How to Identify Bacteria on Agar

Put on a lab coat, safety glasses, gloves, face mask and any other protective gear that is required by the protocol of the particular lab in your high school, college or university.

Examine the agar plate and identify the general form of the bacteria colony. Determine if the form is circular, filamentous, irregular or rhizoid: circular form will appear perfectly round; filamentous form look like several tiny fibers over lapping one another; an irregular form will have several branching paths that give it unnameable shape; and a rhizoid form resembles several thick strings overlapping one another.

Double-check the agar plate for multiple colony forms; if more than one type of form appears then the agar plate is being colonized by more than one type of bacteria. Make separate observations for both bacteria types if this is the case.

Turn the agar plate on its side and flash a light at the bottom. Determine the elevation of the bacterial colony by examining it for a raised, convex, crateriform, umbonate or flat shape: a raised elevation will look like as the bacteria grew uniformly; a convex elevation resembles a tall dome; crateriform elevations appear raised except there will be several small craters on the surface; an umbonate elevation will look raised except there will be several tiny bubbles growing on the surface; and a flat elevation seems as if the bacteria is flush with the bottom of the agar plate.

Examine the surface of the bacteria colony. Determine whether it is smooth, dull, rough or glistening. Examine the pigmentation and color of the bacteria.

Examine the opacity of the colony by determining where it is transparent, translucent, opaque or iridescent: transparent opacity will be completely see-through; translucent opacity appears cloudy but will allow light to pass through; opaque opacity is not see-through at all; and iridescent opacity will appear as luminous colors that change with the vantage point.

Place the agar dish beneath a microscope's 40x observation lens and examine the edge of the bacterial colony. Determine if the edge has a filiform, undulate, lobate, curled or entire shape: a filiform edge will appear like tiny string fibers; an undulate edge will appear like tiny, short bumps; a lobate edge will appear as long several fingers; a curled edge will appear as closely compacted waves; and an entire edge will appear perfectly smooth.

Match the observed characteristics of the bacteria in the agar plate to the characteristic of bacteria in Bergey's Manual of Determinative Bacteriology or your class text book.