Pure Culture Techniques

Question 1

t bacteria exist in natural environments as
__________________. It is only in very rare instances
that they occur as a single species.

Answer 1

mixed populations

Question 2

father of medical microbiology

Answer 2

Robert Koch

Question 3

was one of the first to
recognize that if he was going to prove that a particular bacterium causes a specific disease, it would be
necessary to isolate the agent from all other bacteria
and characterize the pathogen.

Answer 3

Robert Koch

Question 4

From his studies on
pathogenic bacteria, his laboratory contributed many
techniques to the science of microbiology, including
the method for obtaining __________________ of bacteria.

Answer 4

pure cultures

Question 5

contains only a single kind of an organism

Answer 5

pure cultures

Question 6

contains more than one kind of organism

Answer 6

mixed culture

Question 7

contains a desired organism but also unwanted organisms

Answer 7

contaminated culture

Question 8

Several methods for obtaining pure cultures are
available to the microbiologist. Two commonly used
procedures are the

Answer 8

streak plate and the pour plate

Question 9

Both procedures involve diluting the bacterial cells in
a sample to an end point where a single cell divides,
giving rise to a single

Answer 9

pure colony

Question 10

is therefore assumed to be the identical progeny of the
original cell and can be picked and used for further
study of the bacterium.

Answer 10

colony

Question 11

is the procedure most often
used by microbiologists to obtain pure cultures. It is
simple and allows for economy of materials. However, it requires a certain level of skill which is only
obtained through practice.

Answer 11

streak-plate method

Question 12

_______________________ between
colonies on the plate is critical so that a single pure
colony can be aseptically isolated from quadrant
4 and used for further testing and study

Answer 12

Good spacing

Question 13

three procedures for producing
a streak plate that will yield isolated colonies

Answer 13

• Quadrant Streak (Method A)
• Quadrant Streak (Method B)
• Radiant Streak

Question 14

(1) Streak one loopful of organisms over Area 1 near edge of the
plate. Apply the loop lightly. Don’t gouge into the medium.
(2) Flame the loop, cool 5 seconds, and make 5 or 6 streaks from
Area 1 through Area 2. Momentarily touching the loop to a
sterile area of the medium before streaking insures a cool loop.
(3) Flame the loop again, cool it, and make 6 or 7 streaks from
Area 2 through Area 3.
(4) Flame the loop again, and make as many streaks as possible
from Area 3 through Area 4, using up the remainder of the
plate surface.
(5) Flame the loop before putting it aside.

Answer 14

Quadrant Streak
(Method A)

Question 15

(1) Streak one loopful of organisms back and forth over Area 1,
starting at point designated by “s”. Apply loop lightly.
Don’t gouge into the medium.
(2) Flame the loop, cool 5 seconds, and touch the medium in a
sterile area momentarily to insure coolness.
(3) Rotate dish 90 degrees while keeping the dish closed.
Streak Area 2 with several back and forth strokes, hitting
the original streak a few times.
(4) Flame the loop again. Rotate the dish and streak Area 3
several times, hitting the last area several times.
(5) Flame the loop, cool it, and rotate the dish 90 degrees again.
Streak Area 4, contacting Area 3 several times and drag out
the culture as illustrated.
(6) Flame the loop before putting it aside.

Answer 15

Quadrant Streak
(Method B)

Question 16

(1) Spread a loopful of organisms in a small area near the edge
of the plate in Area 1. Apply the loop lightly. Don’t gouge
into the medium.
(2) Flame the loop and allow it to cool for 5 seconds. Touching a
sterile area will insure coolness.
(3) From the edge of Area 1 make 7 or 8 straight streaks to the
opposite side of the plate.
(4) Flame the loop again, cool it sufficiently, and cross streak
over the last streaks, starting near Area 1.
(5) Flame the loop before putting it in its receptacle

Answer 16

Radiant Streak

Question 17

By far
the most popular and most utilized procedure is the

Answer 17

quadrant streak plate

Question 18

This method of separating one species of bacteria
from another consists of diluting out one loopful
of organisms with three tubes of liquefied nutrient
agar in such a manner that one of the plates poured
will have an optimum number of organisms to provide good isolation.

Answer 18

Pour-Plate Method

Question 19

requires somewhat less skill than that required for a
good streak plate; a disadvantage, however, is that it
requires more media, tubes, and plates

Answer 19

Pour-Plate Method