Date: Mon Nov 28 07:53:18 2005
Posted By: Lynn Bry, MD/PhD, Dept. Pathology, Brigham & Women's Hospital, Harvard Medical School
Area of science: Microbiology
There are a few things you can try to get the bacterial species in your mouth
(the"oral flora") to grow. Your normal mouth flora consists of quite a variety
of organisms. While some are areobic (like to grow in our oxygen-containing
atmosphere) or "facultatively anaerobic" (can grow in the presence or absence of
oxygen), many are anaerobic (cannot grow in the presence of oxygen gas), and so
you will not be able to isolate these.
You should be using a nutritious medium such as tryptose soy base agar (TSA),
Sheep's blood agar, chocolate agar or Todd-Hewitt agar.
Incubation at body temperature, 37°C (98.6°F), provides the optimal
temperature at which oral flora grows. However, some species will
be able to grow at room temperature (25°C), just a bit more slowly.
Many organisms like to have an atmosphere rich in CO2, streptococcal species in
particular. Your mouth is filled with what we refer to as "Viridans
streptococci", some species of which are involved in causing tooth decay. You
can create a CO2-rich environment by creating a candle jar in which you'll
culture your plates. Here's what you'll need for your jar:
- A large sturdy, sealable metal tin or large glass jar with an air-tight
screw-on lid. The container needs to be wide enough and tall enough to hold your
plates and still have 2-3 inches of room to spare. Large tins that held popcorn
can be good for this, or large glass jars that held pasta sauce. Don't use
plastic containers, as they might burn.
- A small tea or votive candle - the kind that's about 1/2 inch high and
formed in a small metal container.
If you have a glass jar, you can watch the candle burn for a bit, then
extinguish - it does this once the oxygen supply has gotten too low for it to
continue burning. The oxygen is not completely gone, just much lower that in the
air we breathe. In fact, you couldn't survive on the O2 left in the jar, but
many species of bacteria find this environment quite welcoming! The burning
reaction also increases the concentration of CO2, which will improve the
conditions for isolation of streptococci.
- Streak your plates and label them with the date, source of material and who
streaked them out. For more information on how to streak plates for isolated
colonies, try the information at
How to streak an agar plate.
- Place your plates in the jar or metal container.
- Have an adult help you light the candle and place it on top of the plates
in the container, or on the side in an open area where it won't contact anything.
- Seal the lid immediately.
If you can find an incubator at 37°C, that would be the best plate to put your
candle jar. Incubation overnight, for 18-24h should give you reasonable growth.
If you can't find an incubator, you can try leaving the jar at room temperature
for 2-3 days, then check the growth.
Hope this helps..
-L. Bry, MadSci Admin
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