describe how discrete colonies can appear from a mixed culture using the streak-plate subculture process.?

Answer 1

the key to this technique is to "dilute" the suspension enough to be able to identify individual colonies. Everytime you spread your culture into a new quadrant (and flame the loop in between!) you dilute your suspension.. so by quandrant 4 cells will be separated enough to identify colony types.

This is the entire basis for streak-plating.. given that the individual bacteria you're trying to separate have distinct phenotypes (ie. red vs. white, smooth vs. rough..) you can easily distinguish them, further isolate, subclone a particular genotype etc.

Answer 2

To do this you pipette a small volume of the media onto a plate. This is then streaked around the plate thoroughly. When the plate is incubated many colonies will grow. These colonies come from individual bacteria that got left behind as you streaked the plate. If too many bacteria were in the original media you'll end up with a lawn, where there are so many colonies they're growing together. But if the colonies are well separated each is a clonal line of bacteria descended from an original single bacterium. In this way you can start with a mixed culture of, say, E. coli and S. aureus and end up with a plate with many separate pure E. coli and S. aureus colonies.

Answer 3

Pick a colony, disperse it in a drop of saline (as you might do preparing a Gram stain), then streak a droplet with the loop.

Answer 4

Yes, well, ..that's a very good question. And I just can't quite think at the moment.