Bacterial growth is logarithmic:

Optical Density is a measure of transmitted light:

Absorbance (O.D.) is proportional to cell number over a limited range.

There are four phases in bacterial growth:

• LAG phase - cells recover from stationary phase
• EXPONENTIAL (LOG) phase - cells are dividing by binary fission
• STATIONARY phase - nutrients have been consumed and cells stop growing
• DEATH phase - cells die (and subsequently lyse) from lack of food and accumulation of toxic waste products

Doubling time is the time it takes for the O.D. to double.

DILUTIONS

Serial dilution:

Overall dilution is the product of the individual dilutions.

A 1:10 or ten-fold dilution:

If there are 100 cells / mL in the first tube, then there are 10 cells / mL in the second tube.