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Laboratory Experiments in Microbiology-Exercise 11

front 1

Anton de Bary wrote what about bacteria?

back 1

that these shapes form the characteristic features for the recognition and and differentiation of species. This resulted in the study of the single cell - the pure culture approach.

front 2

What is a pure culture?

back 2

consists of only one species of microorganism

front 3

What were the earliest methods of obtaining a pure culture?

back 3

inoculating the small amounts of a sample taken from a diseased animal into a flak of blood. The microbes were transferred from flask to flask until only one microbe was seen.

front 4

The next development of using agar to solidify samples came from whose lab?

back 4

Robert Koch

front 5

The next development came when the Difco (Differential Ferments Company) began produciing enzymes to help with?

back 5

digestion

front 6

Difco used enzymes to help digest _____________?

back 6

proteins

front 7

What as in the first solid media culture?

back 7

peptone and agar

front 8

What is the role of the peptone?

back 8

furnishes nitrogenous food in the form of amino acids and also a pH buffer substance in the form of phosphate salts

front 9

Name the 3 dilution methods used to for isolating bacteria.

back 9

1. the streak plate
2. the spread plate
3.pour plate

front 10

Streak plate technique

back 10

A loop is used to streak the mixed sample many times over the surface of a solid culture medium in a Petri plate. Theoretically the the process of streaking the loop repeatedly over the agar surface causes the bacteria to fall off the loop one by one and ultimately to be distributed over the agar surface where each cell develops into a colony.

front 11

Which is the most common isolation technique?

back 11

streak plate

front 12

Which plate technique are quantitative?

back 12

spread plate and pour plate

front 13

How can the number of bacteria in a sample be determined?

back 13

counting the number of colony-forming units

front 14

True or false. The sample must be diluted because even a milliliter of of milk or water could yield 20,000 colonies, too many to count.

back 14

true

front 15

A series of ________ is made and cultured because you don't know the _______ of bacteria in a sample.

back 15

dilutions
number

front 16

Spread plate technique is _________________________.

back 16

A small amount of a previously diluted specimen is spread over the surface of a solid medium using a spread rod.

front 17

In the pour plate technique a _____________________________.

back 17

Small amount of diluted sample is mixed with melted agar and poured into an empty sterile Petri dish. After incubation bacterial growth is visible as colonies in and on the agar of of a pour plate.

front 18

What is the number of bacteria used for an original sample of a pour plate?

back 18

between 25 and 250 colonies is selected

front 19

True or false. Fewer than 25 colonies is inaccurate because a single contaminant causes at least a 4% error.

back 19

true

front 20

The number of bacteria in the original sample is calculated as:

back 20

Colony-forming units Number of colonies
_____________________
per ml = Dilution*X Amount plated

front 21

Is this a mixed culture streak plate?

back 21

yes

front 22

Is this a subculture streak plate?

back 22

yes

front 23

This technique involves streaking four sides of a plate, sterlizing the loop between each quadrant and reinoculating it by touching an edge of each quadrant and dragging it to streak the next

back 23

streak plate technique

front 24

How do you know if you have isolated one bacterium in the streak plate technique?

back 24

There should only be one type of colony

front 25

What is a contaminant?

back 25

unwanted organism

front 26

How would you determine whether a colony was a contaminant on a streak plate?

back 26

A contaminant would be one growing in an area where you did not streak

front 27

How would you determine whether a colony was a contaminant on a pour plate?

back 27

By looking at the appearance of the colonies to see if there is one that looks different

front 28

What would happen if a streak plate was incubated for a week longer than it was supposed to be?

back 28

There would be so many colonies growing that none would be isolated

front 29

What is a disadvantage of the streak plate technique?

back 29

You have to be very careful with your streaking to avoid getting groups of cells close that look like a colony but are really multiple types of bacteria

front 30

What is a disadvantage of the pour plate technique?

back 30

It is difficult to isolate aerobes

front 31

Will the isolated colonies always be in the fourth sector on a streak plate?

back 31

Not necessarily; you could get them in the third but usually the fourth