Bacteria That Attack and Live Inside Amoebas

The more I learn about bacteria, the more they fascinate me. They may be small, but they aren't simple. They communicate with one another, mount attacks together or on their own, produce an array of interesting chemicals, and live in an amazing variety of habitats. New research from ETH Zurich has shown that at least one kind of bacterium contains dagger-like structures that attack but don't destroy amoebas. After the attack has finished, the bacterium lives inside the amoeba.

Amoeba proteus by Cymothoa exigua, CC BY-SA 3.0 

Amoebas

Like bacteria, amoebas (or amoebae) are single celled organisms, but their internal structure is more complex. Scientists are discovering that this doesn't necessarily mean that their behaviour is also more complex, however. An amoeba is a predator. It produces extensions from its body called pseudopods ("false feet") which enable it to move and catch prey. The amoeba moves by flowing into the pseudopods. It feeds by surrounding smaller creatures with the pseudopods, engulfing the organisms, and then digesting them with chemicals inside a food vacuole. The feeding process is known as phagocytosis. Bacteria are a major food source for amoebas.

Phagocytosis in an amoeba by Kate Taylor, CC0 public domain license

Attack of Amoebophilus Bacteria

A bacterium called Amoebophilus has developed a way to protect itself from an amoeba attack. It destroys the food vacuole that traps it. After this destruction, the bacterium stays in the amoeba and even reproduces inside it, becoming a symbiont living inside a host.

The "micro-dagger" of the bacterium is located within a sheath attached to the inner membrane of its cell. Although the researchers refer to the dagger in the singular, the structure actually contains multiple piercing devices. The sheath attaching the structure to the membrane is spring-loaded. When the sheath contracts, it sends the dagger through the membrane and into the target. In the case of an Amoebophilus inside an amoeba, the target is the food vacuole. It's vital that the bacterium gets out of the vacuole before the digestive enzymes destroy it.

The mechanism by which the dagger destroys the membrane of the food vacuole is not yet known. The destruction may be simply due to mechanical disruption. There may be another factor at work, however. The dagger may contains enzymes that digest the membrane of the vacuole. Researchers have found that the genome of the bacterium contains instructions for making these enzymes.

It might be expected that once the bacterium is out of the food vacuole it would then break through the membrane of the amoeba and escape into the outside world. This isn't what happens, however. The bacterium stays inside the amoeba and lives there. Further research needs to be done to discover the details of its life inside the amoeba.

Multiple bacteriophages attack a bacterium by Dr. Graham Beards, CC BY 3.0 

A Possible Link to Bacteriophages

The researchers say that the bacterial genes involved in the dagger production are very similar to certain genes found inside bacteriophages, or phages. Phages are viruses that attack bacteria. The bacteriophage genes are needed in order for the virus to pierce the bacterial membrane. Phages pierce the membrane in a similar way to Amoebophilus, except only one spike or piercing device is present. The researchers think that at some time in the past the phage genes were inserted into the bacterial genome.

The ETH Zurich research is exciting because the scientists used a new imaging technique to see the entire dagger structure of the bacterium. Previously only components of the system had been seen. The knowledge that has been gained adds to the growing evidence that despite their microscopic size bacteria are far more complex than we thought.

Reference: The News Release

ETH Zurich. "Bacteria stab amoebae with micro-daggers." ScienceDaily. www.sciencedaily.com/releases/2017/08/170817141531.htm. (accessed August 18, 2017).