Answer: The air surrounding the petri dish and the agar medium are providing the necessary resources required for the growth of the bacteria.
Step-by-step explanation:
Bacteria require food, suitable environment and water necessary for the growth and survival of the bacteria. Nutrient agar is sufficient to provide all these resources to bacteria necessary for survival and reproduction. Also the short generation time of bacteria allows the colonization at a faster pace. The nutrient agar provide resources which sustain the carrying capacity of the agar for longer.