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Flashcards in Chapter 4 Dynamics or Prokaryotic Growth Deck (126):
1

All the bacterial cells that result from the replication of a single original bacterial organism are said to be a


A. population.

B. pure culture.

C. lag culture.

D. mutant culture.

B. pure culture.

2

The scientist that contributed most to the development of pure culture techniques was


A. Alexander Fleming.

B. Louis Pasteur.

C. Edward Jenner.

D. Robert Koch.

D. Robert Koch.


(keep your Koch clean, with pure techniques)

3

The solidifying agent used most successfully in bacterial nutrient media is


A. gelatin.

B. peptone.

C. agar.

D. starch.

C. agar.

4

Prokaryotic cells divide by a process known as


A. conjugation.

B. mitosis.

C. binary fusion.

D. binary fission.

D. binary fission.

5

In nature, bacteria


A. often grow in close association with many other kinds of organisms.

B. may remain in a prolonged exponential phase.

C. frequently synthesize structures such as slime layers.

D. may adhere to surfaces by means of pili and slime layers.

E. All of the choices are correct.

E. All of the choices are correct.

6

Biofilms


A. are a haphazard mixture of bacteria.

B. are a polysaccharide-encased community of microorganisms.

C. may enhance bioremediation efforts.

D. may protect organisms against harmful chemicals.

E. are a polysaccharide-encased community of microorganisms, may enhance bioremediation efforts AND may protect organisms against harmful chemicals.

E. are a polysaccharide-encased community of microorganisms, may enhance bioremediation efforts AND may protect organisms against harmful chemicals.

7

The simplest technique for isolating bacteria in growth media is referred to as the


A. pour plate method.

B. streak plate method.

C. serial dilution method.

D. MPN method.

B. streak plate method.

8

Bacteria may be stored


A. on a slant in the refrigerator.

B. frozen in glycerol solution.

C. lyophilized.

D. in broth at 37°C.

E. on a slant in the refrigerator, frozen in glycerol solution AND lyophilized.

E. on a slant in the refrigerator, frozen in glycerol solution AND lyophilized.

9

In the growth curve of a bacteria population, the bacteria are rapidly increasing in number in the


A. lag phase.

B. exponential (log) phase.

C. stationary phase.

D. decline phase.

E. boomer phase.

B. exponential (log) phase.

10

During which phase of growth are bacteria most susceptible to antibiotics?


A. lag

B. stationary

C. exponential (log)

D. decline

C. exponential (log)

11

When doing experiments with bacteria


A. it is usually not necessary to standardize which stage of growth is used.

B. it is best to use colonies as all the bacteria in a colony are at the same stage of growth.

C. it is best to use bacteria from the same stage of growth.

D. the age of the bacteria is not important.

C. it is best to use bacteria from the same stage of growth.

12

During which phase of the bacterial growth curve does the total number of viable cells decline?


A. stationary

B. lag

C. exponential

D. death

D. death

13

The lag phase of the bacterial growth curve is marked by


A. a decrease in cell mass.

B. dormant, metabolically inactive cells.

C. metabolically active cells.

D. vigorously dividing cells.

C. metabolically active cells.

14

Late log phase of the bacterial growth curve


A. is marked by the production of primary metabolites.

B. is marked by the production of secondary metabolites.

C. is a transition into the death phase.

D. shows a decline in cell numbers.

B. is marked by the production of secondary metabolites.

15

During which phase of the bacterial growth curve does a bacterial population become much more resistant to harmful conditions?


A. lag phase

B. exponential phase

C. stationary phase

D. late log phase

D. late log phase

16

A urine sample with more than 100,000 organisms is considered indicative of infection. A urine sample containing 5,000 bacteria, with a generation time of 30 minutes, sits for 3 hours before finally being assayed. How many bacteria will then be present within the sample?


A. 10,000

B. 64,000

C. 100,000

D. 320,000

D. 320,000

17

A pure culture in exponential growth phase has a bacterial concentration of 6.4 x 108 cells/ml. If the bacterium has a generation time of 1 h, how long ago was the cell concentration 8.0 x 107 cells/ml?


A. 1 h

B. 2 h

C. 3 h

D. 4 h

C. 3 h

18

In a rapidly multiplying bacterial population, cell numbers increase


A. arithmetically.

B. logarithmically.

C. linearly.

D. indirectly.

B. logarithmically.

19

The speed of enzymatic reactions approximately doubles for each


A. 5° C rise in temperature.

B. 10° C rise in temperature.

C. 20° C rise in temperature.

D. 30° C rise in temperature.

B. 10° C rise in temperature.

20

Generally the proteins of thermophiles


A. resist denaturation.

B. react more efficiently with DNA.

C. are easily denatured.

D. have a particular amino acid sequence that restricts bond formation.

E. are easily denatured AND have a particular amino acid sequence that restricts bond formation.

A. resist denaturation.

21

A hot tub (approx. 104 deg. F or 40 deg. C) would most likely contain


A. psychrophiles.

B. partiers.

C. thermophiles.

D. mesophiles.

D. mesophiles.

22

The optimal temperature for most human pathogens might be expected to range from


A. 35-40°C.

B. 20-45°C.

C. 15-25°C.

D. 93-98.6°C.

A. 35-40°C.

23

Bacteria on fish caught in the Arctic Ocean would


A. be psychrophiles.

B. be mesophiles.

C. continue to grow while the fish is in the refrigerator.

D. not grow very well in the refrigerator.

E. be psychrophiles AND continue to grow while the fish is in the refrigerator.

E. be psychrophiles AND continue to grow while the fish is in the refrigerator.

24

Mycobacterium leprae is typically found infecting the ears, toes and fingers of its host due to its


A. requirement for well-oxygenated blood.

B. easy access to those parts.

C. need for cooler temperatures.

D. long incubation period.

C. need for cooler temperatures.

25

Organisms that require gaseous oxygen for metabolism are referred to as


A. facultative aerobes.

B. obligate aerobes.

C. facultative anaerobes.

D. microaerophiles.

B. obligate aerobes.

26

Which of the following is/are obligate aerobes?


A. Clostridium botulinum

B. Escherichia coli

C. Micrococcus luteus

D. Helicobacter pylori

C. Micrococcus luteus

27

Organisms that are indifferent to the presence of oxygen and do not use it are


A. aerotolerant anaerobes.

B. facultative anaerobes.

C. obligate aerobes.

D. microaerophiles.

A. aerotolerant anaerobes.

28

The enzymes that deal with toxic oxygen-containing molecules is/are


A. glycolase.

B. superoxide dismutase.

C. catalase.

D. hydrogen peroxidase.

E. superoxide dismutase AND catalase.

E. superoxide dismutase AND catalase.

29

Shake tubes are used to determine the


A. pH requirements for bacterial growth.

B. temperature requirements for bacterial growth.

C. oxygen requirements for bacterial growth.

D. salt requirements for bacterial growth.

C. oxygen requirements for bacterial growth.

30

The optimum pH for growth of most species of bacteria is


A. pH 5.

B. pH 7.

C. pH 9.

D. pH 6.

B. pH 7.

31

High concentrations of salt and sugar in foods


A. are useful in preserving the food.

B. tend to draw water out of a cell.

C. tend to force water into a cell, causing plasmolysis.

D. have no effect on water availability.

E. are useful in preserving the food AND tend to draw water out of a cell.

E. are useful in preserving the food AND tend to draw water out of a cell.

32

Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur may be considered


A. minor elements.

B. major elements.

C. neutral elements.

D. acidic elements.

B. major elements.

33

Trace elements


A. include zinc, copper and manganese.

B. are required in large amounts.

C. may be needed for enzyme function.

D. are involved in maintaining pH in the cell.

E. include zinc, copper and manganese AND may be needed for enzyme function.

E. include zinc, copper and manganese AND may be needed for enzyme function.

34

Small organic molecules that must be provided to bacteria in order for them to grow are called


A. minerals.

B. growth factors.

C. water.

D. vitamins.

B. growth factors.

35

The prefix photo- indicates that an organism will make use of _______ for energy purposes.


A. chemicals

B. organics

C. light

D. inorganics

C. light

36

Organisms that use organic molecules as their source of carbon are called


A. chemotrophs.

B. organoheterotrophs.

C. heterotrophs.

D. autotrophs.

C. heterotrophs.

37

Organisms may derive energy from


A. sunlight.

B. metabolizing chemical compounds.

C. little tiny AAAAAAAAA batteries.

D. temperature gradients.

E. sunlight AND metabolizing chemical compounds.

E. sunlight AND metabolizing chemical compounds.

38

Organisms that use CO2 as their source of carbon are called


A. organotrophs.

B. heterotrophs.

C. autotrophs.

D. chemotrophs.

C. autotrophs.

39

Chemoheterotrophs


A. use sunlight as an energy source.

B. use pre-formed organic molecules as a carbon source.

C. use pre-formed organic molecules as an energy source.

D. use inorganic chemicals as an energy source.

E. use pre-formed organic molecules as a carbon source AND as an energy source.

E. use pre-formed organic molecules as a carbon source AND as an energy source

40

An organism called Bacillus fastidiosus


A. might be expected to be very flexible as to growth requirements.

B. might be expected to be very strict as to its growth requirements.

C. would probably be grown on a chemically defined media.

D. might be expected to have a rod shape.

E. might be expected to be very strict as to its growth requirements, would probably be grown on a chemically defined media AND might be expected to have a rod shape.

E. might be expected to be very strict as to its growth requirements, would probably be grown on a chemically defined media AND might be expected to have a rod shape.

41

Peptone


A. refers to a hydrolysate of carbohydrates used in growth media.

B. refers to a hydrolysate of proteins used in growth media.

C. consists of a water extract of beef.

D. consists of a mix of monosaccharides and oligosaccharides.

E. refers to a hydrolysate of proteins used in growth media AND consists of a water extract of beef.

B. refers to a hydrolysate of proteins used in growth media.

42

Medically important bacteria are often


A. grown on agar containing blood.

B. grown at 37°C.

C. grown on agar containing chocolate.

D. grown at a pH of 5.

E. grown on agar containing blood AND grown at 37°C.

E. grown on agar containing blood AND grown at 37°C.

43

Products which limit pH changes are often incorporated into media and are referred to as


A. enzymes.

B. bases.

C. buffers.

D. acids.

C. buffers.

44

A medium that inhibits the growth of organisms other than the one being sought is termed a(n)


A. synthetic medium.

B. specific culture medium.

C. selective medium.

D. enrichment medium.

C. selective medium.

45

MacConkey agar is


A. a selective agar.

B. a differential agar.

C. a selective and differential agar.

D. used to distinguish between bacteria by the type of hemolysis observed.

C. a selective and differential agar.

46

Candle jars are usually used to


A. store candles.

B. provide an atmosphere with CO2.

C. stimulate the growth of obligate anaerobes.

D. prevent the growth of obligate aerobes.

E. provide an atmosphere with CO2 AND stimulate the growth of obligate anaerobes.

B. provide an atmosphere with CO2.

47

In the cultivation of microaerophilic and anaerobic bacteria,


A. a(n) candle/anaerobe jar is sufficient.

B. atmospheric oxygen in a(n) candle/anaerobe jar is converted to water.

C. a catalyst is used that makes hydrogen and carbon dioxide in a(n) candle/anaerobe jar.

D. oxidizing agents are incorporated into the media that react with oxygen.

B. atmospheric oxygen in a(n) candle/anaerobe jar is converted to water.

48

Agar


A. is a useful nutrient source for most bacteria.

B. is a hydrolysate of proteins.

C. stays liquid through the typical range of incubation temperatures.

D. has chemical and physical properties that make it almost ideal for solidifying media.

D. has chemical and physical properties that make it almost ideal for solidifying media.

49

Agar replaced gelatin as the gelling (solidifying) agent for media because


A. agar is much cheaper.

B. much fewer bacteria can break down agar than gelatin.

C. agar is solid at body temperature.

D. gelatin became unavailable during World War II.

E. much fewer bacteria can break down agar than gelatin AND agar is solid at body temperature.

E. much fewer bacteria can break down agar than gelatin AND agar is solid at body temperature.

50

It is assumed that every colony observed on a streak plate arose from a single bacterium.

TRUE

51

In microbiology, growth usually refers to an increase in size of the bacteria

FALSE

52

There are 5 stages of growth in an open system of culture.

FALSE

53

Secondary metabolites may be antibiotics

TRUE

54

A single bacterial cell may multiply to form a visible colony

TRUE

55

Prokaryotes are the only organisms able to use atmospheric nitrogen as a nitrogen source

TRUE

56

Freezing is an effective means of destroying bacteria

FALSE

57

One would expect most strict anaerobic organisms to have superoxide dismutase.

FALSE

58

Differential media only allows certain bacteria to grow

FALSE

59

A microbe is discovered growing near a deep sea thermal vent. When researchers bring a sample up to the surface and try to grow it in a lab at room temperature in a normal incubator, they are unsuccessful. Why?


A. The pressure isn't the same at sea-level as it is on the ocean floor.

B. Oxygen concentrations are very different between the two environments-it's possible the microbe is a strict anaerobe and is poisoned by the air (oxygen) in the lab.

C. Salt concentrations might be different in the media the researchers are attempting to use and the salt water the microbe is used to living in. This might be causing osmotic pressure differences that the microbe can't tolerate.

D. The temperature is probably very different-the bottom of the ocean would be very cold, while these researchers are trying to grow this microbe at room temperature. The enzymes in the cells are probably outside of their normal operating range at room temperature, and therefore nonfunctional.

E. All of the above.

A. The pressure isn't the same at sea-level as it is on the ocean floor.

60

You are a microbiologist working for a pharmaceutical company and discover a new secreted metabolite that can serve as a medication. Your company asks you to oversee the production of the metabolite. Which of the following is something that is NOT important to consider if you need to grow 5,000 liter cultures of bacteria for the purpose of harvesting the metabolite they secrete?


A. The death rate of the bacteria after stationary phase is complete.

B. The ideal rate of input of new nutrients into the culture to maintain the cells in log phase.

C. The ideal rate of pulling off some of the culture in order to maintain the cells in log phase.

D. The best way to mix the large vat in order to keep it homogenous (constant throughout) in terms of nutrients, temperature, and oxygen levels.

E. The best way to keep the pH of the entire mixture at the ideal level to promote log phase growth.

A. The death rate of the bacteria after stationary phase is complete.

61

You are working in a clinical laboratory in a hospital setting. You're handed a throat swab from a patient. You are told specifically that the physician is only interested in what Gram positive cells are present. What might you do first to go about working towards this goal? Identification isn't the main goal here-just a first step to work towards determining what Gram positive cells might be there.


A. Perform a Gram stain.

B. Streak the sample for isolation on a tryptic soy agar general purpose medium plate.

C. Streak the sample for isolation on a medium that is selective for Gram positive cells while suppressing Gram negative cell growth.

D. Grow the microbes on the swab by inoculating a tryptic soy broth liquid medium tube.

E. Perform an acid-fast stain.

A. Perform a Gram stain.

62

A physician sends a stool sample to your lab, and wants to know if there are lactose fermenting microbes in the sample. How might you determine if these microbes are present or not from this mixed-microbe specimen?


A. Streak the sample for isolation on Thayer-Martin agar (which contains lactose and particular antibiotics for selectivity).

B. Streak the sample for isolation on a blood agar plate (which contains lactose AND red blood cells that enrich the culture for iron).

C. Streak the sample for isolation on a MacConkey agar plate (which contains lactose and a pH indicator that turns pink when acid byproducts are present).

D. None of the above would work-there's no way to reliably determine this feature from the specimen given.

C. Streak the sample for isolation on a MacConkey agar plate (which contains lactose and a pH indicator that turns pink when acid byproducts are present).

63

You are in charge of water quality for your city's water treatment plant. Of the methods at your disposal, which will be the most efficient and cheapest method of determining the number of viable bacteria in the water coming out of your plant?


A. Direct counts using a microscope and a counting chamber.

B. Using a Coulter counter machine.

C. Performing serial dilutions of your samples and doing spread plate counts.

D. Using membrane filtration followed by placing the membrane in a growth medium for colony counts after incubation.

E. Biochemical analysis of secondary metabolites in the water that are given off by bacteria.

D. Using membrane filtration followed by placing the membrane in a growth medium for colony counts after incubation.

64

You take absorbance readings on a spectrophotometer across a 6 hour culture of E. coli cells growing in tryptic soy broth (TSB). Your absorbance readings clearly indicate a lag phase, a log phase, and a stationary phase. You come back in and take readings at 8, 10, 12, 14, and 16 hours, but the absorbance number remains the same. Shouldn't it start coming down as the closed batch culture enters death phase? What's the most likely thing that is happening?


A. Clearly, something is wrong with the spectrophotometer and it isn't measuring the correct values. Perhaps something is on the detector, making it register falsely high absorbance numbers.

B. When we establish a growth curve, we should actually plot the log of the number of viable cells vs. time. However, a spectrophotometer can only measure absorbance. Absorbance is NOT the same as the number of viable cells. Many of the cells in the tube are most likely dead, but the machine can't discriminate between a live cell and a dead one. This keeps the absorbance high even into the death phase.

C. It'll happen-E. coli just grow slowly. We haven't gotten to the end of the stationary phase yet. Be patient!

D. Perhaps there's a big smudge of something on the tube that is blocking some of the light. This would lead to an elevated reading for every timepoint after the smudge was placed on the tube.

B. When we establish a growth curve, we should actually plot the log of the number of viable cells vs. time. However, a spectrophotometer can only measure absorbance. Absorbance is NOT the same as the number of viable cells. Many of the cells in the tube are most likely dead, but the machine can't discriminate between a live cell and a dead one. This keeps the absorbance high even into the death phase.

65

Binary Fission

Asexual process of reproduction in which one cell divides to form 2 daughter cells.

66

Generation time

The time it takes for the number of cells in a population to double: doubling time.

67

Biofilm

Polymer-encased microbial community.
(slipperiness of rocks in stream beds, plaque on teeth)
They are formed when planktonic (free floating) prokaryotic cells move to a surface and adhere, where they then multiply and release polysaccharides, DNA and other hydrophilic pollers to which unrelated cells may attach and grow. The mesh like accumulation of these polymers, referred to as extracellular polymeric substances (EPS) gives a biofilm its characteristic slimy appearance

68

What gives biofilms the slimy appearance?

EPS, extracellular polymeric substances.

69

Biofilmes are not random mixtures of microbes in a later of EPS, they have channels through which nutrients an wastes pass

Cells communicate with one another by synthesizing and responding to chemical signals.

70

Even though biofilms are bad for humans and even appliances we have where are they useful?

they can be used to degrade harmful chemicals and waste water treatment facilities look for ways to foster their development

71

Pure Culture

a population of organisms descended from a single cell

72

Colony

A distinct mass of cells, about 1 million cells are required for a colony to be easily visible to the naked eye.

73

Agar

Polysaccharide extracted from marine algae that is used to solidify microbiological media

74

What is the simplest and most commonly used technique for isolating prokaryotes?

Streak-plate Method

75

Streak-plate Method

Simplest and most commonly used technique for isolating bacteria; a series of successive streak patterns is used to sequentially dilute an inoculum on the surface of an agar plate

76

what are the steps in performing the streak-plate method?

1. sterilize loop
2. dip loop into culture
3. streak first area
4. sterilize loop
5. streak second area (could be perpendicular to other streak)
6. sterilize loop
7. streak final area (away from the others)
8. Isolated colonies develop after incubation.
(the goal is to reduce the number of cells being spread with each successive series of streaks, by the third streak cells should be separated enough so that distinct, well isolated colonies will form.

77

Batch Cultures

A culture system such as a tube, flask, or agar plate in which nutrients are not replenished and wastes are not removed.

78

what is the name of a state in which the cells are in continuous growth and nutrients are continuously added and waste products are removed?

open system or continuous culture

79

Growth Curve

Growth pattern observed when cells are grown in a closed system; consists of 5 stages -
1. lag phase
2. log phase (exponential phase)
3. stationary phase
4. death phase
5. phase of prolonged decline

80

Lag phase

In a bacterial growth curve, stage characterized by extensive macromolecule and ATP synthesis but no increase in cell number.

81

Exponential phase

in a bacterial growth curve, stage in which cells multiply exponentially; log phase

82

from a medical standpoint, what is most important about the log phase (exponential)

bacteria are most sensitive to antimicrobial medications at this stage. Many medications target processes primarily active when bacteria are multiplying.

83

what is the difference between primary metabolites and secondary metabolites?

first of all these metabolites are molecules that are made by cells.
primary- can be used as nutritional supplements or biofuels
secondary- in the later stages of exponential growth, nutrients gradually become depleted and waste products aciculate. cells activities shift as this occurs. If the cells are able to form endospores, they initiate the process of sporulation. If they cannot they still alter their activities to prepare for starvation conditions. Compounds that begin accumulating at this stage are made for purposed other than growth (these are the secondary metabolites)
COMMERCIALLY THE MOST VALUABLE OF THESE ARE ANTIBIOTICS.

84

Stationary phase

In a bacterial growth curve, stage in which the number of viable cells remains constant.
this is because nutrient levels are too low to sustain growth.
some cells are dying while others are multiplying

85

How can cells multiply when they have exhausted their supply of nutrients?

Dead cells often burst, releasing nutrients that then fuel the growth of other cells.

86

true or false, during the stationary phase do the viable cells continue to synthesize secondary metabolites and maintain the altered properties they demonstrated in late log phase?

true

87

Death phase

the period when the total number of viable cells in the population decreases as cells die off at a constant rate. like cell growth, death is exponential, but the rate is usually much slower.

88

Phase of Prolonged Decline

Final stage of the growth curve; most cells die during this phase, but a few are able to grow
most of the survivors eventually die, although there are a few that make it and they are the ones that are better equipped for survival. this causes successive waves of slightly modified populations that are better fir to survive than previous organisms.

89

Chemostat

A device used to grow bacteria in the laboratory that allows nutrients to be added and waste products to be removed continuously.

90

Extremophiles

Organisms that live under extremes of temperature, pH, or other environmental conditions
PART OF THE DOMAIN ARCHAEA

91

True or false, as a general rule, the optimum growth temperature of an organism is close to the upper limit of the organism's temperature range (close to where it will die if it gets too much hotter)

true

92

Obligate aerobe

requires 02

93

Facultative anaerobe

grows best if o2 is present, but an also grow without it

94

obligate anaerobe

cannot grow in the presence of o2

95

microaerophile

requires small amounts of o2, but higher concentrations are inhibitory

96

aerotolerant anaerobe (obligate fermenter)

indifferent to 02

97

halotolerant

can grow in relatively high salt solutions up to approximately 10% NaCl

98

Halophile

requires high levels of sodium chloride

99

plasmolysis

dehydration and shrinkage of cytoplasm from the cell wall as a result of water diffusing out of a cell

100

heterotroph

an organism that obtains carbon from an organic compound such as glucose

101

autotroph

an organism that uses CO2 as it main carbon source

102

carbon fixation

recycling inorganic carbon (from co2) into an organic form. autotrophs do this. without this the earth would run out of organic carbon.

103

Nitrogen Fixation

Nitrogen is needed to make amino acids and nucleic acids. some prokaryotes use nitrogen gas as a nitrogen source, converting it to ammonia and then incorporating that into cellular material. it is unique only to prokaryotes.

104

limiting nutrients

Nutrient that limits growth because it is present at the lowest concentration relative to need.

105

Growth Factor

compounds that a particular bacterium requires in a growth medium because it cannot synthesize them.

106

fastidious

exacting; refers to organisms that require growth factors

107

photoautotroph

energy source: sunlight
Carbon source: CO2

108

Photoheterotroph

Energy source: sunlight
Carbon Source: Organic compounds

109

Chemolithoautotroph

Energy source: Inorganic chemicals (h2, etc)
Carbon Source: CO2

110

Chemoorganoheterotroph

Energy source: organic compounds
Carbon source: organic compounds
(most associated microorganisms with humans)

111

complex medium

medium for growing bacteria that has some ingredients of variable chemical composition. such as meat juices and digested proteins.
often use peptone which is a mixture of amino acids.

112

Chemically defined media

a culture medium composed of exact quantities of pure chemicals; generally used for specific experiments when nutrients must be precisely controlled.

113

selective media

culture medium that inhibits the growth of certain microorganism and therefore favors the growth of desired microorganisms.

114

differential media

Culture media that contain certain ingredients such as sugars in combination with pH indicators; used to distinguish among organisms based on their metabolic traits.

115

enrichment culture

culture method that provides conditions to enhance the growth of one particular species in a mixed population.

116

Direct cell counts

Used to determine total number of cells; counts include living and dead cells.

117

Direct Microscopic count

a liquid specimen is added to special glade slide designed specifically for counting cells. The slide has a thin chamber that holds a known volume of liquid atop a microscopic grid. the contents of the chamber can be viewed under the light microscope, so the number of cells in a given volume can be counted precisely. at least 10 million bacteria (10^7) per milliliter are usually required for enough cells to be seen in the microscopic field

118

Cell-counting instruments

Couler counters and flow cytometers count total cells in dilute solutions. flow cytometers can also be used to count organisms to which fluorescent dyes or tags have been attached.

119

Viable cell counts

Used to determine the number of viable microorganisms in a sample, but that number includes only those that can grow in given conditions. requires an incubation period of approximately 24 hours or longer. selective and differential media can be used to determine the number of specific microbial species.

120

plate counts

measure the number of viable cells in a sample taking advantage of the fact that an isolated microbial cell on a nutrient agar plate will give rise to one colony.

121

spread plate method

Technique used to cultivate bacteria by uniformly spreading a suspension of cells onto the surface of an agar plate

122

pour plate method

Method of inoculating an agar medium with bacterial cells; the inoculum and melted agar are added to a petri dish, where the agar hardens the clones grow both on the surface and within the medium.

123

membrane filtration

technique used to determine the number of bacterial cells in a liquid sample that has a relatively low number of organisms; concentrates bacteria by filtration before they are plated.

124

most probable number (MPN)

a method for estimating the concentration of cells in a specimen. the procedure used a series of dilutions to determine the point at which subsequent dilutions receive no cells.

125

Turbidity

the cloudiness or turbidity of a microbial suspension is proportional to the concentration of cells, and is measured with spectrophotometer. this instrument shines light through a specimen and measure the percentage that reaches a light detector. that percentage is inversely proportional to the optical density.

126

Total weight (in measuring amount of microorganisms)

the total weight of a culture can be used to measure growth but the method is tedious and time consuming. To measure the wet weight, cells in liquid culture are centrifuged and the liquid supernate removed. the weight of the resulting packed cell mass is proportional the number of cells in a the culture. the dry weight can be determined by heating the centrifuged cells in an oven before weighing them.