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1、Measurement of Microbial Population SizeAfter learning this section,you should be able to:Evaluate direct cell counts,viable counting methods,and cell mass measurements for determining population size,Explain why plate count results are expressed in terms of colony forming units(CFU).There are many

2、ways to measure microbial growth to determine growth rate constants and generation times.Either population number or mass may be followed because growth leads to increases in both.Direct Measurement of Cell NumbersViable Counting MethodsDirect Measurement of Cell NumbersCounting chamber1.Consist of

3、specially designed slides and coverslips;the space between the slide and coverslip creates a chamber of known depth.2.On the bottom of the chamber is an etched grid that facilitates counting the cells3.The number of microorganisms in a sample can be calculated by taking into account the chambers vol

4、ume and any dilutions made of the sample before counting.One disadvantage of using counting chambers is that to determine population size accurately,the microbial population must be relatively large and evenly dispersed because only a small volume of the population is sampled.Viable Counting Methods

5、Several plating methods can be used to determine the number of viable microbes in a sample.These are referred to as either viable counting methods or plate counts because they count only those cells that are able to reproduce when cultured.Two commonly used procedures are the spread-plate and the po

6、ur-plate techniques.Viable Counting MethodsSeveral plating methods can be used to determine the number of viable microbes in a sample.These are referred to as either viable counting methods or plate counts because they count only those cells that are able to reproduce when cultured.Two commonly used

7、 procedures are the spread-plate and the pour-plate techniques.When using these methods to determine population size,the samples should yield between 25 and 250 colonies per plate for accurate counting.Once the number of colonies is known,the original number of viable microorganisms in the sample ca

8、n be calculated from that number and the sample dilution.For example,if 1.0 milliliter of a solution diluted by a factor of 1 106 yielded 150 colonies(i.e.,1.5 102 colonies),then the original sample contained around 1.5 108 cells per milliliter.CFUBecause it is not possible to be certain that each c

9、olony arose from an individual cell,the results are often expressed in terms of colony forming units(CFU),rather than the number of microorganisms.Direct Measurement of Cell NumbersViable Counting Methods are common methods for most microorganisms,but for some special samples they are not suitable.M

10、embrane Filtration ProcedureTo measure the bacteria in aquatic samplesNoticesPlating techniques are simple,sensitive,and widely used for viable counts of bacteria and other microorganisms in samples of food,water,and soil.However,several problems can lead to inaccurate counts.The population will be

11、underestimated if clumps of cells are not broken up and microorganisms well dispersed.The hot agar used in the pour-plate technique may injure or kill sensitive cells;thus spread plates sometimes give higher counts than pour plates.Plate counts will be artificially low if the medium employed cannot

12、support growth of all the viable microorganisms present.Measurement of Cell MassTechniques for measuring changes in cell mass also can be used to determine population size.1.Determination of microbial dry weight.Cells growing in liquid medium are collected by centrifugation,washed,dried in an oven,a

13、nd weighed.This is an especially useful technique for measuring the growth of filamentous fungi.However,it is time-consuming and not very sensitive 2.Spectrophotometry When the concentration of bacteria reaches about a million(106)cells per milliliter,the medium appears slightly cloudy or turbid.Fur

14、ther increases in concentration result in greater turbidity,and less light is transmitted through the medium.The extent of light scattering(i.e.,decrease in transmitted light)can be measured by a spectrophotometer and is called the absorbance(optical density).Absorbance is almost linearly related to

15、 cell concentration at absorbance levels less than about 0.5.If the sample exceeds this value,it must first be diluted and then absorbance measured.Thus population size can be easily measured as long as the population is high enough to give detectable turbidity.Wordlist1.colony forming units(CFU)菌落形

16、成單位菌落形成單位2.Cell mass measurement 計(jì)細(xì)胞（生物）量計(jì)細(xì)胞（生物）量3.direct cell counts直接細(xì)胞計(jì)數(shù)直接細(xì)胞計(jì)數(shù)4.viable counting methods計(jì)算法計(jì)數(shù)計(jì)算法計(jì)數(shù)5.counting chamber計(jì)數(shù)板計(jì)數(shù)板6.hemacytometers血球計(jì)數(shù)板血球計(jì)數(shù)板7.slide 載玻片載玻片8.coverslip蓋玻片蓋玻片9.chamber室室10.an etched grid 蝕刻的網(wǎng)格蝕刻的網(wǎng)格11.Petroff-Hausser Counting Chamber細(xì)菌細(xì)菌計(jì)計(jì)數(shù)板數(shù)板12.Side view側(cè)面觀側(cè)面觀13.spread-plate and the pour-plate techniques 涂布平板法涂布平板法 倒平板法倒平板法14.Membrane Filtration膜過濾法膜過濾法15.Plating techniques 平板技術(shù)平板技術(shù)16.inaccurate counts 不準(zhǔn)確的計(jì)數(shù)不準(zhǔn)確的計(jì)數(shù)17.clumps of cells 成團(tuán)的細(xì)胞成團(tuán)的細(xì)胞18.artificiall