Post-natural bacteria: One of four DNA bases replaced in e.coli strain

In Europe and the US, an international team of researchers have produced a viable bacterium in which one of the four DNA bases has been replaced with 5-chlorouracil.  Applied to Escherichia coli K12 strain, the researchers have developed a bacteria that requires 5-chlorouracil which is not found in nature, resulting in a bacteria that cannot survive in nature, nor can it pass on its genes to other bacteria.