Note on Demonstrating Food Vacuoles
A common technique for demonstrating the food vacuoles in ciliates, found in many of the older books on microscopy, is the use of powdered carmine. The procedures are simple and the results can be elegant. Powdered carmine can be obtained from biological supply houses and is relatively safe to use, but, as with all powdered stains, care should be taken not to inhale any of the powder or let it come in contact with the skin. One can use a flat toothpick or similar implement to transfer a tiny amount of the powder to a drop of water on a slide or a small culture dish containing the organisms. The traditional organism used for this demonstration is Paramecium. In a matter of minutes, the food vacuoles begin to fill up with the carmine granules and, as a consequence, the vacuoles show up bright red. With patience, one can follow the movement of these vacuoles around the cell until they finally discharge the undigestible carmine particles out through the cell wall.
Some ciliates, such as Paramecium and Colpidium readily ingest the carmine, whereas others seem to have mechanisms for screening and rejecting the carmine. I have found that with Stentor and Spirostomum that after 24 to 48 hours, there may be some ingestion, but it is limited and not nearly as dramatic. Bacterial feeders seem to ingest the carmine particles more readily, but it is worthwhile to experiment on a variety of ciliates.
Another stratagem that works with some carnivores is to expose their food organisms to carmine. For example, Stentor and Blepharisma will often ingest Colpidium. Feed a Colpidium culture with carmine particles and then introduce them into a Stentor or Blepharisma culture. As the predators feed on the Colpidium, the carmine which they have ingested shows up in the food vacuoles of the Stentor or Blepharisma.
A technique that can provide interesting contrast is to use a contrasting vital stain, such as, a very dilute solution of methylene blue. (A separate article on vital staining was presented in the February 2000 issue of Micscape.) Almost all so-called vital stains will eventually prove toxic to the cells and some, such as methylene blue are more toxic than others and so must be used in extremely dilute solutions. For methylene blue start with a dilution of 1:100,000. If this proves too strong or too weak, the concentration can be adjusted. The intent is to get a blue stain in major structures of a living organism, such as Paramecium, and then to introduce carmine particles which give the bright red contrast in the food vacuoles. The toxicity varies both with the type of vital stain used and also with the organisms, but with patience and experimentation, one can achieve some spectacular results.
Comments to the author Richard Howey welcomed.
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Published in the May 2000 edition of Micscape Magazine.
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