Microbial Enumeration

Question 1

Methods of Microbial Enumeration

Answer 1

1. Direct
2. Indirect

Question 2

Direct Methods of Microbial Enumeration

Answer 2

• Viable counting
• Serial Dilution
• Spread Plate and Pour Plate
• Filtration
• Direct Microscopic Count

Question 3

Viable count/Plate count Method

Answer 3

• Spread Plate
• Pour Plate

Question 4

It is the most frequently used method of measuring bacterial populations

Answer 4

Viable count/Plate count

Question 5

Advantage of Plate count

Answer 5

It measure the number of viable cells

Question 6

Disadvantage of Plate count

Answer 6

It takes some time, usually 24 hours or more, for visible colonies to form

Question 7

A colony often results, not from a single bacterium, but from short segments of a chain or from a bacterial clump. To reflect this reality, plate counts are often reported as

Answer 7

colony-forming units (CFU)

Question 8

What causes inaccurancies in using viable count method?

Answer 8

• too many colonies are present
• some cells are overcrowded and do not develop

Question 9

original inoculum that is diluted
several times

Answer 9

Serial Dilution

Question 10

• put a 1 ml or 0.1 ml of dilution of the bacterial suspension into the petri dish
• the agar liquid is then poured into the plates which is then mixed into the medium by gentle agitation of the plate

Answer 10

Pour Plate

Question 11

• a 0.1-ml inoculum is added to the surface of a prepoured, solidified agar medium
• the inoculum is then spread unformaly over the surface using a metal rod
• this method positions all the colonies on the surface and avoids contact between the cells and melted agar

Answer 11

Spread Plate

Question 12

When the quantity of bacteria is very small, as in lakes or relatively pure streams, bacteria can be counted by

Answer 12

Filtration method

Question 13

Filtration method is applied frequently to

Answer 13

detection and enumeration of coliform bacteria, which are indicators of fecal contamination of food or water

Question 14

In this method a measured volume of a bacterial suspension is placed within a defined area on a microscope slide

Answer 14

Direct Microscopic Count

Question 15

Direct Microscopic Count is often used to

Answer 15

count the number of bacteria in milk

Question 16

A ______ sample is spread over a marked square centimeter of slide, _____ is added so that the bacteria can be seen, and the sample is viewed under the ___ _________ objective lens

Answer 16

• 0.01 ml
• stain
• oil immersion

Question 17

• a specially designed slide
• used in direct microscopic count

Answer 17

Petroff-Hausser cell counter

Question 18

Disadvantages of Direct Microscopic Count

Answer 18

• motile bacteria are difficult to count
• a rather high concentration of cells is required to be countable

Question 19

Advantage of Direct Microscopic Count

Answer 19

no incubation time is required, usually reserved for applications in which time is the primary consideration

Question 20

• automatically count the number of cells in a measured volume of liquid
• used in some research laboratories and hospitals

Answer 20

Electronic cell counters or Coulter counters

Question 21

Indirect Methods of Mirobial Enumeration

Answer 21

• Most Probable Number (MPN)
• Turbidity
• Metabolic Activity
• Dry Weight

Question 22

• a method for determining the number of bacteria in a sample
• statistical estimating technique is based on the fact that the greater the number of bacteria in a sample, the more dilution is needed to reduce the density to the point at which no bacteria are left to grow in the tubes in a dilution series

Answer 22

Most Probable Number (MPN)

Question 23

When is the MPN most useful?

Answer 23

when the microbes being counted will not grow on solid media (such as the chemoautotrophic nitrifying bacteria)

Question 24

MPN is also useful when

Answer 24

the growth of bacteria in a liquid differential medium is used to identify the microbes (such as coliform bacteria, which selectively ferment lactose to acid, in water testing)

Question 25

The MPN is only a statement that there is a _____ chance that the bacterial population ____ within a certain range and that the MPN is _______ the most ______ _____

Answer 25

• 95%
• falls
• statistically the most probable number

Question 26

For some types of experimental work, estimating _______ is a practical way of monitoring bacterial growth

Answer 26

Turbidity

Question 27

What happens to the medium when a bacteria multiply in a liquid medium?

Answer 27

Turbid or cloudy with cells

Question 28

It is an instrument used to measure turbidity

Answer 28

Spectrophotometer or Colorimeter

Question 29

The change of light as bacterial numbers increase will register on the instrument’s scale as the _____ of ______ (%T)

Answer 29

Percentage of Transmission (%T)

Question 30

• it is printed on the instrument’s scale
• a logartithmic expression

Answer 30

Absorbance or Optical density (OD)

Question 31

The Absorbance is used to

Answer 31

plot bacterial growth

Question 32

Another indirect way to estimate bacterial numbers is to measure a population’s ______ _____

Answer 32

Metabolic Activity

Question 33

An example of a practical application of a metabolic test

Answer 33

Microbiological assay

Question 34

Metabolic test in miobiological assay

Answer 34

acid production is used to determine amounts of vitamins

Question 35

a better way to measure the growth of filamentous organisms

Answer 35

Dry Weight

Question 36

Dry weight procedure

Answer 36

• he fungus is removed from the growth medium
• filtered to remove extraneous material
• dried in a desiccator
• it is then weighed