Final answer:
Metabolic activity is not a direct method to measure microbial growth.
Step-by-step explanation:
The direct method to measure microbial growth is to directly count the cells or determine their presence or activity. The direct microscopic count involves transferring a known volume of a culture to a calibrated slide and counting the cells under a light microscope. The standard plate count method involves diluting a sample and plating it on agar to allow the formation of visible colonies that can be counted.
Filtration is another direct method that involves filtering a sample through a membrane and counting the colonies that grow on a suitable medium. The most probable number (MPN) method is a statistical procedure that estimates the number of viable microorganisms in a sample by observing changes in turbidity or color due to their metabolic activity. Therefore, the method that is NOT a direct method to measure microbial growth is E) metabolic activity.