Life but not as we know it has been discovered on Earth, raising the chances of alien biology evolving on other planets.
The bugs, described as a form of "weird life" by scientists, are able to thrive on arsenic - and even incorporate it into their DNA.
Arsenic is one of the most deadly substances known to most organisms. It usually disrupts metabolic pathways, making it impossible for the normal mechanisms of life to continue.
But the strange microbes scooped from mud at the bottom of a Californian lake not only survive in an arsenic environment but make the chemical part of their molecular structure. The discovery is said to have huge implications for the search for extraterrestrial life.
Dr Felisa Wolfe-Simon, from Arizona State University, who led the US researchers, said: "Our findings are a reminder that life as we know it could be much more flexible than we generally assume or imagine."
When scientists talk about life they tend to mean "life as we know it" - that is, life based on the biological building blocks found on Earth.
But some experts have speculated that there may be alternative kinds of life - dubbed "weird life" - elsewhere in the universe.
The new find lends support to that theory, increasing the prospects of discovering life on Mars or the moons of Saturn or Jupiter, or outside the Solar System.