![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](https://image.slidesharecdn.com/microbialgrowth-240108090944-65ce6fbd/85/Microbial-growth-refers-to-an-increase-in-number-of-cells-rather-than-an-increase-in-cell-size-50-320.jpg)
Microbial growth refers to an increase in number of cells rather than an increase in cell size
- 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.