In the new study, published today (March 14) in the journal Science Advances, the researchers first took samples of whipworms from infected mice. They found that inside the parasites, there were bacteria, which the parasite acquired from its host. (In this case, the parasites acquired the bacteria from the mouse’s gut.) If the parasites were hatched in a bacteria-free environment, they didn’t have any gut bacteria.

What’s more, the parasites needed this gut bacteria to grow and thrive, the researchers said. When the researchers exposed adult whipworms to antibiotics (which have effects on bacteria rather than parasites), the worms died. But when the researchers exposed young whipworms that were free of bacteria to antibiotics, the drugs didn’t have an effect, the researchers said.

In another experiment, the researchers looked at mice that didn’t have any gut bacteria (called germ-free mice), and infected the mice with sterile whipworm larvae (whipworm larvae with no bacteria). Two weeks later, these mice had “barely detectable” levels of worms, while mice with normal gut bacteria had high levels of worms.

Interestingly, the researchers found that the composition of gut bacteria inside the adult whipworms was quite different from that of its host. This finding suggests that the whipworm “selects and maintains its own distinct microbiota regardless of the surrounding bacterial populations,” the researchers said.

The researchers also found that, once a whipworm infection is established inside a host, the infection results in changes to the host’s gut bacteria.This altered gut microbiome reduces the number of new whipworm eggs that can hatch. While this may seem counterproductive for the worm, it keeps amount of the worms from getting too high, and prevents the host’s immune system from removing the worms, the researchers said.

Which of the following can logically be inferred from this passage?

 A. Researchers found that there were bacteria inside the parasites, which the host acquired from the parasite.

 B. Parasites do not need gut bacteria to grow and thrive.

 C. The composition of gut bacteria inside the adult whipworms was a mirror-image of that of its host.

 D. Almost no levels of worms were noticed two weeks after germ-free mice were infected with sterile whipworm larvae.