If you’re trapped in a building, it’s probably not the best time to start setting fire to things. But this is exactly what some bacteria do when they find themselves in a human; they cause diseases that are potentially fatal but not contagious. Without an escape, they risk going down with their host. This seems like a ludicrous strategy but we’re looking at it from the wrong perspective – our own. In truth, humans often have nothing to do with the diseases that plague us; we’re just collateral damage in an invisible war.
Like all living things, bacteria have to defend themselves against predators like amoebas. Some species do so using resistance genes that turn them from passive victims into aggressive fighters. And by coincidence, these same adaptations make them more virulent (good at causing disease) in human bodies. We’re just caught in the crossfire.
This post is part of a celebration of the 2-year anniversary of open-access journal PLoS ONE.
Gathering in large numbers is usually a good way of protection yourself against predators, and it’s no surprise that mass defence is a common strategy in the natural world. But it doesn’t always work. There is one hunter that has found a way to use group defence to its advantage. It allows its prey to gather in large numbers and then freezes them in place with a chemical weapon, providing it with a bountiful banquet to eat at its leisure. It’s called Dictyostelium caveatum.
D.caveatum is a member of the dictyostelids, a group that also goes by the names of “slime moulds” or “social amoebae”. It consists of a single cell and its rather unassuming amoeba-like appearance hides the fact that it is a predator par excellence. Lacking fangs, claws or any of the weapons of multi-celled creatures, it nonetheless has highly effective ways of killing its prey – other very closely related social amoebae.
The majority of dictyostelids, such as D.discoideum, are some of nature’s most vivid examples of cooperation. They live most of their lives as single cells that eat bacteria, but a lack of food drives them to seek out company. Like shattered pieces of a T-1000, single cells move towards each other and stick together to form clumps (left image). These clusters elongate to become multi-celled “slugs” (middle), which in turn reach for the sky and transform into “fruiting bodies” (right) – a long stalk topped by a ball of spores. When food is available again, the spores are released and become new amoebae, starting the cycle all over again. The cells that make up the stalk are left to die, sacrificing themselves for the future of their peers.
It’s a lovely story, but add D.caveatum into the mix and you get a very different ending. If a group of 10,000 D.discoideum cells is invaded by even a single D.caveatum one, they are doomed. The lone invader eventually eats the other species, using their cells as fuel to produce its own fruiting bodies. After 48 hours, only D.caveatum remains. This extraordinary behaviour was discovered about two decades ago by one David Waddell but only last year, Clement Nizak from Rockefeller University managed to observe it for the first time under the microscope and work out how it happens.
(Watch D.caveatum (the red arrows) devour D.discoideum (the green arrows).)