HONG KONG, July 3 (Reuters) - Some of the nastiest bacteria that thrive in the human gut and make us sick may have evolved from hardy ancestors living deep under the sea, a group of Japanese scientists found.

Writing in the latest issue of the Proceedings of the National Academy of Sciences, the scientists said they had analysed the genetic sequences of two well-known disease-causing gut bugs and compared them to two closely related but harmless bacteria found deep on the ocean bed.

The scientists found that they shared many similar genes which enabled them to grow in extreme environments.

They also had few DNA repair genes, allowing frequent mutations to occur, and adapted quickly to changing conditions and to the immune response of a symbiotic host.

Such characteristics allowed the gut bacteria to "persist in infections", the scientists wrote.

"The researchers suggest that human pathogens (gut bugs) evolved from a deep-sea ancestor, and acquired further virulence factors while living in symbiosis with invertebrates," they added.

The two gut bugs the scientists selected were the helicobacter, which causes ulcers, and the campylobacter, which causes food-borne diarrhoea.

The two proteobacteria, sulfurovum and nitratiruptor, are found in very deep seas, areas on the sea floor so hostile that only the hardiest micro-organisms can survive.

Recent technological advances allowed scientists to culture these bacteria. They found the micro-organisms could grow at temperatures ranging anywhere between 4 and 70 degrees Celsius (39 to 158 Fahrenheit).

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