Most bacterial and archaeal cells reproduce by binary fission

1. What is microbial
growth?
Microbial Growth
1
Microbial growth refers to an increase
in number of cells rather than an
increase in cell size
Most bacterial and archaeal cells
reproduce by binary fission

2. Mechanism of binary fission
Simple type of cell division
The cell elongates as new cell envelope
material is synthesized, and subcellular
structures like ribosomes and inclusions are
abundant enough to be evenly distributed in
the cytoplasm.
Only the nucleoid is present as a single entity,
and its replication and partitioning into each
half of the elongated cell is a critical step in
binary fission
A septum (cross wall) is formed at
midcell, dividing the parent cell into two
progeny cells, each having its own nucleoid and
a complement of other cellular constituents

3. Other reproductive strategies in
bacteria
Some bacteria reproduce by forming a bud.
Certain cyanobacteria undergo multiple fission.
The progeny cells, called baeocytes, are held within the cell wall
of the parent cell until they mature.
Other bacteria, such as members of the genus Streptomyces,
form multinucleoid filaments that eventually divide to form
uninucleoid spores.
These spores are readily dispersed, much like the dispersal
spores formed by filamentous fungi

4. Bacterial Cell Cycles
Divided into 3 phases.
The cell cycle is the complete sequence of events extending
from formation of a new cell through the next division
a fundamental biological process
understanding the cell cycle has practical importance
Eg: Synthesis of peptidoglycan during the cell cycle
is the target of numerous antibiotics used to treat bacterial infections.
4

5. Bacterial cell cycle
Extensively studied cell cycles—Escherichia coli, Ba-
cillus subtilis, and the aquatic bacterium Caulobacter crescentus
The bacterial cell cycle consists of three phases:
(1) a period of growth after the cell is born, which is similar
to the G1 phase of the eukaryotic cell cycle;
(2) Chromosome replication and partitioning period, which
functionally corresponds to the S and mitosis events of the M phase of
the eukaryotic cycle;
(3) cytokinesis, during which a septumand daughter cells are formed
5

6. Eukaryotic cell cycle
and
bacterial cell cycle
◉ In the eukaryotic cell cycle, the S phase is separated from the M
phase by another period called G2.
◉ In G2, chromosome replication is completed and some time passes before
chromosome segregation occurs.
◉ This is not the case for bacteria. Chromosome replication and partitioning occur
concurrently.
◉ Furthermore, the initial events of cytokinesis actually occur before chromo
some replication and partitioning are complete.
◉ Finally, some bacteria are able to initiate new rounds of replication before
the first round of replication and cytokinesis is finished
6

7. 7
Cell cycle of E coli
Chromosome Replication
and
Partitioning
Most bacteria have a single circular
chromosome.
Each circular chromosome has a
single site at which replication starts
called the origin of replication, or
simply the origin

8. Chromosome Replication
and Partitioning
DNA synthesizing machinery is called the replisome,
DNA replication proceeds in both directions from the origin.
As the progeny chromosome is synthesized, the two origins move
toward opposite ends of the cell, and the rest of each chromosome
follows in an orderly fashion.
The process of DNA synthesis and movement
seems rather similar to most bacteria but the mechanism by which
chromosomes are partitioned to each daughter cell
varies some what among bacterial species
8

9. Septation is the process of forming a cross wall between two daughter cells.
Cytokinesis, a term that was once used to describe the formation of two eukaryotic daughter cells,
is now used to describe this process in all cells.
In bacteria and archaea, septation is divided into several steps:
(1) selection of the site where the septum will be formed;
(2) assembly of the Z ring, which is composed of the cytoskeletal protein FtsZ;
(3) assembly of the cell wall–synthesizing machinery (i.e.,
for synthesis of peptidoglycan and other cell wall constituents);
(4) constriction of the cell and septum formation
Cytokinesis
9

10. Cellular Growth and Determination of Cell Shape
Correct septation requires that the Z ring form at the proper
place at the proper time.
A mechanism called nucleoid occlusion
helps ensure that the Z ring forms only after most of the daughter
chromosomes have separated from each other.
This is important because the septum might otherwise guillotine the chromosome
Division varies according to species of bacteria
2
Bacterial and archaeal cells have species specific defined shapes.
These shapes are neither accidental nor random,
Some microbes change their shape under certain circumstances.
For instance, Sinorhizobium meliloti : from rods to Yshaped cells when living symbiotically with plants.
Helicobacter pylori, the causative agent of gastric ulcers and stomach cancer, changes from its
characteristic helical shape to a sphere in stomach infections and in prolonged culture

11. Growth Curves Consist of
Five Phases
11
Long-Term
Stationary Phase
Death phase
Exponential/log phase
Lag Phase
Stationary Phase

12. Growth curve
◉ Population growth is often studied by analyzing the growth
of microbes in liquid (broth) culture.
◉ When microorganisms are cultivated in broth, they usually are grown in a batch culture
Batch culture: incubated in a closed culture vessel like a test tube or a flask with a single batch
of medium.
◉ Fresh medium is not provided during incubation
◉ so as nutrients are consumed, their concentrations decline, and wastes accumulate.
◉ Population growth of microbes reproducing by binary fission in a batch culture can be plotted as
the logarithm of the number of viable cells versus the incubation time.
◉ The resulting curve has five distinct phases

13. Lag Phase
◉ When microorganisms are introduced into fresh culture medium, usually no
immediate increase in cell number occurs
◉ It is not a time of inactivity
◉ Cells are synthesizing new components.
◉ Reasons: The cells may be old and depleted of ATP, essential cofactors, and
ribosomes; these must be synthesized before growth can begin.
◉ The medium may be different from the one the microorganism was growing in
previously.
◉ In this case, new enzymes are needed to use different nutrients.
◉ Possibly the microorganisms have been injured and require time to recover.
◉ cells begin to replicate their DNA, increase in mass, and divide.

14. Exponential Phase
◉ Exponential phase: microorganisms grow and divide at the maximal rate possible.
◉ Factors: their genetic potential, the nature of the medium, nutrient availability. and the
environmental conditions.
◉ The rate of growth is constant during the exponential phase;
◉ The population is most uniform in terms of chemical and physiological properties
during this phase; therefore exponential phase cultures are usually used in biochemical and
physiological studies.
◉ When microbial growth is limited by the low concentration of a required nutrient, the final
net growth or yield of cells increases with the initial amount of the limiting nutrient present

15. The rate of growth also increases with nutrient concentration but it saturates,
much like what is seen with many enzymes
The shape of the curve is thought to reflect the rate of nutrient uptake by
microbial transport proteins.
At sufficiently high nutrient levels, the transport systems are saturated, and
the growth rate does not rise further with increasing nutrient concentration

16. Mathematics of Growth
◉ During the exponential phase, each microorganism is dividing at constant
intervals.
◉ Thus the population doubles in number during a specific length of time
called the generation (doubling) time .
◉ Suppose that a culture tube is inoculated with one cell that divides every 20
minutes
◉ The population will be 2 cells after 20 minutes, 4 cells after 40 minutes, and
so forth.
◉ Because the population is doubling every generation, the increase in
population is always 2 n where n is the number of generations.
◉ The resulting population increase is exponential

17. ◉ The mathematics of growth during the exponential phase: calculation of
two important values.
◉ The growth rate constant (k) : the number of generations per unit time
and is often expressed as generations per hour (hr−1).
◉ It can be used to calculate the generation time.
◉ The generation time is simply the reciprocal of the growth rate constant.
The generation time can also be determined directly from a semilogarithmic
plot of growth curve data
◉ Once this is done, it can be used to calculate the growth rate constant

18. Generation times vary markedly with the microbial species and environmental conditions.
They range from less than 10 minutes (0.17 hours) to several days
Generation times in nature are usually much longer than in laboratory culture

19. Stationary Phase
In a closed system such as a batch culture, population growth eventually ceases and the growth curve
becomes horizontal
This stationary phase is attained by some bacteria at a population level of around 109 cells per milliliter.
Final population size depends on nutrient availability and other factors, as well as the type of
microorganism.
In stationary phase, the total number of viable microorganisms remains constant.
This may result from a balance between cell division and cell death, or the
population may simply cease to divide but remain metabolically active.

20. Reasons for Stationary
phase
One important reason is nutrient limitation; if an essential
nutrient is severely depleted, population growth will slow and
eventually stop.
Aerobic organisms often are limited by O 2 availability.
Oxygen is not very soluble and may be depleted so quickly that only the surface of a
culture will have an O 2 concentration adequate for growth.
Population growth also may cease due to the accumulation of toxic waste products.
For example, streptococci can produce so much acid from sugar fermentation that
growth is inhibited
critical population level is reached

21. Death Phase
Cells growing in batch culture cannot remain in stationary phase indefinitely.
Eventually they enter a phase known as the death phase
During this phase, the number of viable cells declines exponentially, with
cells dying at a constant rate.
Detrimental environmental changes such as nutrient deprivation and the
buildup of toxic wastes cause irreparable harm to the cells

22. Long-Term
Stationary Phase
After a period of exponential death some microbes have a long period where the
population size remains more or less constant.
This long-term stationary phase (also called extended stationary
phase) can last months to years
During this time, the bacterial population continually evolves so that
actively reproducing cells are those best able to use the nutrients released by their dying
brethren and best able to tolerate the accumulated toxins.
This dynamic process is marked by successive waves of genetically distinct variants.
Thus natural selection can be witnessed within a single culture vessel

23. Environmental
Factors Affect
Microbial Growth
Yellow stone
National Park

24. Extreme environments
◉ Rich microbial community lives in these same springs, as well as in the
hot pots, fumaroles, and other thermal features of the park.
◉ Clearly, the adaptations of some microorganisms to what humans
perceive as inhospitable, extreme environments are truly remarkable.
Indeed, microbes are thought to be present nearly everywhere on
Earth
◉ Extremophiles: Microorganisms that grow in such harsh conditions

25. Optimum Growth
◉ If the conditions exceed their ability to respond, they will not grow and eventually they
may die.
◉ Thus for each environmental parameter, all microbes have a characteristic range at which
growth occurs defined by high and low values beyond which the microbe cannot survive.
◉ Within the range is an optimal value at which growth is best.
◉ To study the ecological distribution of microbes,
◉ it is important to understand the strategies they use to survive.
◉ An understanding of the environmental influences on microbes and their activity also aids
in the control of microbial growth

27. Temperature
◉ Microorganisms :susceptible to external temperatures
◉ They cannot regulate their internal temperature.
◉ The effect of temperature on growth: Temperature sensitivity of enzyme catalyzed reactions.
Each enzyme has an optimum temperature
◉ Beyond a certain point, further temperature increases actually slow growth
◉ sufficiently high temperatures :lethal.
◉ High temperatures: denature enzymes, inactivates transport system
◉ Inactivates other proteins.
◉ Temperature has significant effect on microbial membranes.
◉ At very low temperatures, membranes solidify.
◉ At high temperatures, the lipid bilayer simply melts and disintegrates.
◉ Organisms above or below their optimum temperature: cell function and structure are affected

28. Cardinal temperatures
◉ —minimum, optimum, and maximum growth temperatures
◉ The temperature optimum is always closer to the maximum than to the
minimum.
◉ The cardinal temperatures are not rigidly fixed.
◉ They depend to some extent on other environmental factors such as pH and
available nutrients, water.
Ex: Crithidia fasciculata, a flagellated protist living in the gut of mosquitoes,
grows in a simple medium at 22° to 27°C.
If the medium is supplemented with extra metals, amino acids, vitamins, and
lipids, it can grow at 33° to 34°C

30. ◉ Microbes that grow in cold environments are either
psychrotolerants or psychrophiles.
◉ Psychrotolerants (sometimes called psychrotrophs) grow at 0°C
or higher and typically have maxima at about 35°C.
◉ Psychrophiles (sometimes called cryophiles) grow well at 0°C
and have an optimum growth temperature of 15°C; the maximum
is around 20°C.
◉ They are readily isolated from Arctic and Antarctic habitats.
◉ Oceans constitute an enormous habitat for psychrophiles because
90% of ocean water is 5°C or colder.
◉ Psychrophiles are widespread among bacterial taxa and are found
in such genera as Pseudomonas, Vibrio, Alcaligenes, Bacillus,
Photobacterium, and Shewanella
The psychrophilic protist Chlamydomonas nivalis can actually
turn a snowfield or glacier pink with its bright red spores (a
phenomenon called “watermelon snow”).

31. Adaptations of
psychrophiles
◉ Enzymes, transport systems, and protein synthetic machinery function well at low
temperatures.
◉ The membranes of psychrophilic microorganisms have high levels of unsaturated
fatty acids and remain semifluid when cold.
◉ Many psychrophiles begin to leak cellular constituents at temperatures higher than
20°C because of membrane disruption.
◉ Many psychrophiles accumulate compatible solutes.
◉ Rather than protecting against osmotic stress, in this case the compatible solutes
decrease the freezing point of the cytosol.
◉ Still other psychrophiles use antifreeze proteins to decrease the freezing point of the
cytosol.
◉ Psychrophilic bacteria and fungi are major causes of refrigerated food spoilage.

32. Mesophiles
◉ Mesophiles are microorganisms that grow in
moderate temperatures.
◉ They have growth optima around 20° to 45°C and
often have a temperature minimum of 15° to
20°C and a maximum of about 45°C.
◉ Almost all human pathogens are mesophiles, as
might be expected because the human body is a
fairly constant 37°C.

33. Thermophiles
and
Hyperthermophiles
◉ Microbes that grow best at high temperatures are thermophiles and
hyperthermophiles.
◉ Thermophiles grow at temperatures between 45° and 85°C, and they often have optima
between 55° and 65°C.
◉ The vast majority are members of Bacteria or Archaea, although a few photosynthetic
protists and fungi are thermophilic.
◉ Thermophiles flourish in many habitats including composts, self-heating hay stacks, hot
water lines, and hot springs.
◉ Hyperthermophiles have growth optima between 85° and 113°C.
◉ They usually do not grow below 55°C.
◉ Pyrococcus abyssi and Pyrodictium occultum are examples of marine
hyperthermophiles found in hot areas of the seafloor.

34. Adaptations
◉ They have heat stable enzymes and protein synthesis systems that function at high temperatures.
◉ Heat stable proteins have highly organized hydrophobic interiors and many hydrogen and other
noncovalent bonds to stabilize their structure.
◉ Large quantities of amino acids such as proline make polypeptide chains less flexible and more heat stable.
◉ The proteins are stabilized and aided in folding by proteins called chaperones.
◉ Nucleoid associated proteins appear to stabilize the DNA of thermophilic bacteria
◉ Hyperthermophiles have an enzyme called reverse DNA gyrase that changes the topology of their DNA and
enhances its stability.
◉ The membrane lipids of thermophiles and hyperthermophiles are also quite temperature stable.
◉ They tend to be more saturated, more branched, and of higher molecular weight.
◉ This increases the melting points of membrane lipids.
◉ The archaea have membrane lipids with ether linkages.
◉ Such lipids are resistant to hydrolysis at high temperatures.
◉ The diglycerol tetraethers observed in the membranes of some archaeal thermophiles span the
membrane to form a rigid, stable monolayer

35. Oxygen Concentration
• The importance of oxygen to the
growth of an organism correlates with
the processes it uses to conserve
energy
• Five types of relationships to oxygen
• Completely dependent on atmospheric O2
for growth: obligate aerobes
• Microaerophiles are damaged by the
atmospheric level of O2 (20%) and require
O2 levels in the range of 2 to 10% for
growth.
• Facultative anaerobes do not require O2
for growth but grow better in its presence
• Aerotolerant anaerobes grow equally well
whether O2 is present or not; they can
tolerate O2, but they do not make use of it
• Obligate anaerobes: Need absence of
oxygen for growth O2 is toxic, and they are
usually
killed by prolonged exposure to O2
Oxygen Concentration

36. Thioglycollate broth
SOD: Super Oxide Dismutase

37. Why oxygen is toxic???
Oxic O 2 derivatives are formed when cellular proteins such as
flavoproteins transfer electrons to O 2.
These toxic O 2 derivatives are called reactive oxygen species
(ROS), and they can damage proteins, lipids, and nucleic acids.
ROS include the superoxide radical, hydrogen peroxide, and the
most dangerous hydroxyl radical.
O 2 + e− → O 2 −• (superoxide radical)
O 2 −• + e− + 2H+ → H 2 O 2 (hydrogen peroxide)
H 2 O 2 + e− + H+ → H 2 O + OH• (hydroxyl radical)

38. A microorganism must be able to protect
itself against ROS or it will be killed.
Many microorganisms possess enzymes that protect against
toxic O2 products
Obligate aerobes and facultative anaerobes usually contain the enzymes
superoxide dismutase (SOD): Destruction of super oxide radicals
Catalase: catalyze the destruction of hydrogen peroxide
Peroxidase: also can be used to destroy hydrogen peroxide.
2O2−• + 2H+ O2 + H2O2
2H2O2 2H2O + O2
H2O2 + NADH + H+ 2H2O + NAD+
Strict anaerobes lack these enzymes or have them in very low
concentrations and therefore cannot tolerate O2.

39. Pressure
◉ Organisms that spend their lives on land or the surface of water:
subjected to a pressure of 1 atmosphere (atm; 1 atm is ~0.1
megapascal, or MPa for short) and are never affected significantly
by pressure.
◉ Many bacteria and archaea, live in the deep sea (ocean depths of
1,000
m or more):hydrostatic pressure can reach 600 to 1,100 atm and the
temperature is about 2° to 3°C.
◉ These high hydrostatic pressures affect membrane fluidity and
membrane
associated function

40. ◉ barotolerant—increased pressure adversely affects them but not
as much as it does nontolerant microbes.
◉ Piezophilic (barophilic): An organism that has a maximal growth
rate at pressures greater than 1 atm.
For instance, a piezophile recovered from the Mariana trench near
the Philippines (depth about 10,500 m) grows only at pressures
between about 400 to 500 atm at 2°C

41. Adaptation observed in
piezophiles
◉ Change their membrane lipids in response to increasing pressure.
For instance, bacterial piezophiles increase the amount of unsaturated
fatty acids in their membrane lipids as pressure increases.
◉ Shorten the length of their fatty acids.
◉ Piezophiles play an important roles in nutrient cycling in the deep sea.
◉ Bacterial genera :e.g., Photobacterium, Shewanella, Colwellia

42. Radiation
◉ Radiation similar to waves like those traveling on the surface of
water.
◉ The distance between two wave crests or troughs is the
wavelength.
◉ As the wavelength of electromagnetic radiation decreases,
the energy of the radiation increases
◉ gamma rays and X rays are much more energetic than visible
light or infrared waves.
◉ Electromagnetic radiation also acts like a stream of energy
packets called photons, each photon having a quantum of
energy whose value depends on the wavelength of the radiation

43. Sunlight
◉ Sunlight is the major source of radiation on Earth.
◉ Includes visible light, ultraviolet (UV) radiation, infrared
rays, and radio waves.
◉ Visible light is a most conspicuous and important
aspect of our environment
◉ Life on Earth depends on the ability of photosynthetic
organisms to trap the energy
◉ The visible spectrum consists of the ROYGBV

44. ◉ Sunlight is evenly distributed across these wavelengths : appears
white.
◉ Visible light: not the predominant component of sunlight.
◉ Infrared rays : almost 60% of the sun’s radiation.
◉ Infrared radiation : major source of Earth’s heat.
◉ 3% of the light reaching Earth’s surface is UV radiation
◉ There are three major types of UV radiation: UVA, UVB, and UVC,
which range from longest (UVA) to shortest (UVC) wavelengths.
◉ At sea level, there is very little UV radiation at wavelengths below
about 290 nm (UVC and some UVB)
◉ Wavelengths shorter than 290 nm are absorbed by O2 in Earth’s
atmosphere: this process forms a layer of ozone (O3) between 40
and 48 km above Earth’s surface.
◉ The ozone layer absorbs somewhat longer UV-rays and reforms O 2.
◉ Thus the major form of UV radiation reaching Earth’s surface is
UVA radiation

45. Radiation and
microorganisms
◉ Electromagnetic radiation : very harmful to microorganisms.
◉ Most damaging: ionizing radiation (radiation of very short wavelength
and high energy): causes atoms to lose electrons (ionize).
◉ Two major forms of ionizing radiation are X rays (artificially produced) &
gamma rays : emitted during natural radioisotope decay.
◉ Low levels of ionizing radiation : produce mutations that indirectly
result in death and higher levels are directly lethal.
◉ Ionizing radiation causes a variety of changes in cells.
◉ It breaks hydrogen bonds, destroys ring structures, and polymerizes
some molecules but the most severe effect is protein oxidation

46. Deinococcus radiodurans
◉ microorganisms are more resistant to ionizing radiation than larger
organisms
◉ They are still destroyed by a sufficiently large dose.
◉ Ionizing radiation can be used to sterilize items.
◉ Bacterial endospores and some bacteria such as Deinococcus radiodurans
are extremely resistant to large doses of ionizing radiation.
◉ D. radiodurans : microbe is able to piece together its genome after it is
blasted apart by massive doses of radiation.
◉ Mn+2 ions exerta protective role, when they are complexed with small
metabolites.
◉ Radiation resistant bacteria

47. Ultraviolet (UV) radiation
◉ Very damaging form of radiation.
◉ It can kill microorganisms due to its short wavelength (approximately from 10 to 400
nm) and high energy.
◉ Most lethal UV radiation: wavelength of 260 nm, the wavelength most effectively
absorbed by and damaging to DNA.
◉ The damage caused by UV light can be repaired by several DNA repair mechanisms:
Photoreactivation using the enzyme photolyase
◉ Excessive exposure to UV light outstrips the organism’s ability to repair the damage and
death results.
◉ Longer wavelengths of UV light (near UV radiation; 325 to 400 nm) can also harm
microorganisms because they induce the breakdown of the amino acid tryptophan to
toxic photoproducts.
◉ These toxic photoproducts plus the near UV radiation itself produce breaks in DNA
strands

48. Visible light
◉ Even visible light, when present in sufficient intensity, can
damage or kill microbial cells.
◉ Usually pigments called photosensitizers and O2 are involved.
◉ Photosensitizers include pigments such as chlorophyll, bacteriochlorophyll, cytochromes, and
flavins, which can absorb light energy and become excited or activated.
◉ The excited photosensitizer (P) transfers its energy to O2, generating singlet oxygen (1O2).
◉ P P(activated)
P(activated) + O2 P + 1O2

49. ◉ Singlet oxygen is a very reactive, powerful oxidizing agent that
quickly destroys a cell.
◉ Many microorganisms that are airborne or live on exposed surfaces
use carotenoid pigments for protection against photooxidation.
◉ Carotenoids effectively quench singlet oxygen; that is, they absorb
energy from singlet oxygen and convert it back into the unexcited
ground state. Both phototrophic and nonphototrophic
microorganisms employ pigments in this way.

50. pH
◉ pH is a measure of the relative acidity of a solution
◉ Definition: as the negative logarithm of the hydrogen ion concentration (ex-
pressed in terms of molarity).
pH = − log [H+] = log(1/[H+])
The pH scale extends from pH 0.0 (1.0 M H+) to pH 14.0 (1.0 ×
10−14 M H +), and each pH unit represents a 10 fold change
in hydrogen ion concentration.
◉ Microbial habitats vary widely in pH—from pH 0 to 2 at
the acidic end to alkaline lakes and soil with pH values between 9 and 10

51. ◉ Each species has a definite pH growth range and pH growth
optimum.
◉ Acidophiles: growth optimum between pH 0 and 5.5
◉ Neutrophiles: between pH 5.5 and 8.0
◉ Alkaliphiles (alkalophiles): between pH 8.0 and 11.5.

52. ◉ Many archaea are acidophiles.
◉ For example, the archaeon Sulfolobus acidocaldarius is
a common inhabitant of acidic hot springs
◉ it grows well from pH 1 to 3 and at high temperatures.
◉ The archaea Ferroplasma acidarmanus and Picrophilus
oshimae can actually grow very close to pH 0.
◉ Alkaliphiles are distributed among all three domains of life.
◉ They include bacteria belonging to the genera Bacillus,
Micrococcus, Pseudomonas, and Streptomyces; yeasts
◉ Marine bacteria: alkalophiles due to alkaline condition

54. Tolerance mechanism
◉ When the external pH is low, the concentration of H+ is much
greater outside than inside, and H+ will move into the cytoplasm
and lower the cytoplasmic pH.
◉ Drastic variations in cytoplasmic pH can harm microorganisms by
disrupting the plasma membrane or inhibiting the activity of enzymes and
membrane transport proteins.
◉ Most microbes die if the internal pH drops much below 5.0 to 5.5.
◉ Changes in the external pH also can alter the ionization of nutrient molecules
and thus reduce their availability to the organism.
◉ Microorganisms respond to external pH changes using mechanisms that maintain
a neutral cytoplasmic pH.
◉ Several responses to small changes in external pH have been identified.
◉ Neutrophiles appear to exchange potassium for protons using an antiport system

55. ◉ Extreme alkaliphiles such as Bacillus alcalophilus
maintain their internal pH close to neutrality by
exchanging internal sodium ions for external protons.
◉ Acidophiles use a variety of measures to maintain a neutral
internal pH.
◉ These include the transport of cations (e.g., potassium
ions) into the cell, thus decreasing the movement of H+
into the cell; proton transporters that pump H+ out if
they get in; and highly impermeable cell membranes

56. Solutes Affect Osmosis and
Water Activity
◉ Water is critical to the survival of all organisms
◉ water can also be destructive.
◉ Solutes in an aqueous solution alter the behavior of water.
◉ Osmosis, which is observed when two solutions are separated by a
semipermeable membrane that allows movement of water but not solutes.
◉ If the solute concentration of one solution is higher than the other, water moves to equa
the concentrations.
◉ Water moves from solutions with lower solute concentrations to those with higher solute
concentrations.
◉ Because a selectively permeable plasma membrane separates a cell’s cytoplasm
from its environment, microbes can be affected by changes in the solute concentration of
their surroundings.

57. If a microorganism is placed in a hypotonic solution (one with a lower solute
concentration; solute concentration is also referred to as osmotic concentration or
osmolarity), water will enter the cell and cause it to burst unless something prevents
the influx of water or inhibits plasma membrane expansion
If the microbe is placed in a hypertonic solution (one with a higher osmotic con
centration), water will flow out of the cell.
In microbes that have cell walls, the membrane shrinks away from the cell wall.
Dehydration of the cell in hypertonic environments: damage the plasma
membrane and cause the cell to become metabolically inactive.
Uncontrolled osmosis: Potential damaging effects it is important that microbes be
able to respond to changes in the solute concentrations of their environment.
Microbes in hypotonic environments are protected in part by their cell wall,
which prevents overexpansion of the plasma membrane.
Wall-less microbes can be protected by reducing the osmotic concentration of their
cytoplasm
This protective measure is also used by many walled microbes to provide
protection in addition to their cell walls.

58. mechanosensitive (MS)
channels
◉ Microbes use several mechanisms to lower the solute concentration of
their cytoplasm.
◉ For example, some bacteria have mechanosensitive (MS) channels in
their plasma membrane.
◉ In a hypotonic environment, the membrane stretches due to an increase
in hydrostatic pressure and cellular swelling.
◉ MS channels then open and allow solutes to leave.
◉ Thus MS channels act as escape valves to protect cells from bursting.
◉ Many protists use contractile vacuoles to expel excess water

59. Some microbes are
adapted to extreme
hypertonic
environments
◉ Osmophiles.
◉ Definition: osmophiles include halophiles, which require the presence of
NaCl at a concentration above about 0.2 M
◉ Osmophile: organisms that require high concentrations of sugars.
◉ Halophiles: Extreme halophiles have adapted so completely to hypertonic,
saline conditions that they require NaCl concentrations between about 3 M
and saturation (about 6.2 M).
◉ Archaeal halophiles can be isolated from the Dead Sea (a salt lake between
Israel and Jordan), the Great Salt Lake in Utah, and other aquatic habitats
with salt concentrations approaching saturation

60. Halophiles :able to live in their high-salt habitats
They synthesize or obtain some molecules from their environment: compatible solutes.
Compatible solutes: kept at high intracellular concentrations without interfering with
metabolism and growth.
Compatible solutes : Two types
Inorganic molecules & organic molecules
Inorganic molecules :potassium chloride (KCl).
Organic molecules : choline, betaines (neutral molecules having both negatively charged
and positively charged functional groups)
and amino acids such as proline and glutamic acid.
Use of compatible solutes : also a strategy utilized by many osmophiles.
Many micro organisms, whether in hypotonic or hypertonic environments, use compatible
solutes to keep the osmotic concentration of their cytoplasm somewhat above that of the
habitat so that the plasma membrane is always pressed firmly against the cell wall.
For instance, fungi and photosynthetic protists employ the compatible solutes sucrose and
polyols (e.g., arabitol, glycerol, and mannitol) for this purpose.

61. best studied halophiles:
Halobacteriales.
◉ Accumulate potassium and chloride ions to remain hypertonic to their environment; the
internal potassium concentration may reach 4 to 7 M
◉ Their nzymes, ribosomes, and transport proteins are unusual because they require high
potassium levels for stability and activity.
◉ In addition, their plasma membranes and cell walls are stabilized by high concentrations
of sodium ion.
◉ If the sodium concentration decreases too much, the wall and plasma membrane
disintegrate.
◉ Although extreme halophiles have successfully adapted to environmental conditions
that would destroy most organisms, they have become so specialized that they lack
ecological flexibility.