Final answer:
A calibration curve correlates turbidity with cell density, enabling quick estimates of bacterial counts. Newer indirect methods monitor metabolic products to assess cell activity and estimate live cell density.
Step-by-step explanation:
The purpose of a calibration curve when estimating cell count from turbidity measurements is to establish a relationship between the known concentration of a bacterial culture (as determined by direct methods like viable plate counts) and the turbidity measured by a spectrophotometer. Once this correlation is established, the turbidity can be used to estimate the number of cells in samples that are prepared under similar conditions.
Newer indirect methods for counting live cells measure cell activity by monitoring the production of metabolic products like adenosine triphosphate (ATP), biosynthesis of proteins and nucleic acids, or consumption of oxygen. With these methods, live cell density can be estimated quickly and easily without the labor-intensive process of culturing and directly counting cells.