Direct isolation direct isolation is used if the

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Direct Isolation Direct Isolation is used if the desired Bacterium is present as a High Percentage of the Organisms in its Natural Environment. We’ll use Direct Isolation to isolate Bacillus sp. from Soil, and Skin Bacteria (Micrococcus luteus & Staphylococcus epidermidis ) from “Patient” Skin Swabs and Enteric Bacteria (E. coli and Enterobacter aerogenes ) from “Patient” Rectal Swabs. Enrichment Enrichment is used if the Bacterium is present as a Low Percentage of the Organisms in its Natural Environment. Think of Enrichment as a “Boost” or an “Amplification” Step. We’ll use Enrichment to isolate Pseudomonas sp. from Soil and as a Back-Up Plan for Enteric Bacteria. A Series of Steps is used in Isolations to obtain and identify a Pure Culture of the desired Bacterium. These Steps are outlined below. You’ll notice that they vary a bit depending on the Bacterium of Interest. 1A. Direct Isolation The Bacterium is present as a Large Percentage of a Heterogeneous Population and so can be isolated directly from its Natural Environment and inoculated directly into Selective Media or exposed directly to Selective Techniques. 1B. Enrichment The Bacterium is present as a Small Percentage of a Heterogeneous Population and so is introduced into an Enrichment Medium containing various Components that favor the Growth of the desired Bacterium, and in so doing amplifying the Proportion of the desired Bacterium in a Mixed Culture.
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Lab 1 Page 23 2. Selection The Bacterium is inoculated into a Selective Medium containing Components that only allow the Growth of the desired Bacterium while limiting the Growth of other Bacteria. Some Selective Media may also be Differential Media in that they allow the desired Bacterium to be visually distinguished from other Bacteria on the same Medium. 3. Purification The selected Bacterium is inoculated into a Non-Selective Medium so that any contaminating Bacteria inhibited by the Selective Media but carried along can now be forced out of Hiding. One isolated Colony is chosen from this Plate and re-streaked to obtain a Pure Culture. When you’ve got 30 to 40 isolated Colonies on such Non-Selective Media that appear to be alike in all respects except Size (more crowded Colonies are always smaller than well-isolated ones), then you’ve almost certainly got a Pure Culture. If a Gram Stain shows only one type of Morphology, one can assume the Culture is Pure. The Bacterium must be obtained in Pure Culture before any Biochemical Tests can be performed. 4. Identification Biochemical Tests can be performed on the Pure Culture to confirm (in our Situation) or determine (in Clinical or Industrial Situations) the Identity of the Bacterium. The Bacterium can be tested for its Ability to utilize various Substrates and for its Production of different Products.
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