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(Source: B. Trevarrow from Zebrafish Book 5th Edition)

Contamination is a recurrent problem with paramecia cultures.  Here is a simple technique that allows the isolation of paramecia or other motile micro-organisms from non-motile or more slowly moving organ­isms that may contaminate the cul­ture.
1.     Prepare a number of small diam­eter glass tubes (about 5 cm long and 2-3 mm inside diameter) that are filled with loosely com­pacted glass floss.  The floss should be inserted 3-5 mm in from each end (see below).  To insert it, cut a strip about 2-3 mm wide and about 5 cm long and draw through the tube with a wire hook or tubing with a notch in it.

Device for purifying paramecia.

2.   Sterilize the tubes.
3.   Suspend a sterile floss-containing tube in a sterile 60 mm Petri plate with a piece of tape.  The tape does not need to be sterile.  Avoid touching the ends of the tube.
4.   Using a sterile pipette tip, add sterile 10% Hank's to one end of the tube.  Leave space (about 2-3 mm) in the end of the tube for the addition of the impure para­mecia culture.
5.   Add a droplet of the im­pure para­mecia culture to the same end of the tube filling the 2-3 mm space.
6.   Occlude the culture-con­taining end of the tube with a small piece of mod­eling clay (e.g. Plasticine).
7.   Wait a few hours and the para­mecia will appear at the open end of the tube where they are ob­servable with a dis­secting micro­scope and can be isolated with a sterile pipette tip.  Do not wait too long or other slower mov­ing organisms may catch up.
8.   Add the isolated paramecia to a sterile culture and no other or­ganisms should be present.