《微生物学英文教学课件：chapter6 Microbial Growth》由会员分享，可在线阅读，更多相关《微生物学英文教学课件：chapter6 Microbial Growth（158页珍藏版）》请在金锄头文库上搜索。
1、C H A P T E R 6C H A P T E R 6Microbial GrowthMicrobial GrowthMicrobialgrowthTheGrowthCurveCelllifecycleMeasurementofMicrobialGrowthMeasurementofCellMassGrowthYieldsandtheEffectsofaLimitingNutrientTheContinuousCultureofMicroorganismsTheChemostatTheInfluenceofEnvironmentalFactorsonGrowthOutline1.Grow
2、thisdefinedasanincreaseincellularconstituentsandmayresultinanincreaseinanorganismssize,populationnumber,orboth.2.Whenmicroorganismsaregrowninaclosedsystem,populationgrowthremainsexponentialforonlyafewgenerationsandthenentersastationaryphaseduetofactorslikenutrientlimitationandwasteaccumulation.Ifapo
3、pulationisculturedinanopensystemwithcontinualnutrientadditionandwasteremoval,theexponentialphasecanbemaintainedforlongperiods.Concepts3.Awidevarietyoftechniquescanbeusedtostudymicrobialgrowthbyfollowingchangesinthetotalcellnumber,thepopulationofviablemicroorganisms,orthecellmass.4.Wateravailability,
4、pH,temperature,oxygenconcentration,pressure,radiation,andanumberofotherenvironmentalfactorsinfluencemicrobialgrowth.Yetmanymicroorganisms,andparticularlybacteria,havemanagedtoadaptandflourishunderenvironmentalextremesthatwoulddestroymostorganisms.Growthmaybegenerallydefinedasasteadyincreaseinallofth
5、echemicalcomponentsofanorganism.Growthusuallyresultsinanincreaseinthesizeofacellandfrequentlyresultsincelldivision.Growthdefinition:G1Primarygrowthphaseofthecellduringwhichcellenlargementoccurs,agapphaseseparatingcellgrowthfromreplicationofthegenomeSphaseinwhichreplicationofthegenomeoccursG2Phaseinw
6、hichthecellpreparesforseparationofthereplicatedgenomes,thisphaseincludessynthesisofmicrotubulesandcondensationofDNAtoformcoherentchromosomes,agapphaseseparatingchromosomereplicationfrommiosis.Mphasecalledmiosisduringwhichthemicrotubularapparatusisassociatedandsubsequentlyusedtopullapartthesisterchro
7、mosomes.CelllifecycleinEukaryoticcellsG1SG2MG1RDEukaryoticcell:Prokaryoticcell:Mostbacterialcellsreproduceasexuallybybinaryfission,aprocessinwhichacelldividestoproducetwonearlyequal-sizedprogenycells.Binaryfissioninvolvesthreeprocesses:Increaseincellsize(cellelongation),DNAreplicationCelldivisionBin
8、aryfission When microorganisms are cultivated in liquid medium, they usually are grown in a batch culturebatch culture or closed systemthat is, they are incubated in a closed culture vessel with a single batch of medium. 6.1 Growth Curve6.1 Growth Curve Because no fresh medium is provided during inc
9、ubation, nutrient concentrations decline and concentrations of wastes increase. The growth of microorganisms reproducing by binary fission can be plotted as the logarithm of the number of viable cells versus the incubation time. The resulting curve has four distinct phases.Growthcurveofbacteria1.Lag
10、Phase2.ExponentialPhase3.StationaryPhase4.DeathPhase(a)Lagphase:cellsbegintosynthesizeinducibleenzymesandusestoredfoodreserves.(b)Logarithmicgrowthphase:therateofmultiplicationisconstant.(c)Stationaryphase:deathrateisequaltorateofincrease.(d)Deathphase:cellsbegintodieatamorerapidratethanthatofreprod
11、uction.LagPhaseStationaryPhaseDeathPhaseLogarithmicgrowthphase When microorganisms are introduced into fresh culture medium,usually no immediate increase in cell number occurs, and therefore this period is called the lag lag phasephase. . Although cell division does not take place right away and the
12、re is no net increase in mass, the cell is synthesizing new components. 1.Lag Phase1.Lag Phase A lag phase prior to the start of cell division can be necessary for a variety of reasons. The cells may be old and depleted of ATP, essential cofactors,and ribosomes; these must be synthesized before grow
13、th can begin. The medium may be different from the one the microorganism was growing in previously. Here new enzymes would be needed to use different nutrients. Possibly the microorganisms have been injured and require time to recover. Whatever the causes, eventually the cells retool, replicate thei
14、r DNA, begin to increase in mass, and finally divide. The lag phase varies considerably in length with the condition of the microorganisms and the nature of the medium. This phase may be quite long if the inoculum is from an old culture or one that has been refrigerated. Inoculation of a culture int
15、o a chemically different medium also results in a longer lag phase. On the other hand, when a young, vigorously growing exponentialphase culture is transferred to fresh medium of the same composition,the lag phase will be short or absent. During the exponentialexponential or log phaselog phase, , mi
16、croorganisms are growing and dividing at the maximal rate possible given their genetic potential, the nature of the medium, and the conditions under which they are growing. Their rate of growth is constant during the exponential phase; that is, the microorganisms are dividing and doubling in number
17、at regular intervals.2.Exponential Phase2.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.Thetimerequiredforacelltodivide(anditspopulat
18、iontodouble)iscalledthegenerationtime.Supposethatabacterialpopulationincreasesfrom103cellsto109cellsin10hours.Calculatethegenerationtime.NumberofcellsTimeNt=Nox2nNo=numberofbacteriaatbeginningoftimeinterval.Nt=numberofbacteriaatendofanyintervaloftime(t).G=generationtimeT=time,usuallyexpressedinminut
19、esn=numberofgenerationG=tlog2/logNtlogNo Generation times vary markedly with the species of microorganism and environmental conditions. They range from less than 10 minutes (0.17 hours) for a few bacteria to several days with some eucaryotic microorganisms. Generation times in nature are usually muc
20、h longer than in culture. Eventually population growth ceases and the growth curve becomes horizontal. This stationary phasestationary phase usually is attained by bacteria at a population level of around 109 cells per ml. Other microorganisms normally do not reach such high population densities, pr
21、otozoan and algal cultures often having maximum concentrations of about 106 cells per ml. 3.Stationary Phase3.Stationary Phase Of course final population size depends on nutrient availability and other factors, as well as the type of microorganism being cultured. In the stationary phase the total nu
22、mber 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 though remaining metabolically active. Microbial populations enter the stationary phase for several reasons. One obvious factor is nutrien
23、t limitation; if an essential nutrient is severely depleted, population growth will slow. Aerobic organisms often are limited by O2 availability. Oxygen is not very soluble and may be depleted so quickly that only the surface of a culture will have an O2 concentration adequate for growth. The cells
24、beneath the surface will not be able to grow unless the culture is shaken or aerated in another way. Population growth also may cease due to the accumulation of toxic waste products. This factor seems to limit the growth of many anaerobic cultures. For example, streptococci can produce so much lacti
25、c acid and other organic acids from sugar fermentation that their medium becomes acidic and growth is inhibited. Detrimental environmental changes like nutrient deprivation and the buildup of toxic wastes lead to the decline in the number of viable cells characteristic of the death phasedeath phase.
26、 . The death of a microbial population, like its growth during the exponential phase,is usually logarithmic (that is, a constant proportion of cells dies every hour). 4.Death Phase4.Death Phase This pattern in viable cell count holds even when the total cell number remains constant because the cells
27、 simply fail to lyse after dying. Often the only way of deciding whether a bacterial cell is viable is by incubating it in fresh medium; if it does not grow and reproduce, it is assumed to be dead. That is, death is defined to be the irreversible loss of the ability to reproduce. There are many ways
28、 to measure microbial growth to determine growth rates and generation times. Either population mass or number may be followed because growth leads to increases in both. No single technique is always best; the most appropriate approach will depend on the experimental situation.6.2 6.2 Measurement of
29、Microbial GrowthMeasurement of Microbial Growth The most obvious way to determine microbial numbers is through direct counting. Using a counting chamber is easy, inexpensive, and relatively quick; it also gives information about the size and morphology of microorganisms. Petroff-Hausser counting cha
30、mbers can be used for counting procaryotes; hemocytometers can be used for both procaryotes and eucaryotes. 1.Measurement of Cell Numbers1.Measurement of Cell Numbers Procaryotes are more easily counted in these chambers if they are stained, or when a phase-contrast or a fluorescence microscope is e
31、mployed. These specially designed slides have chambers of known depth with an etched grid on the chamber bottom.The number of microorganisms in a sample can be calculated by taking into account the chambers volume and any sample dilutions required. There are some disadvantages to the technique. The
32、microbial population must be fairly large for accuracy because such a small volume is sampled. It is also difficult to distinguish between living and dead cells in counting chambers without special techniques. Larger microorganisms such as protozoa, algae, and nonfilamentous yeasts can be directly c
33、ounted with electronic counters such as the Coulter Counter. The microbial suspension is forced through a small hole or orifice. An electrical current flows through the hole, and electrodes placed on both sides of the orifice measure its electrical resistance. Every time a microbial cell passes thro
34、ugh the orifice, electrical resistance increases(or the conductivity drops) and the cell is counted. The Coulter Counter gives accurate results with larger cells and is extensively used in hospital laboratories to count red and white blood cells. It is not as useful in counting bacteria because of i
35、nterference by small debris particles, the formation of filaments,and other problems. Coulter counter. (a) Coulter counter. (a) As cells pass through this device, they trigger an electronic sensor that tallies their numbers. A flow cytometer works on a similar principle, but in addition to counting,
36、 it can measure cell size and even differentiate between live and dead cells. When used in conjunction with fluorescent dyes and antibodies,it has been used to differentiate between gram-positive and gram-negative bacteria. It is being adapted for use as a rapid method to identify pathogens in patie
37、nt specimens and to compare bacterial species on the basis of genetic differences such as guanine and cytosine content. A variation on this machine,called a flow cytometer, can record the number, size, and types of cells by tagging them with fluorescent substances and passing them through a beam of
38、light. Shown here is a fluorescence signature of three species, which are differentiated on the basis of guanine and cytosine percentages. Counting chambers and electronic counters yield counts of all cells, whether alive or dead. There are also several viable counting techniques, procedures specifi
39、c for cells able to grow and reproduce. In most viable counting procedures, a diluted sample of bacteria or other microorganisms is dispersed over a solid agar surface. Each microorganism or group of microorganisms develops into a distinct colony. The original number of viable microorganisms in the
40、sample can be calculated from the number of colonies formed and the sample dilution. For example,if 1.0 ml of a 1106 dilution yielded 150 colonies, the original sample contained around 1.5108 cells per ml. Usually the count is made more accurate by use of a special colony counter. In this way the sp
41、read-plate and pour-plate techniques may be used to find the number of microorganisms in a sample. Plating techniques are simple, sensitive, and widely used for viable counts of bacteria and other microorganisms in samples of food, water, and soil. Several problems, however, can lead to inaccurate c
42、ounts. Low counts will result if clumps of cells are not broken up and the microorganisms well dispersed. Because it is not possible to be absolutely certain that each colony arose from an individual cell, the results are often expressed in terms of colony forming colony forming units (CFU)units (CF
43、U) rather than the number of microorganisms. The samples should yield between 30 and 300 colonies for best results. Of course the counts will also be low if the agar medium employed cannot support growth of all the viable microorganisms present. The hot agar used in the pour-plate technique may inju
44、re or kill sensitive cells; thus spread plates sometimes give higher counts than pour plates. Microbial numbers are frequently determined from counts of colonies growing on special membrane filters having pores small enough to trap bacteria. In the membrane filter technique,a sample is drawn through
45、 a special membrane membrane filterfilter. The filter is then placed on an agar medium or on a pad soaked with liquid media and incubated until each cell forms a separate colony. A colony count gives the number of microorganisms in the filtered sample, and special media can be used to select for spe
46、cific microorganisms. This technique is especially useful in analyzing aquatic samples. Microbiologists have developed several alternative ways of analyzing bacterial growth qualitatively and quantitatively. One of the simplest methods for estimating the size of a population is through turbidometry.
47、 2.Measurement of Cell Mass2.Measurement of Cell Mass This technique relies on the simple observation that a tube of clear nutrient solution loses its clarity and becomes cloudy, or turbidturbid, , as microbes grow in it. In general, the greater the turbidity,the larger the population size, which ca
48、n be measured by means of sensitive instruments. Increases in the total cell mass, as well as in cell numbers, accompany population growth. Therefore techniques for measuring changes in cell mass can be used in following growth. The most direct approach is the determination of microbial dry weight.
49、Cells growing in liquid medium are collected by centrifugation,washed, dried in an oven, and weighed. This is an especially useful technique for measuring the growth of fungi. It is time consuming,however, and not very sensitive. Because bacteria weigh so little, it may be necessary to centrifuge se
50、veral hundred milliliters of culture to collect a sufficient quantity. If the amount of a substance in each cell is constant, the total quantity of that cell constituent is directly related to the total microbial cell mass. For example, a sample of washed cells collected from a known volume of mediu
51、m can be analyzed for total protein or nitrogen. An increase in the microbial population will be reflected in higher total protein levels. Similarly, chlorophyll determinations can be used to measure algal populations, and the quantity of ATP can be used to estimate the amount of living microbial ma
52、ss. Up to this point the focus has been on closed systems called batch cultures in which nutrient supplies are not renewed nor wastes removed. Exponential growth lasts for only a few generations and soon the stationary phase is reached. 6.3 6.3 The Continuous Culture of The Continuous Culture of Mic
53、roorganismsMicroorganisms However, it is possible to grow microorganisms in an open system, a system with constant environmental conditions maintained through continual provision of nutrients and removal of wastes. These conditions are met in the laboratory by a continuous culture systemcontinuous c
54、ulture system. . A microbial population can be maintained in the exponential growth phase and at a constant biomass concentration for extended periods in a continuous culture system. Two major types of continuous culture systems commonly are used: (1) chemostats and (2) turbidostats. A chemostat che
55、mostat is constructed so that sterile medium is fed into the culture vessel at the same rate as the media containing microorganisms is removed. The culture medium for a chemostat possesses an essential nutrient (e.g., an amino acid) in limiting quantities. 1.The Chemostat1.The ChemostatChemostatused
56、forcontinuouscultures.Rateofgrowthcanbecontrolledeitherbycontrollingtherateatwhichnewmediumentersthegrowthchamberorbylimitingarequiredgrowthfactorinthemedium.Chemostat Because of the presence of a limiting nutrient, the growth rate is determined by the rate at which new medium is fed into the growth
57、 chamber, and the final cell density depends on the concentration of the limiting nutrient. The rate of nutrient exchange is expressed as the dilution rate (D), the rate at which medium flows through the culture vessel relative to the vessel volume, where f is the flow rate (ml/hr) and V is the vess
58、el volume (ml).D =f/V For example, if f is 30 ml/hr and V is 100 ml, the dilution rate is 0.30 hr-1. Both the microbial population level and the generation time are related to the dilution rate. The microbial population density remains unchanged over a wide range of dilution rates. The generation ti
59、me decreases (i.e., the growth rate rises) as the dilution rate increases. The limiting nutrient will be almost completely depleted under these balanced conditions. If the dilution rate rises too high, the microorganisms can actually be washed out of the culture vessel before reproducing because the
60、 dilution rate is greater than the maximum growth rate. The limiting nutrient concentration rises at higher dilution rates because fewer microorganisms are present to use it. At very low dilution rates, an increase in D causes a rise in both cell density and the growth rate. This is because of the e
61、ffect of nutrient concentration on the growth rate, sometimes called the Monod relationship. Only a limited supply of nutrient is available at low dilution rates. Much of the available energy must be used for cell maintenance, not for growth and reproduction. As the dilution rate increases, the amou
62、nt of nutrients and the resulting cell density rise because energy is available for both maintenance and growth. The growth rate increases when the total available energy exceeds the maintenance energy.maintenance energy. The second type of continuous culture system, the turbidostatturbidostat, , ha
63、s a photocell that measures the absorbance or turbidity of the culture in the growth vessel. The flow rate of media through the vessel is automatically regulated to maintain a predetermined turbidity or cell density. 2.The Turbidostat2.The Turbidostat The turbidostat differs from the chemostat in se
64、veral ways. The dilution rate in a turbidostat varies rather than remaining constant, and its culture medium lacks a limiting nutrient. The turbidostat operates best at high dilution rates; the chemostat is most stable and effective at lower dilution rates. Continuous culture systems are very useful
65、 because they provide a constant supply of cells in exponential phase and growing at a known rate. They make possible the study of microbial growth at very low nutrient levels, concentrations close to those present in natural environments. These systems are essential for research in many areasfor ex
66、ample, in studies on interactions between microbial species under environmental conditions resembling those in a freshwater lake or pond. Continuous systems also are used in food and industrial microbiology. Microbes are exposed to a wide variety of environmental factors in addition to nutrients. Mi
67、crobial ecology focuses on ways that microorganisms deal with or adapt to such factors as heat, cold, gases, acid, radiation, osmotic and hydrostatic pressures, and even other microbes. 6.4 6.4 The Influence of EnvironmentalThe Influence of EnvironmentalFactors on GrowthFactors on GrowthBacteriagrow
68、overarangeoftemperatures;theydonotreproducebelowtheminimumgrowthtemperaturenorabovethemaximumgrowthtemperature.Withinthetemperaturegrowthrangethereisanoptimumgrowthtemperatureatwhichbacterialreproductionisfastest.three cardinal temperaturescardinal temperatures: (1) The minimum temperatureminimum te
69、mperature is the lowest temperature that permits a microbes continued growth and metabolism; below this temperature, its activities are inhibited. TEMPERATURETEMPERATURE (2)The maximum temperaturemaximum temperature is the highest temperature at which growth and metabolism can proceed. If the temper
70、ature rises slightly above maximum, growth will stop,but if it continues to rise beyond that point, the enzymes and nucleic acids will eventually become permanently inactivated and the cell will die. This is why heat works so well as an agent in microbial control. (3)The optimum temperatureoptimum t
71、emperature covers a small range, intermediate between the minimum and maximum, which promotes the fastest rate of growth and metabolism (rarely is the optimum a single point). Depending on their natural habitats, some microbes have a narrow cardinal range, others a broad one. Strains of Staphylococc
72、us aureus grow within the range of 646C, and the intestinal bacterium Enterococcus faecalis grows within the range of 044C.EnzymesexhibitaQ10sothatwithinasuitabletemperaturerangetherateofenzymeactivitydoublesforevery10riseintemperature.Microorganismsareclassifiedaspsychrophiles,mesophiles,thermophil
73、es,andextremethemophilesbasedontheiroptimalgrowthtemperature. Another way to express temperature adaptation is to describe whether an organism grows optimally in a cold, moderate, or hot temperature range. The terms used for these ecological groups are psychrophile, mesophile, and thermophile respec
74、tively. A psychrophile psychrophile is a microorganism that has an optimum temperature below 15C and is capable of growth at 0C.It is obligate with respect to cold and generally cannot grow above 20C. Laboratory work with true psychrophiles can be a real challenge. Inoculations have to be done in a
75、cold room because room temperature can be lethal to the organisms. Unlike most laboratory cultures, storage in the refrigerator incubates, rather than inhibits, them. As one might predict, the habitats of psychrophilic bacteria, fungi, and algae are snowfields, polar ice, and the deep ocean. Rarely,
76、 if ever, are they pathogenic. True psychrophiles must be distinguished from psychrotrophs or facultative psychrophiles that grow slowly in cold but have an optimum temperature above 20C. Bacteria such as Staphylococcus aureus and Listeria monocytogenes are a concern because they can grow in refrige
77、rated food and cause food-borne illness. The majority of medically significant microorganisms are mesophilesmesophiles,organisms that grow at intermediate temperatures. Although an individual species can grow at the extremes of 10C or 50C, the optimum growth temperatures (optima) of most mesophiles
78、fall into the range of 2040C. Organisms in this group inhabit animals and plants as well as soil and water in temperate, subtropical, and tropical regions. Most human pathogens have optima somewhere between 30C and 40C (human body temperature is 37C). Thermoduric microbes, which can survive short ex
79、posure to high temperatures but are normally mesophiles, are common contaminants of heated or pasteurized foods. Examples include sporeformers such as Bacillus and Clostridium. A thermophile thermophile is a microbe that grows optimally at temperatures greater than 45C. Such heat-loving microbes liv
80、e in soil and water associated with volcanic activity and in habitats directly exposed to the sun. Thermophiles vary in heat requirements, with a general range of growth of 4580C. Most eucaryotic forms cannot survive above 60C, but a few thermophilic bacteria called hyperthermophiles, grow between 8
81、0C and 110C (currently thought to be the temperature limit endured by enzymes and cell structures). Strict thermophiles are so heat-tolerant that researchers may use an autoclave to isolate them in culture. Currently, there is intense interest in thermal microorganisms by biotechnology companies. On
82、e of the earliest thermophiles to be isolated was Thermus aquaticus. It was discovered by Thomas Brock in Yellowstones Mushroom Pool in 1965 and was registered with the American Type Culture Collection. Developers of the polymerase chain reaction(PCR), a versatile tool for making multiple copies of
83、DNA fragments, found that the technique would work only if they performed the test at temperatures around 6572C. The mesophilic enzymes they tested were destroyed at such high temperatures, but the Thermus DNA-copying enzyme (Taq polymerase) functioned splendidly. The PCR technique became the basis
84、for a variety of test procedures in forensics and gene detection and analysis.Effectofoxygenconcentrationonthegrowthofvariousbacteriaintubesofsolidmedium.(a)Obligateaerobes-growthoccursonlyintheshortdistancetowhichtheoxygendiffusesintothemedium.(b)Facultativeanaerobesgrowthisbestnearthesurface,where
85、oxygenisavailable,butoccursthroughoutthetube.(c)Obligateanaerobes-oxygenistoxic,andthereisnogrowthnearthesurface.(d)Aerotolerantanaerobes-growthoccursevenlythroughoutthetubebutisnotbetteratthesurfacebecausetheorganismsdonotuseoxygen.(e)Microaerophiles,aerobicorganismsthatdonottolerateatmosphericconc
86、entrationsofoxygen-growthoccursonlyinanarrowbandofoptimaloxygenconcentration. In general, microbes fall into one of three categories: those that use oxygen and can detoxify it; those that can neither use oxygen nor detoxify it; and those that do not use oxygen but can detoxify it. As oxygen enters i
87、nto cellular reactions, it is transformed into several toxic products. Singlet oxygen (1O2) is an extremely reactive molecule produced by both living and nonliving processes. Notably, it is one of the substances produced by phagocytes to kill invading bacteria. The buildup of singlet oxygen and the
88、oxidation of membrane lipids and other molecules can damage and destroy a cell. The highly reactive superoxide ion (O2-), peroxides (H2O2-), and hydroxyls (OH-) are other destructive metabolic by-products of oxygen. To protect themselves against damage, most cells have developed enzymes that go abou
89、t the business of scavenging and neutralizing these chemicals. The complete conversion of superoxide ion into harmless oxygen requires a two-step process and at least two enzymes: In this series of reactions (essential for aerobic organisms), the superoxide ion is first converted to hydrogen peroxid
90、e and normal oxygen by the action of an enzyme called superoxide dismutase. Because hydrogen peroxide is also toxic to cells (it is a disinfectant and antiseptic), it will be degraded by the enzyme catalase into water and oxygen. If a microbe is not capable ofdealing with toxic oxygen by these or si
91、milar mechanisms, it is forced to live in habitats free of oxygen. With respect to oxygen requirements, several general categories are recognized. An aerobeaerobe(aerobic organism) can use gaseous oxygen in its metabolism and possesses the enzymes needed to process toxic oxygen products. An organism
92、 that cannot grow without oxygen is an obligate aerobeobligate aerobe. . Most fungi and protozoa,as well as many bacteria (genera Micrococcus and Bacillus),have strict requirements for oxygen in their metabolism. A facultative anaerobefacultative anaerobe is an aerobe that does not require oxygen fo
93、r its metabolism and is capable of growth in the absence of oxygen. This type of organism metabolizes by aerobic respiration when oxygen is present, but in its absence, it adopts an anaerobic mode of metabolism such as fermentation. Facultative anaerobes usually possess catalase and superoxide dismu
94、tase. A large number of bacterial pathogens fall into this group (for example, gram-negative entericenteric bacteria and staphylococci). A microaerophilemicroaerophile does not grow at normal atmospheric tensions of oxygen but requires a small amount of it in metabolism. Most organisms in this categ
95、ory live in a habitat (soil, water, or the human body) that provides small amounts of oxygen but is not directly exposed to the atmosphere. An anaerobeanaerobe (anaerobic microorganism) lacks the metabolic enzyme systems for using oxygen in respiration. Because strictstrict, , or obligateobligate, a
96、naerobes , anaerobes also lack the enzymes for processing toxic oxygen,they cannot tolerate any free oxygen in the immediate environment and will die if exposed to it. Strict anaerobes live in highly reduced habitats, such as deep muds, lakes, oceans, and soil. Aerotolerant anaerobes do not utilize
97、oxygen but can survive and grow to a limited extent in its presence. These anaerobes are not harmed by oxygen, mainly because they possess alternate mechanisms for breaking down peroxides and superoxide. Certain lactobacilli and streptococci use manganese ions or peroxidases to perform this task. De
98、termining the oxygen requirements of a microbe from a biochemical standpoint can be a very time-consuming process. Often it is illuminating to perform culture tests with reducing media(those that contain an oxygen-absorbing chemical). One such technique demonstrates oxygen requirements by the locati
99、on of growth in a tube of fluid thioglycollate.Bacteria:NeutralconditionFungi:AcidicconditionActinomycetes:Alkalinecondition Each species has a definite pH growth range and pH growth optimum. AcidophilesAcidophiles have their growth optimum between pH 0 and 5.5; neutrophilesneutrophiles, , between p
100、H 5.5 and 8.0; and alkalophilesalkalophiles prefer the pH range of 8.5 to 11.5. Extreme alkalophiles have growth optima at pH 10 or higher. In general, different microbial groups have characteristic pH preferences. Most bacteria and protozoa are neutrophiles. Most fungi prefer slightly acid surround
101、ings, about pH 4 to 6; algae also seem to favor slight acidity. The majority of organisms live or grow in habitats between pH 6 and 8 because strong acids and bases can be highly damaging to enzymes and other cellular substances. Although microorganisms will often grow over wide ranges of pH and far
102、 from their optima, there are limits to their tolerance. Drastic variations in cytoplasmic pH can harm microorganisms by disrupting the plasma membrane or inhibiting the activity of enzymes and membrane transport proteins. Procaryotes die if the internal pH drops much below 5.0 to 5.5. Changes in th
103、e external pH also might alter the ionization of nutrient molecules and thus reduce their availability to the organism. Although most microbes exist under hypotonic or isotonic conditions, a few, called halophileshalophiles, live in habitats with a high solute concentration. Obligate halophiles such
104、 as Halobacterium and Halococcus inhabit salt lakes, ponds, and other hypersaline habitats. 4.OSMOTIC PRESSURE4.OSMOTIC PRESSURE They grow optimally in solutions of 25% NaCl but require at least 9% NaCl (combined with other salts) for growth. These archaea have significant modifications in their cel
105、l walls and membranes and will lyse in hypotonic habitats. Facultative halophiles are remarkably resistant to salt, even though they do not normally reside in high-salt environments. For example, Staphylococcus aureus can grow on NaCl media ranging from 0.1% up to 20%. Although it is common to use h
106、igh concentrations of salt and sugar to preserve food ( jellies, syrups, and brines), many bacteria and fungi actually thrive under these conditions and are common spoilage agents. The term to describe microbes that withstand and grow at high osmotic pressures is osmophileosmophile. .Iftheconcentrat
107、ionofsolutes,suchassodiumchloride,ishigherinthesurroundingmedium(hypertonic),thenwatertendstoleavethecell.Thecellmembraneshrinksawayfromthecellwall(anactioncalledplasmolysis),andcellgrowthisinhibited.NormalcellPlasmolyzedcell Various forms of electromagnetic radiation (ultraviolet, infrared, visible
108、 light) stream constantly onto the earth from the sun. Some microbes (phototrophs) can use visible light rays as an energy source, but non-photosynthetic microbes tend to be damaged by the toxic oxygen products produced by contact with light.5. 5. OTHER ENVIRONMENTAL FACTORSOTHER ENVIRONMENTAL FACTO
109、RS Some microbial species produce yellow carotenoid pigments to protect against the damaging effects of light by absorbing and dismantling toxic oxygen. Other types of radiation that can damage microbes are ultraviolet and ionizing rays (X rays and cosmic rays). Descent into the ocean depths subject
110、s organisms to increasing hydrostatic pressure. Deep-sea microbes called barophilesbarophiles exist under pressures that range from a few times to over 1,000 times the pressure of the atmosphere. These bacteria are so strictly adapted to high pressures that they will rupture when exposed to normal a
111、tmospheric pressure.Thewateractivityofasolutionis1/100therelativehumidityofthesolution(whenexpressedasapercent),oritisequivalenttotheratioofthesolutionsvaporpressuretothatofpurewater.aw =Psolution/PwaterApproximatelowerawlimitsformicrobialgrowth:0.901.00formostbacteria,mostalgaeandsomefungiasBasidio
112、mycetes, Mucor, Rhizopus.0.75forHalobacterium,Aspergillus0.60forsomesaccharomycesspecies. Because of the high water content of cytoplasm, all cells require water from their environment to sustain growth and metabolism. Water is the solvent for cell chemicals, and it is needed for enzyme function and
113、 digestion of macromolecules. A certain amount of water on the external surface of the cell is required for the diffusion of nutrients and wastes. Even in apparently dry habitats, such as sand or dry soil, the particles retain a thin layer of water usable by microorganisms. Dormant, dehydrated cell
114、stages (for example,spores) tolerate extreme drying because of the inactivity of their enzymes.1. Growth is an increase in cellular constituents and results in an increase in cell size, cell number, or both.2. When microorganisms are grown in a closed system or batch culture, the resulting growth cu
115、rve usually has four phases: the lag,exponential or log, stationary, and death phases.SummarySummary 3. In the exponential phase, the population number doubles at a constant interval called the doubling or generation time. The mean growth rate constant (k) is the reciprocal of the generation time. 4
116、. Exponential growth is balanced growth, cell components are synthesized at constant rates relative to one another. Changes in culture conditions lead to unbalanced growth. A portion of the available nutrients is used to supply maintenance energy.5. Microbial populations can be counted directly with
117、 counting chambers, electronic counters, or fluorescence microscopy. Viable counting techniques such as the spread plate, the pour plate, or the membrane filter can be employed.6. Population changes also can be followed by determining variations in microbial mass through the measurement of dry weigh
118、t,turbidity, or the amount of a cell component.7. Microorganisms can be grown in an open system in which nutrients are constantly provided and wastes removed.8. A continuous culture system is an open system that can maintain a microbial population in the log phase. There are two types of these syste
119、ms: chemostats and turbidostats.9. The environmental factors that control microbial growth are temperature, pH, moisture,radiation, gases, and othermicroorganisms. Environmental factors control microbial growth by their influence on microbial enzymes.10. Three cardinal temperatures for a microorgani
120、sm describe its temperature range and the temperature at which it grows best. These are the minimum temperature, the maximum temperature, and the optimum temperature. Microorganisms are classified by their temperature requirements as psychrophiles, mesophiles, or thermophiles.11. Most eucaryotic mic
121、roorganisms are aerobic, but bacteria vary widely in their oxygen requirements from facultative to anaerobic.12. Each species of microorganism has an optimum pH for growth and can be classified as an acidophile, neutrophile, or alkalophile.13. Most deep-sea microorganisms arebarotolerant, but some a
122、re barophilic and require high pressure for optimal growth.14. High-energy or short-wavelength radiation harms organisms in several ways. Ionizing radiationX rays and gamma raysionizes molecules and destroys DNA and other cell components.I.MULTIPLE-CHOICE QUESTIONSI.MULTIPLE-CHOICE QUESTIONS1. The t
123、ime required for a cell to undergo binary fission is called thea. stationary period b. growth curvec. generation time d. lag periodHOMEWORKHOMEWORK2. In a viable plate count, each ( ) represents a ( ) from the sample population.a. cell, colony b. hour, generationc. colony, cell d. cell, generation 3
124、. During the ( ) phase, the rate of new cells being added to the population has slowed down.a. stationary b. lagc. death d. exponential growth4. Psychrophiles would be expected to grow ( ).a. in hot springsb. on the human bodyc. at refrigeration temperaturesd. at low pH1. Explain the relationship be
125、tween colony counts and colony-forming units. Why can one use the number of colonies as an index of population size?2. Why is growth called exponential? What makes it a geometric progression?II.CONCEPT QUESTIONSII.CONCEPT QUESTIONS3. Explain what is happening to the population at points A, B, C, and
126、 D in the following diagram.1.Is there a microbe that could grow on a medium that contains only the following compounds dissolved in water: CaCO3, MgNO3, FeCl2, ZnSO4, and glucose? Defend your answer.2. How can you explain the observation that unopened milk will spoil even while refrigerated?III. CRITICAL-THINKING QUESTIONSIII. CRITICAL-THINKING QUESTIONS
