The idea of living inside another species, or intimately attached to it, may seem strange to you — it might even make you think "yuck". But it will interest you to know that there are probably more species of parasites living on earth, then there are species of "free-living" organisms. Parasitism is a dominant way of life.

Think of it. Two thousand of species of mammals and each has its very own group of parasites — worms, ticks, bacteria, fungi and protozoans. To be sure, some parasites are versatile, able to live with several host species, but most are specialists, infecting only one or perhaps a small number of closely related species. If the number of host-specific parasites is say 10 on average, then the parasite diversity for two thousand mammals is 20,000 parasitic species. And many of these parasites have their own parasites—and on it goes.

The notion that microorganisms have parasites was slow to take hold among biologists. But with the advent of better and better microscopy, biologists began seeing structures that seemed out of place in the cells they studied.

Not long ago, one of these mysterious parasites showed up in my lab, in a population of wild Paramecium caudatum growing in a jar of rotting pond vegetation. The first thing I noticed was that some of the cells contained a large round body filled with refractive structures that looked like bacteria.

Observation #1

Lightly staining the cells with neutral red vital stain and flattening them by cover glass pressure made the macronucleus stand out, along with the ball of supposed bacteria.

Observation #2

Observation #3

Observation #4

Following division, the micronucleus swelled with bacteria—the structures seen in observation #1. The photos are of live paramecia taken from video frames.

Two French investigators have discovered that this "vertical transfer" of Holospora bacteria from generation to generation is the norm for well-fed, rapidly-dividing paramecia. When the food is exhausted and starvation sets in, the parasite switches to infective mode and releases infective bacteria into the water where they can be picked up by other paramecia. This horizontal transfer of the infection explains how paramecia become infected in the first place.

But my paramecia seem to be thriving, in spite of the heavy parasite load. However, the illusion of well-being is temporary. In the case of the bacteria that infect the micronucleus, eventual extinction of the infected population is assured. This is because the micronucleus plays the key role in conjugation, the sexual exchange of genetic programs that establishes a new, genetically vitalized clonal population of paramecia. By incapacating the micronucleus, the parasite brings on the eventual genetic death of the clone. However, by then, the bacterium has broadcast its infective stages, which will eventually be eaten by a new population of host cells, where they will transfer from asexual generation to asexual generation. (P.caudatum may undergo from 200 to 1,000 clonal generations between conjugations.)

Internet research: Investigate the parasites of Paramecium using the internet. To start you on this adventure, check out the species of bacteria I have described here, or search on one or both of these well-studied parasites of Paramecium: Holospora and Caedibacter.

For more on Paramecium, check our Classics of Biology Gallery.