Methods for Measurement of Cell mass and Cell Numbers

Methods for measurement of cell mass

1.Direct physical measurement of dry weight, wet weight, or volume of cells after centrifugation

2.Direct chemical measurement of some chemical component of the cell such as total N, total protein, or total DNA content

3.Indirect measurement of chemical activity such as rate of O2 production or consumption, CO2 production or consumption, etc

4.Turbidity measurements employ a variety of instruments to determine the amount of light scattered by a suspension of cell. Particulate objects such as bacteria scatter light in proportion to their numbers. The turbidity of optical density of a suspension of cell is directly related to cell mass or cell number, after construction and calibration of a standard curve. The method is simple and nondestructive, but the sensivity is limited to about 107 cells per ml for most bacteria

Methods for measurement of cell numbers

1.Direct microscopic counts are possible using special slides known as counting chambers. Dead cell cannot be distinguished from living ones. Only dense suspensions can be counted (>107 cells per ml), but samples can be concentrated by centrifugation or filtration toincrease sensitivity

2.Electronic counting chambers counts numbers and measure size distribution of cells. For cells the size of bacteria the suspensing medium must be very clean. Such electronic devices are more often used to count eukaryotic cells such as blood cells

3.Indirect viable cell counts, also called plate counts, involve plating out (spreading) a sample of a culture on a nutrient agar surface. The sample or cell suspenction can be diluted in a nontoxic diluent (e.g water or saline) before plating. If plated on a suitable medium, each viable unit grows and forms a colony. Each colony that can be counted is called a colony forming unit (cfu) and the number of cfu’s is related to the viable number of bacteria in the sample.