sentences:  Samples taken from the lake so far contain about one part of kerosene per 1000 of water, and they are contaminated with bacteria previously present in the drill bit and the kerosene drilling fluid.So far, the scientists have been able to identify 255 contaminant species, but also have found an unknown bacterium when they initially drilled down to the lake's surface in 2012, with no matches in any international databases, and they hope it may be a unique inhabitant of Lake Vostok.However, Vladimar Korolev, the laboratory head of the study at the same institution, said that the bacteria could in principle be a contaminant that uses kerosene—the antifreeze used during drilling—as an energy source.