 The size of this zone depends on many factors, one being how effective the antibiotic is at stopping the growth of the bacterium.Another factor that will influence the size of a zone is the diffusion of the antibiotic within the agar medium and varies based on the molecular configuration of the antibiotic.Once the zone diameter is measured it must be compared to a database of zone standards to determine if the bacterium being studied is susceptible, moderately susceptible or resistant to the antibiotic in question.