An isolated colony on a selective medium is not considered a pure culture because inhibited organisms might be masked by the colonies. This statement is true.
On the basis of their constitution or usage, culture media can be classified into various groups; these include defined, complex, selective, as well as enrichment medium.
A selective medium consists of dyes or noxious substances/chemicals which inhibit the growth of specific microorganisms but promote the growth of other microbes.
A pure culture is a laboratory culture which contains a single species of microorganism. It is free from other microorganisms.
Since selective media affects the growth of distinct organisms differently, therefore, an isolated colony formed in such type of medium is not considered a pure culture.
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The organisms which appeared on earth first was bacteria.
<u>Explanation:</u>
- The first-ever living organism on Earth was a bacteria known as cyanobacteria. It is believed by scientists that it first formed around 3 billion years ago.
- When the Earth was first formed, it was inhospitable for a long time until 3.8 billion years ago approximately, the core cooled down and the level of oxygen had risen. This, along with photosynthesis resulted in the formation of Cyanobacteria.
- These organisms were capable of carrying out the process of photosynthesis by which they prepared their food. These were known as common ancestors. All the living organisms thereafter are known to be their descendants.
