This completely contradicts the classical judgment of immunological science – that the evolutionary consequence of the immune system is the ability to sense every microorganism.
At the Time, scientists had to find bacteria that were unlikely to have had previous contact with the mammalian immune system in order to test this basic hypothesis. They chose the middle of the Pacific Ocean, 2,655 kilometers southwest of Hawaii, in the Phoenix Islands Reserve of Kiribati.
“That’s not just the deep ocean, but the deepest, oldest, most remote and most protected part of the ocean.” study co-author Randi Rotjan, a marine ecologist at Boston University, told Live Science.
“It’s 4,000 meters deep; there are no resident mammals; and it’s on the equator, and there won’t even be any whales in residence.” Rotjan said, “Whales tend to breed in one hemisphere and feed in the other, so they only cross the equator on migration. It’s a good place to find bacteria that are completely different from those on land.”
The researchers used a remote submarine to collect marine bacteria from water, sponge, starfish and sediment samples, then identified 117 culturable strains.
After characterizing the bacteria, the researchers introduced 50 of the strains into immune cells of mice and humans. To their surprise, they found that 80% of the microbes (mostly belonging to the genus Moritella) escaped immunity. The receptors on the mammalian bone marrow immune cells used in the study were unable to see them.
To figure out which qualities of marine bacteria allow them to escape immunity, the team also exposed mouse and human cells to just a specific part of the bacterial cell wall, that is the component known as lipopolysaccharide (LPS). It is known that the mammalian immune system uses this outermost part of the bacterial cell wall to recognize so-called Gram-negative bacteria.
The researchers found that the receptors of mammalian cells are also blind to LPS itself.
“LPS molecules look similar to what we find in terrestrial bacteria, but many of them are completely cloaked to the immune system.” Kagan says, “This is because the lipid chains on the LPS are originally much longer than what our immune system is used to, but we still don’t know why that evades immunity.”
Despite their bizarre ability to evade detection, the researchers say the deep-sea bacteria do not pose any risk of infection.
“For one thing, they didn’t evolve deliberately to evade the mammalian immune system, so if there was any pathogenicity, it would simply be an accident.” Rotjan said.
“The second reason, to infect humans that’s highly unlikely because the temperature, pressure and chemical environment in our bodies is very different from what you’d find on the sea floor. These bacteria would die in minutes outside of their normal habitat.”
The researchers published their findings March 12 in the online edition of Science Immunology.
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