Final answer:
A colony on a selective medium is not considered pure because selective media may favor the growth of various closely related organisms, not just a single type. Colony morphology can vary on different media due to varying nutritional components and growth conditions that influence gene expression and metabolic activities.
Step-by-step explanation:
A colony on a selective medium is not considered pure because selective media are designed to favor the growth of certain microorganisms while inhibiting others, rather than isolating a single type of microorganism. Selective media may still allow the growth of a few different types of microorganisms that share similar resistance to the selective agents used in the medium. However, when you pick a colony from a streak plate and transfer it to a new medium, you can achieve a pure culture. This colony, when grown on a selective medium, may consist of the descendants of a single bacterial cell, but depending on the specificity of the medium, it may also include other closely related species or strains that have similar growth requirements and resistances.
Differences in colony morphology on various media, like minimal media with glucose compared to trypticase soy agar plates, can be attributed to different nutritional components and growth conditions provided by each medium. This can alter the expression of specific genes related to metabolism and cellular structures, thus affecting the appearance of the colonies. Additionally, differential media can cause observable changes in the color of colonies or the medium based on the metabolic reactions occurring, further aiding in distinguishing between different bacterial types.