Chapter-7--Microbial-Growth-and-Growth-control--微生.ppt
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1、Chapter 7 Microbial Growth and Growth control7.1 Overview of Cell Growth7.2 Population Growth 7.3 Measurement of Growth7.4 Continuous Culture:The Chemostat 7.5 Effect of Environment on Growth7.6 Growth Control 7.7 Viral Control7.8 Fungal Control Chapter outline Concepts Microbial growth is defined a
2、s an increase in cellular constituents and may result in an increase in a microorganisms size,population number,or both.A wide variety of techniques can be used to study microbial growth by following changes in the total cell number,the population of viable microorganisms,or the cell mass.Solid obje
3、cts can be sterilized by physical agents such as heat and radiation;Liquids and gases are sterilized by heat,radiation,and filtration through the proper filter.A knowledge of methods used for microbial control is essential for personal and public safety.7.1 Overview of microbial growthThe bacterial
4、cell is a synthetic machine that is able to duplicate itself.The processes involve as many as 2000 chemical reactions of a wide variety of types:Energy transformations.Biosynthesis of small molecules-the building,blocks of macromolecules-as well as the various cofactors and coenzymes needed for enzy
5、matic reactions.G1 Primary growth phase of the cell during which cell enlargement occurs,a gap phase separating cell growth from replication of the genomeS Phase in which replication of the genome occursG2 Phase in which the cell prepares for separation of the replicated genomes,this phase includes
6、synthesis of microtubules and condensation of DNA to form coherent chromosomes,a gap phase separating chromosome replication from miosis.M Phase called miosis during which the microtubular apparatus is associated and subsequently used to pull apart the sister chromosomes.Cell life cycle in Eukaryoti
7、c cellsEukaryotic cell:Prokaryotic cell:G1 S G2 M G1 R DMost bacterial cells reproduce asexually by binary fision,a process in which a cell divides to produce two nearly equal-sized progeny cells.Three processes:Increase in cell size(cell elongation)DNA replication Cell divisionBinary fision7.2 Popu
8、lation GrowthGrowth is defined as an increase in the number of microbial cells in a population.Growth rate is the change in cell number or cell mass per unit time.The interval for the formation of two cells from one is called a generation.The time required for this to occur is called the generation
9、time.A growth experiment beginning with a single cell having a doubling time of 30 min is presented.This pattern of population increase,where the number of cells doubles during each unit time period,is referred to as exponential growth.Exponential GrowthGrowth Cycle of PopulationsA typical growth cu
10、rve for a population of cells can be divided into several distinct phases called the lag phase,exponential phase,stationary phase,and death phase.Lag Phase When a microbial population is inoculated into a fresh medium,growth usually does not begin immediately but only after a period of time called t
11、he lag phase,which may be brief or extended depending on the history of the culture and growth conditions.This happens because for growth to occur in a particular culture medium the cells must have a complete complement of enzymes for synthesis of the essential metabolites not present in that medium
12、.Exponential Phase It is a consequence of the fact that each cell divides to form two cells,each of which also divides to form two more cells,and so on.Most unicellular microorganisms grow exponentially,but rates of exponential growth vary greatly.In general,prokaryotes grow faster than eukaryotic m
13、icroorganisms If incubation continues after a population reaches the stationary phase,the cells may remain alive and continue to metabolize,but they may also die.If the latter occurs,the population is said to be in the death phase.Death Phase Population growth is measured by following changes in the
14、 number of cells or weight of cell mass.7.3 Measurement of Growth The number of cells in a population can be measured by counting a sample under the microscope,either on samples dried on slides or on samples in liquid.With liquid samples,special counting chambers must be used.Total Cell CountDirect
15、microsopic counting procedure using the Petroff-Hausser counting chamber.The usual practice,which is the most valid statistically,is to count colonies only on plates that have between 30 and 300 colonies.To make a 10-fold(101)dilution,one can mix 0.5 ml of sample with 4.5 ml of diluent,or 1.0 ml sam
16、ple with 9.0 ml diluent.Viable Counting methodThe usual way to perform a viable count is to determine the number of cells in the sample capable of forming colonies on a suitable agar medium.There are two ways of performing a plate count:the spread plate method and the pour plate method.In either cas
17、e the sample must usually be diluted before plating.The number of colonies obtained in a viable count depends not only on the inoculum size but also on the suitability of the culture medium and the incubation conditions used;It also depends on the length of incubation.The length of incubation.Some t
18、iny colonies may be missed during the counting.The incubation conditions(medium,temperature,time).Key dilutions must be prepared.Sources of Error in Plate Counting A cell suspension looks cloudy(turbid)to the eye because cells scatter light passing through the suspension.The more cells present,the m
19、ore light scattered and hence the more turbid the suspension.Turbidimetric Measurements of Cell Number The most common type of continuous culture device used is a chemostat,which permits control of both the population density and the growth rate of the culture.Both parameters can be set by the exper
20、imenter.The ChemostatChemostat used for continuous cultures.Rate of growth can be controlled either by controlling the rate at which new medium enters the growth chamber or by limiting a required growth factor in the medium.Steady-state relationships in the chemostat.The dilution rate is determined
21、from the flow rate and the volume of the culture vessel.Thus,with a vessel of 1000ml and a flow rate through the vessel of 500ml/hr,the dilution rate would be 0.5hr-1.Note that at high dilution rates,growth cannot balance dilution,and the population washes out.Note also that although the population
22、density remains constant during steady state,the growth rate(doubling time)can vary over a wide range.Thus,the experimenter can obtain populations with widely varying growth rates without affecting population density.Even over rather wide ranges,any desired growth rate can be obtained in the chemost
23、at by simply varying the dilution rate.A practical advantage to the chemostat is that a population may be maintained in the exponential growth phase for long periods,for days and even weeks.The experimenter using the chemostat can have such cells available at any time.Experimental Uses of the Chemos
24、tatThe activities of microorganisms are greatly affected by the chemical and physical conditions of their environments.Understanding environmental in fluences helps us to explain the distribution of microorganisms in nature and makes it possible for us to devise methods for controlling microbial act
25、ivities and destroying undesirable organisms.Four main factors:Temperature,pH,water availability,and oxygen.7.5 Effect of Environment on Growth Bacteria grow over a range of temperatures;they do not reproduce below the minimum growth temperattire nor above the maximum growth temperature.Within the t
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