Final answer:
The size of a zone of inhibition in a disk-diffusion assay indicates the effectiveness of an antimicrobial agent, but it does not directly correlate with the agent's effectiveness due to various influencing factors. True comparisons of antimicrobial effectiveness require standardized charts and additional laboratory methods like MIC and MBC determination.
Step-by-step explanation:
A zone of inhibition does indicate the effectiveness of an antimicrobial agent against a specific microbe when using the disk-diffusion assay. However, the statement that the size of the zone of inhibition is more an indication of molecular weight and the rate of diffusion through the agar, and does not necessarily rank effectiveness, is true. The size of a zone of inhibition can be influenced by various factors, including the antimicrobial agent's solubility, its diffusion rate through the agar, the agar medium's thickness, and the concentration of the drug on the disk used in the assay. Therefore, a larger zone does not automatically denote a more effective antimicrobial agent. In order to accurately determine the susceptibility of a microbe to antimicrobial drugs, the sizes of zones of inhibition must be compared to standardized charts that correlate zone sizes with susceptibility or resistance. Furthermore, other laboratory methods, such as determining the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC), can provide a more definitive indication of an antimicrobial drug's potency.