As stated in laboratory 2, microbe exist in nature as mixed populations. However, to examine microorganisms in the activities we must have them in the type of a pure culture, the is, one in which every organisms are descendants of the very same organism. Two significant steps are affiliated in obtaining pure societies from a blended population:

First, the mixture have to be diluted till the miscellaneous individual microorganisms come to be separated far sufficient apart on an agar surface ar that after ~ incubation they kind visible colonies isolated native the colonies of various other microorganisms. This bowl is called an isolation plate.


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B. THE pour PLATE and also SPIN PLATE methods OF ISOLATION

Another technique of separating bacteria is the pour plate method. V the pour plate method, the bacteria are mixed with melted agar until evenly distributed and also separated throughout the liquid. The melted agar is then poured into an empty plate and allowed to solidify. ~ incubation, discrete bacterial colonies can climate be found cultivation both ~ above the agar and also in the agar.

The spin plate method entails diluting the bacter sample in tubes of sterile water, saline, or broth. Small samples of the diluted bacteria room then pipetted ~ above the surface of agar plates. A sterile, bent-glass stick is then used to spread the bacteria evenly end the whole agar surface ar (see Fig. 4) in bespeak to see isolated swarms (see Fig. 5). In lab 4 us will usage this technique as component of the plate count method of enumerating bacteria.

C. Use OF devoted MEDIA

To complement mechanical approaches of isolation such together the streak bowl method, countless special-purpose media are accessible to the microyellowcomic.comlogist to assist in the isolation and also identification of specific microorganisms. These special objective media autumn into four groups: selective media, differential media, enrichment media, and combination selective and differential media.

1. Selective media: A selective medium has actually agents added which will certainly inhibit the expansion of one group of organisms while allow the development of another. For example, Columbia CNA agar has the antiyellowcomic.comtics colistin and also nalidixic acid added which inhibit the growth of Gram-negative bacteria yet not the growth of Gram-positives. It is, therefore, said to be selective because that Gram-positive organisms, and also would be beneficial in separating a mixture the Gram-positive and Gram-negative bacteria.

2. Differential media: A differential medium consists of additives that cause an observable shade change in the medium when a particular chemical reaction occurs. Castle are valuable in distinguishing bacteria according to part yellowcomic.comchemical characteristic. In other words, they show whether or no a certain organism can bring out a specific yellowcomic.comchemical reaction during its regular metabolism. Numerous such media will certainly be provided in future labs to aid in the identification of microorganisms.

3. Enrichment media: An enrichment medium contains additives that enhance the expansion of specific organisms. This is useful when the biology you wish to culture is existing in fairly small numbers compared to the various other organisms growing in the mixture.

4. Combination selective and also differential media: A combination selective and also differential medium permits the expansion of one group of organisms while inhibiting the development of another. In addition, the differentiates those organisms that grow based on whether they deserve to carry out particular chemical reactions.

For example, MacConkey agar (see Fig. 6) is a selective tool used because that the isolation of non-fastidious Gram-negative rods, an especially members of the family Enterobacteriaceae and the genus Pseudomonas, and the differentiation of lactose fermenting from lactose non-fermenting Gram-negative bacilli. MacConkey agar contains the dye decision violet well together bile salts the inhibit the growth of most Gram-positive bacteria yet do not affect the development of most Gram-negatives. If the Gram-negative bacterium ferments the street lactose in the medium, the acid end products lower the pH that the medium. The neutral red in the agar transforms red in color once the pH drops listed below 6.8. Together the pH drops, the neutral red is took in by the bacteria, leading to the swarms to show up bright pink to red.

Results are interpreted as follows:

Typical swarm morphology on MacConkey agar is as follows:

PROCEDURE (to be done in pairs)

1. Top top the bottom of every of the three petri bowl you are using today, divide the plate into thirds with your wax marker and also label as displayed below. This will guide your streaking.


2. Return Trypticase soy agar (TSA), which grows both Gram-positive and Gram-negative bacteria, is not normally used together an isolation medium, we will attempt to acquire isolated colonies of the 2 organisms in your mixture by making use of strictly mechanical methods. Often, however, one bacter overgrows another in a mixture and by the moment you spread out out the much more abundantant organism sufficient to gain isolated colonies, the one in smaller sized numbers is no longer on the loop therefore you might not see solitary colonies of each on the TSA following time.

Streak your mixture top top a key of Trypticase soy agar using among the 2 streaking patterns depicted in rap 2, Fig. 4 and also Fig. 5. agar

YouTube movie showing how to streak one agar plate for isolation: 4 sector method. Blue Ridge neighborhood College, Virginia

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2. Using any type of of the 3 plates on which they room growing: