Use of the plates within a specific time is necessary as time of incubation is related to growth of the bacterial cells which grows with time initially and after a particular period starts declining. Moisture can increase the process of diffusion and also can spread bacterial cells into a zone which is known as zone of inhibition. Solution Use of the plates within a specific time is necessary as time of incubation is related to growth of the bacterial cells which grows with time initially and after a particular period starts declining. Moisture can increase the process of diffusion and also can spread bacterial cells into a zone which is known as zone of inhibition..