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by Stan Zahler
When you streak out Your Favorite Bacterium on an appropriate agar plate, the colonies reach an expected diameter and stop growing. Why?
Corollary: If you inoculate YFB in the center of an agar plate and incubate it, it would reach its expected diameter. What do you predict would happen if you touch the tip of a sterile hair to the edge of the colony, draw it out on the agar radially, and re-incubate the plate?
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Stan comments on these comments:
By definition, a Talmudic Question shouldn't have a simple answer. This one surely depends greatly on what Your Favorite Bacterium is, and what medium you grow it on, and how you incubate it.
If YFB is a swarmer, you may be able to inhibit swarming (say, by making the agar 5% NaCl) and then ask the Question.
Some combinations of bacterium and medium can indeed give bacterial growth that is inhibited only by the drying-out of the agar. You can control for this by putting a stack of filter paper in the lid of the Petri dish and keeping it soaked for days or weeks, until the colony covers the whole plate.
Some combos really give a finite colony size. This seems generally to result from toxic products (poorly diffusible?) released from the bacteria. Perhaps a better medium - say, one without sugar, if YFB is a fermenter - will give bigger colonies. Do near-by colonies of YFB fuse as they grow, or inhibit each other?
Do not try to answer this Talmudic Question if YSB produces an agarase.
-- Stan Zahler
Posted by: mschaech@sunstroke.sdsu.edu | June 29, 2010 at 03:41 PM