By Levi Clancy for Student Reader on
Fundamentally, there are 4 ways to purify a culture so that it contains only only the organism(s) desired: enrichment, screening, selection and counterselection.
The environment will favor growth of the organism of interest. For example, if you want to isolate a photoautotroph then you will incubate MB in a well-lit condition. If you want to isolate a thermophile then you will incubate the culture at 50ºC.
You isolate your organism using visual cues. For example, if you want to isolate a lacZ WT, then you will inoculate LMA + X-gal or LMA + ONPG.
You add chemicals which kill undesired microbes. For example, if you want to isolate an AmpR mutant then you will inoculate broth containing ampcillin. If you want to isolate only prokaryotic organisms, then you will add cycloheximide to your culture.
You must perform two steps. First, you inoculate a culture and add penicillin. You incubate this in such a way that growth of your desired organism is prevented. Second, you replace the media with media lacking antibiotic. You then incubate in such a way that growth of your desired organism is supported. This process works because in the first step, you are killing all actively dividing cells. This means that your desired microbe will not be harmed (the environment is not suitable for it) but all other organisms will get killed. In the second step, you grow up whatever is still alive.