Final answer:
Indirect microbial growth estimation includes measuring liquid culture turbidity with a spectrophotometer, analyzing metabolic products, and using the Most Probable Number method for dilute samples.
Step-by-step explanation:
An indirect estimation of microbial growth refers to methods that measure aspects of cell presence or activity without counting each cell individually. One common indirect method involves using a spectrophotometer to measure the turbidity (cloudiness) of a culture in liquid suspension; turbidity increases with cell density. Another indirect approach is to analyze the presence of cell metabolic products, such as adenosine triphosphate (ATP), proteins, and nucleic acids, or monitor the consumption of oxygen. The Most Probable Number (MPN) method is especially useful for estimating viable cell numbers in dilute samples like water, as it relies on observing changes in turbidity or color due to metabolism.