I wonder why bacteria form colonies in soft agar plates and why those colonies grow independently from each other. Are those colonies DNA different from each other? Thank you.

No, at least not if you are plating from a homogeneous culture. The colony is simply formed because the bacteria are diluted on the plate. That is you usually dilute it until there are only single bacteria on the plate, which then start to divide. Each colony is the result of continuous division starting from (ideally) a single bacterium.

Colonial variants can occur. I believe it is down to the expression and non expression of particular genes. The bacteria making the colonies will all have the same DNA just just aren't using all the same genes or they may have some kind of defective gene which ends up changing the morphological appearance of the colony.

 

I don't think it is that common to see colonial variants but when you do see it it is often down to there being a capsule present or absent.

Thank you CharonY and Callie, I have learnt something new from you. I believe you have the right answer. I agree with Callie in that each colony is meant to have the same genome. In the process of transformation or while bacteria grow on the plate they may be forced to face different conditions that may lead to either differnt gene expressions or even point mutations. Thank you.

if you use the same strain of bacteria on one type of agar they should all be same and express the same genes because there is nothing biochemically different on any part of the agar to make them produce different proteins