Question

i would like to know what proedure is best used to produce and maintain pure cultures of two diffrent orgamisms ? i belive its aseptic technique on a nutrient agar slant but not sure if im correct

Answer 1

A pure culture means all the organisms present in the culture are descendants of the same organisl. To grow these organisms, it is important to have a sterile nutrient containing medium. During culture, it is also important to have a way of transferring these organisms from a pure culture to a sterile medium without any contaminants. This method of preventing unwanted micro- organisms from entering into the pure culture is called aseptic technique or sterile technique. It is used to avoid contamination of sterile media and equipment during cell culture.

Two main steps involved in obtaining pure cultures of multiple organisms are as follows:

First step involves the dilution of the organism mixture until each type of micro-organism get seperated far enough on an agar surface. Thus, once incubated, they form different isolated colonies that are visible. This plate is called the isolation plate.

The second step involves picking the isolated colony aseptically from the isolation plate in order to be transferred on to the new sterile medium. Again, once the incubation is done, the final result would be a pure culture.

The other ways of getting pure cultures include:

1. Streak Plate method: This is the most commong method used to seperate bacterial cells on the agar surface. In this method, a small amount of the organisms are placed on the tip of an inoculation needle and streaked across the surface of the agar medium. During this, the successive streaks cause thinning of the inoculum and thus causing the seperation of different micro- organism from each other.

2. The Pour Plate Method: This is another method used to seperate bacteria. In this, the bacteria are mixed with melted agar unless they are evenly distributed and seperated in all of the liquid. This melted agar is then poured in to an empty plate so as to allow it fo solidify. Then after incubation, seperated bacterial colonies will be found on the agar agar.

3.Spread Plate Method: In this method, the mixed culture is not diluted in agar but in a series of tubes that are filled with sterile liquid ( usually water or saline). A drop of this liquid is dropped at the center of an agar plate and spread evently. Once incubation is done at this stage, well isolated colonies will be visible.

4. Serial dilution method: This is most commonly used to obtain pure culture of the micro- organisms that grow only in liquid media. The micro- organism that predominates in a mixed culture can be isolated by performing a series of dilutions in a sterlie liquid medium.

5. Single cell isolation method: In this method, an individual cell of the required kind of micro -organism is chosen and picked from the mixed culture and is allowed to grow.

6. Enrichment culture method: This is a method used mainly for organisms that are present in a relatively small number or have slow growth rates compared to the rest of the micro- organisms in the culture. This method  incorporates a specific nutrient medium that favors the growth of only the desired species of organsim.