**4.3 Indirect cell counting methods**

*Cell Growth*

whereas *M. tuberculosis* and mammalian cells have longer doubling time of several hours or a day. The growth and reproduction of cells can be further altered by changes in temperature, nutrition, viral infection, and/or the presence or absence of inhibitors [46]. Therefore, the growth curve is important in guiding clinical drug usage, investigating gene functions, and understanding drug mechanism of action. Various methods have been developed to measure the absolute number of cells or

Traditionally, cell numbers are counted by taking an aliquot of a homogenous cell suspension and plating on a hemocytometer to count the numbers under a light microscope. The obtained cell number in a certain volume of the suspension is then converted into the cell concentration (cells per ml) in the stock solution. Bacteria are counted by a Petroff-Hausser bacterial counter, a Hawksley counter, and/or the plate colony formation method. The plate colony counting method often gives a lower cell number than the actual value, because it is often difficult to disperse bacteria into a single cell and to make sure that a single colony is not derived from

The most commonly used automatic cell counting methods are direct electrical impedance, flow cytometry, computer-aided image analysis, and serological counting. Through changes in electrical properties, the direct electrical impedance method quantifies the number and the volume of cells in the blood. Using a photomultiplier to filter and detect the signal, flow cytometry records both the density and height of fluorescent pulses and then converts them to the number of bacteria; the method is fast and sensitive and can simultaneously analyze the cell morphology and protein biomarkers. Computer-aided image analysis [47] and serology [48] counting methods analyze the image or 2D picture to obtain accurate quantification and morphological structure. So far, both methods have been used successfully in

biology, materials science, mineralogy, and neurological science.

the changes in cell number, as shown in **Figure 4**.

**4.1 Manual cell counting**

several bacteria.

**4.2 Automated mechanical counting**

**44**

**Figure 4.**

*The main methods for cell growth measurement.*
