McDonald's restaurants in Australia are fighting back against thieves by blasting suspected robbers with an invisible DNA spray as they attempt to flee.The spray, which remains on the suspect's skin for two weeks and on clothes for up to six months, has been introduced in some of the chain's busiest stores in the state of New South Wales.If the SelectaDNA "forensic marking" spray proves successful in apprehending bandits, McDonald's will introduce the system across all its 780 Australian outlets.Developed in the United Kingdom by a police officer and a chemist, the spray has been used by McDonald's outlets in Britain and Europe.Each outlet keeps the details of its distribution a close secret, but one McDonald's restaurant in The Netherlands installed an orange device above the main door which was electronically linked to a panic alarm system. Staff could activate the device in an emergency."Once there has been a security breach, the hi-tech spray unit will douse fleeing robbers with an invisible, synthetic DNA solution," McDonald's Australia's chief restaurant support officer, Jackie McArthur, said."The solution is invisible to the naked eye and unique to each location. It stays on clothing for up to six months and on skin for up to two weeks."Using a UVA light, police can see the markings left by the system and link the offender back to the scene.