Virulent bugs are spreading outside hospitals and inside the community and may put lives at risk, experts say.
MRSA is not usually associated with youngsters
They want doctors to be alert to a potentially lethal form of MRSA which can infect the lungs, and may strike young people in particular.
At the same time, Irish researchers say a drug-resistant bug behind bladder infections is becoming widespread.
The findings are being presented at the Federation of Infections Societies Conference in Cardiff.
Panton Valentine leukocidin (PVL) strains of community-acquired MRSA can cause a condition called necrotizing pneumonia, which destroys lung tissue.
This only affects a minority of those infected, but can be deadly.
The condition is spread outside of hospitals via skin-to-skin contact and appears as sores which look like insect bites. In the very worst cases, it can kill in a day.
"These new strains of bacteria appear to be able to stick to damaged skin and airways better than the hospital MRSA strains, and they can also multiply at a faster rate," says Dr Marina Morgan, of the Royal Devon and Exeter Foundation NHS Trust.
So far these strains are mainly spreading in the US, where 12% of all MRSA cases are community-acquired, but the UK has seen an increasing number of cases.
It is unclear why children seem to be at particular risk, but the speculation is that older people in the community have fewer cuts and abrasions - a key transmission route - and have less contact with other people.
Professor Kevin Kerr, consultant microbiologist at Harrogate District Hospital, said: "MRSA is often thought as a hospital superbug, but we are becoming increasingly aware of strains which are causing infections outside hospital.
"The emergence of community MRSA underlines just how good bacteria are at evolving to present us with new and difficult problems to solve."
The Department of Health noted the condition was treatable, and that it was currently trying to establish prevalence.
"Clinicians have already been asked to be extra vigilant and report cases direct to the Health Protection Agency," a spokesperson said.
Nursing home threat
Meanwhile, Irish researchers are examining a new breed of bacteria which carry enzymes called extended spectrum beta lactamases (ESBLs), which are capable of destroying a many common antibiotics.
They include a strain of E. Coli, which is spreading into nursing homes and communities across Europe.
This was held responsible for a severe outbreak of cystitis, a bladder infection, in the UK between 2003 and 2004.
"Although cystitis is not life threatening, it is the most common form of urinary tract infection, and the economic consequences of failing to treat an outbreak quickly and properly are considerable," said Dr Dearbhaile Morris, of the National University of Ireland.
"In severe infections, patients may suffer serious complications if the first antibiotic given to them does not work."
Mark Enright, professor of molecular epidemiology at Imperial College, said he was "not surprised" by the findings.
"The emergence and spread of ESBL E. Coli does give physicians problems in providing proper initial care for some patients especially those with urinary tract infections."
He added: "The control of infections in many nursing homes is inferior to hospitals despite the medication and specialist care required by some residents."