*4.3.1 Turbidity assay by spectrophotometry*

Turbidity can be observed when the cell density reaches certain level; within a certain range, the number of cells is proportional to the turbidity of the bacterial culture. The cell turbidity is measured by a spectrophotometer or a colorimeter, and a standard curve is generated by plotting the absorbance at OD600nm and the actual cell numbers in the sample. Photoelectric turbidimetric counting is a simple, rapid, and continuous measurement suitable for high-throughput screening. However, its optical density is less sensitive, cannot differentiate between dead or live bacteria, and is greatly affected by cell size, morphology, and the color of the culture solution [49].
