Streaking is a technique used to isolate a pure strain of microorganisms, often bacteria, from a sample containing multiple strains. A wire loop is dipped into the sample and used to streak bacteria in successive passes on an agar plate, diluting the concentration with each pass so that isolated colonies can grow from single bacteria. The plate is then incubated, allowing each colony to multiply from a single parent bacterium. This process isolates pure strains that can be identified, studied, or tested.

2. Streaking
is a technique used to isolate a
pure strain from a single species of
microorganism, often bacteria. Samples
can then be taken from the resulting
colonies and a culture can be grown on a
new plate so that the organism can be
identified, studied, or tested.

3. PURPOSE
• The purpose of making a streak plate is to isolate a single
colony of bacteria on an agar filled petri dish.
• You may need to culture this colony for a later experiment.
• A colony is created after a single bacteria divides many
times creating a mass of genetically identical bacteria.
• When performing this procedure keep in mind that you want
to dilute a small sample to a point where 1 portion of a petri
dish has just 1 bacteria.

4. MATERIALS
Wire loop
Petri dish

5. PROCEDURES
To cool the sterilized
loop, touch it to a
sterile agar plate,
then dip the loop
into a sample
containing a mixture
of bacteria to pick
them up.

6. Before continuing to
streak the plate, the
remaining bacteria on the
loop are first killed in the
flame, after cooling the
sterilized loop, drag it
through the previous
path, picking up a small
number of bacteria and
spreading them into a
new area of the plate.

7. After sterilizing and
cooling the loop again,
the procedure is
repeated. With each
new path, the loop
picks up a smaller
number of bacteria,
and therefore can
spread them farther
and farther apart.

8. During incubation,
the bacteria multiply and
produce colonies. Each colony
consists of millions of cells
derived from a single parent
bacterium. The isolated
colonies found in the last
streak represent isolated
strains to ensure the
strains purity, the streak plate procedure is usually repeated a few
more times, using an isolated colony as the source of starting
bacteria.