The alarming US government report, which its authors describe as a "wake up call", has led to predictions of untreatable epidemics sweeping around the world and will add urgency to calls to ban the routine feeding of antibiotics to farm animals.
US government health officials are putting doctors across the country on alert, after hundreds of people have contracted it - and four children have died - in the Midwest farm states of Minnesota and North Dakota.
The report by the Center for Disease Control and Prevention (CDP) says that the bug has infected people from all racial groups in both rural and urban parts of the States. It adds: "It is unclear how to limit its spread within the community." And it says that it seems to have reached other countries.
The bug, a strain of Staphylococcus aureus - which is spread by skin- to-skin contact - has long been prevalent in hospitals, including some in the UK, where antibiotics are heavily used. But Dr Jay Todd Weber, of the CDP, says: "These were the first deaths that we were aware of among previously healthy people in the general public."
The four dead children - a seven-year-old black girl from urban Minnesota, a 16-month- old American Indian girl from North Dakota, a 13-year-old white girl from rural Minnesota and a 12-month-old white boy from rural North Dakota - were all previously healthy, the report says. Neither they nor any of their family had recently been in hospital and none of their close relatives worked in healthcare, where they might have picked up the bug. The children were all given the right antibiotics but their doctors were unaware that they were dealing with resistant bacteria.
The CDP adds that 200 people have become ill in the US over the past two years after contracting the superbug and that cases have also been found in Australia, New Zealand and Canada. Another report, from the Minnesota Department of Health, says that antibiotic-resistant bacteria were found on the skins of a high proportion of chickens in local supermarkets.
The report says the superbug can still be treated with Vancomycin, often described as the "antibiotic of last resort". But there is evidence that bacteria are now developing resistance even to this.
Ten days ago, Britain's official Advisory Committee on the Microbiological Safety of Food reported an "apparently inexorable growth in the number of resistant bacteria" as a result of the practice of feeding antibiotics to animals raised for meat to make them grow faster.
It adds: "We believe that the evidence shows conclusively that giving antibiotics to animals results in the emergence of some resistant bacteria which infects humans."