Question

Difference between colonies and contamination on plates

Answer 1

Some investigators consider that pour plates have advantages in the isolation of highly motile organisms that form spreading colonies on surface culture.  Colonies are trapped in the medium.  In my view use of media with higher concentrations of agar is easier.  The effectiveness of the pour plate for producing pure colonies depends on the uniformity on the suspension used for inoculation and adequate dilution to ensure colonial separation. 
You also need to conditions of growth.  Aerobic organisms grow better on surface culture.  Hungate role tubes  (a form of pour plate) are very effective for the isolation of anaerobes; it is claimed that better than spread plates in an anaerobic chamber.