How Scientists Count Bacteria

Direct Microscopic Count

• Scientists use a glass slide with a known volume of culture and they count the number of visible bacteria. This method does not distinguish between dead and living microbes and requires advance training and time.

Plate Counts

• Also called the serial dilution method, plate counts are performed by diluting the sample of bacteria with a sterile solution and spreading it onto an agar plate. After a period of time, the bacteria colonies that grow are counted, and the number of bacteria in the original sample is calculated based on the dilution factor.

Most Probable Number Method

• The most probable number method (MPN) approximates the count by diluting the sample of bacteria and then growing the cultures in saline tubes. The MPN method is used to count bacteria for cases where the type of bacteria do not readily form colonies.

Turbidity

• The amount of turbidity, or cloudiness, of the bacteria sample can be directly related to the mass of cells. The bacterial cell count is estimated based on the average cell mass.

Metabolic Activity

• Metabolic methods are based on the understanding that bacteria emits gas, acids or uses up oxygen. The rate at which the bacteria in the sample metabolize these various constituents allows scientists to estimate the number of bacteria present.

Dry Mass

• The bacterial cells are separated out from the sample using a centrifuge or a filter. Then, the bacterial cells are dried and the dry mass is weighed to estimate the cell count.

Membrane Filters

• Bacteria is filtered from a sample of water containing bacteria and the filter is set on an agar plate. Scientists count the number of bacteria that grow on the filter after a period of time.