Counting bacteria

A colony of bacteria growing on agar contains identical cells that have all descended from a single ancestor cell. When a microbiologist inoculates agar, individual bacteria disperse in the medium. During incubation, each cell from the inoculum doubles in number every half hour or so, depending on species, until they form the visible mass of cells known as a colony. Microbiologists call the colonies CFUs for colony-forming units and count them either manually under a magnifying glass or electronically by scanning the agar plate with a laser beam.

Samples containing several thousand to millions of cells would create an almost contiguous sheet of colonies unless the microbiologist serially dilutes the sample before inoculating the plates. Serial dilution produces plates containing between 30 and 300 CFUs, most of which are spatially separated from each other and easy to count. Microbiologists prefer plates with this many colonies because CFU numbers of less than 30 do not give consistently accurate results, and plates with 300 or more colonies are too dense to count. On densely populated plates, bacteria begin inhibiting the growth of nearby colonies by using up nutrients and excreting antimicrobial substances.