Bursting with parasites

You know things are going badly when two thirds of your body cavity is filled with parasitic bacteria. This is the unfortunate fate of a creature called a Paramecium (Paramecium caudatum) when infected by a nasty bacterial parasite called Holospora undulata.

For my current research project I’m working with these species to study evolution in the context of disease epidemiology (more on that another time) and as such I spend a considerable portion of my day staring down the microscope at these little critters.

Healthy Paramecium / Antonio Guillén / CC BY-NC-SA 2.0

A Healthy Paramecium / Antonio Guillén / CC BY-NC-SA 2.0

Here is a normal healthy Paramecium seen under the microscope. Each individual is just a single cell, albeit quite a big one on the cellular scale – about 0.17 to 0.29 mm – just about visible if you hold a tube of them up to the light. For comparison, we humans are each made up of about 10 trillion cells.

Paramecia live in freshwater and eat bacteria, tiny algae and the like. To find their prey they swim using small hair like protrusions called cilia which line the cell and beat in synchrony to propel them through the water. In the two boxes marked in red you can see what are called “digestive vacuoles”. When the Paramecium finds something to eat it absorbs it into its body and packages it inside these round cases. These are then filled with digestive enzymes to break down the food.

If the Paramecium is having an unlucky day it might accidentally eat a Holospora bacterium. When ingested they are enveloped in the digestive vacuole like any other food item, but these bacteria have an escape mechanism. It’s not exactly clear how they do it but somehow they break open the vacuole and escape into the main body of the cell. Then they make their way to their new home, a place called the micronucleus.

The nucleus of course is where most DNA is stored. But Paramecium are unusual in that they have two nuclei – one, called the macronucleus, is used for asexual reproduction, which is the normal method of reproduction for a Paramecium. A single cell simply divides into two. The second nucleus, called the micronucleus, is used for a form of sexual reproduction and it is here that the Holospora parasite likes to live. Once it reaches the micronucleus it reproduces.  FAST.

Paramecium 3 days post infection

Paramecium 3 days post infection / Image Credit Oliver Kaltz

Above you can see a zoomed in photograph of another Paramecium (this is at 100x magnification). It’s pinkish because we stain them to make it easier to distinguish the different structures. The large dark part is the DNA in the macronucleus and the smaller round part nestled just above is the infected micronucleus. Each of those dark dashes is one of the parasites and this is about 3 days after infection – as you can see there are quite a few already.

But wait…

Paramecium 7-10 days post infection

Paramecium 7-10 days post infection / Image Credit Oliver Kaltz

This is an infected Paramecium after 7-10 days. That huge mass taking up most of the photo is all parasites. The majority of the cell is absolutely packed with them, bloating the Paramecium from its usual streamlined shape and forcing the macronucleus against the cell wall. It’s amazing how powerful the tendency to anthropomorphise is when you see these in the lab. Despite Paramecium being single-celled and almost indistinguishable to the naked eye you still feel a little sad for them when you see these infections. Amazingly even at this stage the Paramecium still swims and feeds, but the infection will eventually kill it and the parasites will burst alien-style from the cell, to await their next victim.

So consider yourself lucky you are not a Paramecium in my lab!

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