colony counter

from the GNU version of the Collaborative International Dictionary of English.

• noun (Microbiology) an instrument designed to conveniently count or assist counting colonies{9} of microorganisms on a plate containing a gelled growth medium. One variety uses a pencil-like rod with a metal tip, which is connected by an electrical connection to the gelled growth medium; when touched to a colony{9} on the plate, the completion of the electrical circuit causes an increment of 1 unit on the readout of the colony counter.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun microbiology laboratory equipment used to count colonies of bacteria, or other microorganisms, growing on agar, usually in a Petri dish.