusion
- Help protect the cell from osmotic stress
16
Minimizing Osmotic Stress
In addition to moving water, microbes have at
least two mechanisms to minimize osmotic
stress
- In hypertonic media, bacteria protect their
internal water by synthesizing or importing
compatible solutes (E.g.: Proline or K+)
- In hypotonic media, pressure-sensitive or
mechanosensitive channels can be used to
leak solutes out of the cell
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Changes in Salt Concentrations
Halophiles require high salt concentrations
- From 2-4 M NaCl (10-20% NaCl)
- For comparison, seawater is ~ 3.5% NaCl
Figure 5.8
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Changes in pH
Figure 5.11
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Changes in pH
All enzyme activities exhibit optima, minima,
and maxima with regard to pH
Bacteria regulate internal pH
- When environment is in a similar pH range
Weak acids can pass through membranes
- Disrupt cell pH homeostasis, and thus will
kill cells
- This phenomenon is used to preserve
foods
20
Changes in pH
Three classes of organisms are differentiated
by the pH of their growth range
- Neutralophiles grow at pH 5-8
- Include most pathogens
- Acidophiles grow at pH 0-5
- Are often chemoautotrophs
- Alkaliphiles grow at pH 9-11
- Typically found in soda lakes
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The cyanobacterium
Spirulina has high
concentrations of
carotene, giving it
a distinct pink color
- It is also a major
food for the famous
pink flamingo
Figure 5.15
22
pH Homeostasis
When cells are placed in pH conditions below
the optimum, protons can enter the cell and
lower internal pH to lethal levels
Microbes can prevent the unwanted influx of
protons by exchanging extracellular K+ for
intracellular H+ when the internal pH becomes
too low
Under extremely alkaline conditions, the cells
can use the Na+/H+ antiporter to bring protons
into the cell in exchange for expelling Na+
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Figure 5.17
24
Oxygen As An Electron Acceptor
Many microorganisms use oxygen as a
terminal electron acceptor in a process
called aerobic respiration
Figure 5.18
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Microbial Responses to Oxygen
Strict aerobes can only grow in oxygen
Microaerophiles grow only at lower O2 levels
Strict anaerobes die in least bit of oxygen
Facultative anaerobes can live with or
without oxygen
Aerotolerant anaerobes grow in oxygen
while retaining a fermentation-based
metabolism
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Oxygen-related growth zones in a standing
test tube
Figure 5.19
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Generation and destruction of reactive
oxygen species (ROS)
Figure 5.20
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Culturing Anaerobes in the Lab
Three oxygen-removing techniques are
used today
1. Special reducing agents (thioglycolate)
or enzyme systems (Oxyrase) can be
added to ordinary liquid media
2. An anaerobe jar
3. An anaerobic chamber with glove ports
- O2 is removed by vacuum and
replaced with N2 and CO2
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Figure 5.21
30
Microbial Response to Starvation
Starvation is a stress that can elicit a
“starvation response” in many microbes
- Enzymes are produced to increase the
efficiency of nutrient gathering and to
protect cell macromolecules from damage
This response is usually triggered by the
accumulation of small signal molecules
such as cAMP or guanosine tetraphosphate
31
Microbial Response to Starvation
Some organisms growing on nutrient-limited
agar can even form colonies with intricate
geometrical shapes that help the population
cope, in some unknown way, to food stress
Figure 5.22
32
Oligotrophic Bacteria
In natural ecosystems, most microbes appear
to be oligotrophs, organisms with a high
rate of growth at low solute concentrations
- Indeed, they require low nutrient levels to
survive
Some oligotrophic bacteria have thin
extensions of their membrane and cell wall
called prothecaes (stalks)
- These expand the surface area of the cell
and increase nutrient-transport capacity
33
Humans Influence Microbial Ecosystems
Maximum diversity
in an ecosystem
is maintained, in
part, by the
different nutrientgathering profiles
of competing
microbes
Figure 5.23
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Humans Influence Microbial Ecosystems
Eutrophication is the sudden infusion of
large quantities of a formerly limiting
nutrient
- It can lead to a
“bloom” of microbes,
which can threaten
the existence of
competing species
Figure 5.24
35
Control of Microbes
A variety of terms are used to describe
antimicrobial control measures
- Sterilization: Killing of all living organisms
- Disinfection: Killing or removal of
pathogens from inanimate objects
- Antisepsis: Killing or removal of
pathogens from the surface of living tissues
- Sanitation: Reducing the microbial
population to safe levels
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Microbes die at a logarithmic rate
Decimal reduction time (D value) is the
length of time it takes an agent or condition
to kill 90% of the population
Figure 5.25
37
Physical Agents
High temperature
- Moist heat is more effective than dry heat
- Boiling water (100o C) kills most cells
- Killing spores and thermophiles usually
requires a combination of high pressure and
temperature
- Steam autoclave
- 121o C at 15 psi for 20 minutes
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Figure 5.26
39
Physical Agents
Pasteurization
- Many different time and temperature
combinations can be used
- LTLT (low-temperature/long-time)
- 63o C for 30 minutes
- HTST (high-temperature/short-time)
- 72o C for 15 seconds
- Both processes kill Coxiella burnetii, the
causative agent of Q fever
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Physical Agents
Cold
- Low temperatures slow down growth and
preserve strains
- Refrigeration temperatures (4-8o C) are
used for food preservation
- For long-term storage of cultures
- Placing solutions in glycerol at -70o C
- Lyophilization or freeze-drying
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Physical Agents
Filtration
- Micropore filters with pore sizes of 0.2 mm
can remove microbial cells, but not viruses,
from solutions
Figure 5.27
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Air can also be sterilized by filtration
Laminar flow biological safety cabinets
force air through HEPA filters
Figure 5.28
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Physical Agents
Irradiation
- Ultraviolet light
- Has poor penetrating power
- Used only for surface sterilization
- Gamma rays, electron beams and X-rays
- Have high penetrating power
- Used to irradiate foods and other heatsensitive items
44
Chemical Agents
A number of factors influence the efficacy of a
given chemical agent, including:
- The presence of organic matter
- The kinds of organisms present
- Corrosiveness
- Stability, odor, and surface tension
45
The Phenol Coefficient
The phenol coefficient test compares the
effectiveness of disinfectants
Table 5.3
46
Commercial Disinfectants and
Antiseptics
These include:
- Ethanol
- Iodine (Wescodyne and Betadine)
- Chlorine
- All of the above damage proteins, lipids,
and DNA
- Are used to reduce or eliminate
microbial content from objects
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Figure 5.30
48
Antibiotics
Antibiotics are chemical compounds synthesized
by one microbe that kill or inhibit the growth of
other microbial species
Penicillin mimics part of the bacterial cell wall
- Prevents cell wall formation and is bactericidal
Figure 5.31
49
Effect of ampicillin (a penicillin derivative) on
E. coli
Figure 5.32
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Biological Control of Microbes
Biocontrol is the use of one microbe to control
the growth of another
- Probiotics contain certain microbes that,
when ingested, aim to restore balance to
intestinal flora
- Lactobacillus and Bifidobacterium
- Phage therapy aims to treat infectious
diseases with a virus targeted to the pathogen
- A possible alternative to antibiotics in the
face of rising antibiotic resistance
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Chapter Summary
Global analysis of genes and proteins allow us to
study how microbes react to environmental changes
● Microbes are classified by growth temperature:
- Psychrophiles, mesophiles, and thermophiles
● Barophiles can grow at very high pressures
● Halophiles require high salt concentrations
● Microbes are classified by pH range:
- Acidophiles, neutralophiles, and alkaliphiles
● Microbes are classified by their O2 requirements:
- Aerobes, facultative, microaerophiles, and
anaerobes
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●
Chapter Summary
Cells treated with antimicrobials die at a logarithmic
rate
● Physical agents used to control microbes include:
- Autoclaving, Pasteurization, refrigeration, filtration,
and irradiation
● Chemical agents used to control microbes include:
- Antiseptics and disinfectants
● Antibiotics selectively control microbial growth
● Biological control of microbes include the use of
probiotics and phage therapy
●
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Concept Quiz
Microbes that grow at temperatures between
40°C and 80°C are called:
a) psychrophiles.
b) mesophiles.
c) thermophiles.
d) extreme thermophiles.
54
Concept Quiz
Bacteria cannot grow in solutions with very
high concentrations of sugar because
a) bacteria cannot digest pure sugar.
b) sugar raises the solution’s osmolarity.
c) sugar lowers the solution’s osmolarity
d) sugar raises the solution’s pH.
e) sugar lowers the solution’s pH.
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Concept Quiz
Physical agents used to prevent bacterial
growth include:
a) pasteurization, freezing, phages.
b) irradiation, probiotics, filtration.
c) autoclaving, irradiation, freezing.
d) antibiotics, refrigeration, pasteurization.
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Concept Quiz
The ______ coefficient test is used to
compare disinfectants.
a) ethanol
b) iodine
c) phenol
d) chlorox
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Concept Quiz
Microbes that grow at very high pressures
are called:
a) osmophiles.
b) mesophiles.
c) barophiles.
d) halophiles.
58
Concept Quiz
The D-value refers to the length of time it
takes an agent to kill ___% of the microbial
population.
a) 50
b) 90
c) 99
d) 100
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