STREAKING METHODS

 

STREAKING METHODS

INTRODUCTION:

Streaking is a common method used to isolate a particular bacterial colony from a mixture of bacteria that is pure culture. In the streak plate method, the concentration of bacterial colonies is more at the starting point and it goes on decrease toward the last point of the streak. It helps to isolate individual colonies from other colonies and each colony is considered a pure colony.

During the identification of a microorganism, the first and important step is to isolate the individual species from a mixed sample. This is mainly done by the streak plate method; therefore streak plate method is an isolation technique.

In this method a sterile inoculating loop is first dipped into a diluted bacterial culture; then the culture-containing loop is streaked on the surface of a solidified agar plate to make a series of parallel, non-overlapping streaks.

As the culture is diluted before streaking on solid agar, the organism number will decrease by the third or fourth quadrant. Therefore only a few bacterial cells are transferred on the solidified agar medium as a result it will give discrete colony forming units (CFUs).

During the streaking an agar plate different patterns are used, depends on the source of inoculum and the microbiologist’s preference. The patterns of streaking patterns range from simple to more complex, these are designed to separate deposited cells (CFUs) on the agar surface so individual cells (CFUs) grow into isolated colonies.

A quadrant streak pattern is used for samples suspected of high cell density, while a simple zigzag pattern used for samples containing lower cell densities.

AIM: To perform the Streak plate method on different media

REQUIREMENT:

• Bacterial culture: 24 hours bacterial culture
• Apparatus: Sterile Petri dishes, inoculating nichrome wire loop, burner, marking pen.
• Media: Nutrient agar media.
• Equipments: Autoclave, Colony Counter, Hot air Oven, Incubator.

General Procedure:

1. Petri dish is labeled on the bottom.
2. Sterilize the nichrome wire loop on the flame of the bunsen burner.
3. Open the bacterial culture tube and collect a sample of bacterial culture with the help of a sterile nichrome wire loop.
4. Streak the nutrient agar plate with bacterial culture. The lid of the agar plate has to be opened between the bunsen burner and streak out the labeled quadrants.
5. Make sure that incinerate the loop before and after inoculating the bacterial culture.
6. All the process is done in a strictly aseptic condition in a laminar airflow cabinet.

Three Sector Streak (t- streak) & Zig - Zag Streak Method:

1. Sterilize the nichrome wire loop on the flame of a bunsen burner.
2. Cool the wire loop between the bunsen burner.
3. Dip the wire loop into the broth culture containing the mixture of bacteria.
4. Insert the nichrome wire loop on the nutrient agar plate and streak the bacterial suspension in a zigzag manner and forms T-shaped streaking.
5. Incubate the plate for 24 hours and you will see isolated colonies in the third sector. There will be less growth in the second sector and the heaviest growth in the first sector.

Four Quadrant Streak method:

1. Sterilize the Nichrome wire loop on the flame of a bunsen burner.
2. Cool the wire loop between the burner.
3. Label the petri dish with a marker and mark four-quadrant on the base of the petri dish.
4. Flame the test tube which contains bacterial culture.
5. Insert the nichrome wire loop into the culture tube.
6. Streak the bacterial suspension in the four-quadrant of the plate between the two burners.
7. Incubate plate at 37°C for 24 hours.

 

Observation: Examine the growth of isolated colonies on the surface of the nutrient agar plate.

Result: Few numbers of isolated colonies appear along with the points of the streak.