uniform colony distribution

Robert Preston rapr at med.pitt.edu
Tue Dec 13 14:38:17 EST 1994

A method that seems not to be widely known, but works very well and is easy
to learn, and requires no beads or sticks (it does require sterile
pipettes, though, OR sterile rims on your transformation sample tubes).

Arrange (plan ahead for this) to have the volume to be plated = 0.2 - 0.3 ml 

Place the plate on a spinner/turntable that has good rotational inertia.

Deposit the sample in the center of the plate, and then spin the plate
(not too fast) while pushing the tip of the pipette slowly toward the far
side of the plate.  The tip, in collaboration with centrifugal force, will
push the sample uniformly out to the edge.  When it reaches the edge, STOP
the rotation (else, centrifugal force moves the bulk of the sample to the 
edge).  Let the sample soak in.

Say goodbye to hockey sticks and stick-sterilization time-wasting, and hello
to uniform colony distribution.  

If you do transformations in glass 10mm tubes, the rim of the tube can be kept
sterile and used as a pusher in place of the pipette (if the tube has
only one plateful of sample).

Beware: plastic dispo pipettes usually have sharp-edged tips, unlike the
old fire-polished glass ones, so be very light-handed to avoid gouging the
agar surface. Also, avoid burrs on the plastic tip.

A few minutes practice on discarded plates should suffice to learn the
right combination of rotational rate and pipette pressure etc.

Robert Preston                                    rapr at med.pitt.edu
U. Pittsburgh Pathology Dept.                   Pittsburgh PA 15261
vox 412-648-9573                                   fax 412-648-1916

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