Estimating turbidity is a practical way to monitor bacterial growth. As bacteria multiply in a liquid medium, the medium becomes turbid or cloudy with cells. Scales of spectrophotometers (galvanometers) are measured in percent transmittance (% T) and/or absorbance. Bacterial numbers are directly proportional to absorbance and indirectly proportional to % T. In order to quantify the absorbance reading, a quantitative enumeration must be made of the number of bacteria in unit volume. A unit volume of sample is usually 1 ml or 1 g. If absorbance readings are matched with plate counts of the same culture, this correlation can be used in future estimations of bacterial numbers obtained by measuring turbidity. Enzymatic and biochemical reactions are used to identify microorganisms once they have been isolated. Some rapid methods and basic concepts of the commercial systems are currently available for performing these tests.