Only two oral antibiotics and a few intravenous antibiotics remain effective in treating many of the infections caused by the new strains of E. coli, say British health officials.

The Health Protection Agency called for more research, saying there is no reliable estimate of the number of cases involved and it is not certain how the bug is transmitted.

E. coli are very common bacteria that normally live harmlessly in the gut, and are one of the most common causes of urinary tract infections.

More Resistant to Antibiotics

In a report to a health conference in Warwick, central England, the HPA said the new E. coli strains produce an enzyme called Extended-Spectrum Beta Lactamase, or ESBL, which makes them more resistant to antibiotics and therefore makes the infections harder to treat. In many cases, only two oral antibiotics and a few intravenous antibiotics remain effective.

"Voluntary national surveillance of blood poisoning [in England] caused by E. coli from 1994 to 2004 indicates a recent increase in the numbers of infections that are multi-resistant and therefore likely to be ESBL-producing strains," said Dr. Georgia Duckworth, who compiled the report.

"There is no comprehensive surveillance of urinary tract infections in the community so there is no reliable estimate of the number of infections caused by ESBL-producing E. coli strains in the community."