BISC209: Rhizobium isolation from soil

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Wellesley College-BISC 209 Microbiology -Spring 2010


Rhizobium is a Gram negative bacterium associated with the roots of some plants, such as legumes. The plant and the bacterium establish a symbiotic relationship with the plant forming root nodules. The bacterium traps atmospheric nitrogen and makes it available to the plant in the form of ammonia. Rhizobia can be cultivated on Emerson agar. If you are sampling from an area in the greenhouse (near a Mimosa perhaps?) and suspect Rhizobium might be present, use Emerson agar as one of your enrichment medium choices.

Protocol for collecting Rhizobium:

Locate a root nodule of a likely symbiot, or take a soil sample close to the roots of a clover or legume relative (peas, beans, locust, Mimosa).
Clean the root nodule of surface particles as thoroughly as possible
Aseptically crush the nodule into a small drop of sterile water until you see a milky suspension.
An alternate technique is to shake 1g of soil collected from near the roots of the plant in a small tube of sterile water and then streak for isolation on Emerson Agar. Incubate at RT for 1 week
Look for small glistening white colonies.
Microscopic examination should show pleomorphic Gram negative rods.

Links to Labs

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