(Lab Periods 2 and 3)
Determining the Number of Bacteria in a Colony
Bacteria are found throughout the biosphere, inhabiting places that eukaryotes find
uninhabitable, sharing all the places where eukaryotes live, and living in and on the bodies of
Most of the time, however, their presence is not obvious, and detection of live
bacteria requires the employment of special techniques that must achieve two ends at once:
(l) Allowing bacteria in a given sample to multiply until a number sufficient for them to be
observable is attained, while
(2) Avoiding allowing bacteria from any other material to grow and multiply.
A commonly-used technique for the first purpose is the viable count, in which a sample of
bacteria is mixed with a liquid (and diluted further in liquid if necessary) then spread out on the
surface of a growth- supporting medium, usually agar in a petri plate.
The bacteria are then
allowed to grow and reproduce until each bacterium has produced so many offspring that the
accumulated mass is macroscopically visible.
Such a mass of bacteria is a colony, and the cell
or cluster of cells that initiated development of the colony is a colony-forming unit (cfu).
container of growth medium plus growing bacteria is a culture, whether it is liquid or solidified
with agar.) The number of colonies on the plate’s surface tells you how many bacterial cells
were in the solution you added.
The methods for achieving the second purpose are collectively known as aseptic
technique, or asepsis (literally, without infection).
The first basic rule for asepsis is that all
materials that come into contact your sample must be sterile.
With reasonable care, this is
possible except for air, which contains bacteria and fungi.
Therefore, the second basic rule of
asepsis is that all containers must be kept closed except when the experimenter must work
within the containers, e.g., to inoculate (introduce the bacteria into) the medium or to remove
some of the culture for one purpose or another.
In this experiment, you will perform a viable count aseptically in order to determine how
many live bacteria (cfu) are in a colony that you scrape off of an agar plate. Petri plates
containing colonies different microorganisms (one organism per plate) will be distributed to the
The colonies formed by different species of bacteria look different--in shape, size
(diameter), color, whether they are shiny or matte in appearance, and the smoothness of the
edge of the colony.
At some time during the laboratory period, examine the colonies of all the
organisms and record a description (including maximum diameter, in mm) of a typical colony of
each organism; your description should be adequate to help you recognize the colonies of each
Each group of four students will be assigned to work with one of the organisms.