Which of the following methods is NOT a form of isolation on a petri dish?

The method of isolation that is not performed on a petri dish is the slant culture. Slant cultures are created in test tubes that contain agar that has been solidified at an angle, creating a slanted surface where microorganisms can be inoculated and grown. This technique is commonly used for maintaining microbial cultures and for storage purposes, rather than for isolating single colonies from a mixture.

In contrast, the three-zone streak plate, spread plate, and pour plate methods all utilize petri dishes and are specifically designed to isolate individual colonies from a mixed culture. The three-zone streak plate involves streaking a loopful of culture in three sections to achieve dilution across the agar surface, while the spread plate method distributes a diluted microbial sample evenly across the surface using a glass spreader. The pour plate technique involves mixing diluted samples with molten agar and then allowing them to solidify in a petri dish, resulting in colonies growing both on the surface and within the agar. These techniques focus on achieving isolation, making them suitable for use in petri dishes.