Medical researchers have hit upon a new idea for combating the growing problem of antibiotic resistance, an ability that many disease-causing bacteria acquire, often confounding attempts to treat infection.

Antibiotic resistance results when bacteria undergo a form of evolution in which the drug kills all the vulnerable individuals, leaving alive one that happens to be resistant. Without competition from the other bacteria, the resistant organism is free to multiply unchecked.

Bacteriologists have long known that resistance is conferred by certain genes carried on rings of deoxyribonucleic acid known as plasmids. The genes work differently in different cases but one common type directs the bacterium to make an enzyme that breaks down antibiotic molecules.

Scientists had generally assumed that resistance to a given antibiotic emerged in many places, but new findings suggest that it may happen only once and then spread widely.

A seven-person team, led by Thomas F. O'Brien of Harvard Medical School, has reported in Science that resistance to gentamicin arose once, probably in the early 1970s, and subsequently spread to at least five very different kinds of bacteria all over the United States and Venezuela.

It is known that plasmids can move from one bacterium to another, crossing not only from one species to another but also from one genus to another. The findings are based on the fact that plasmids bearing the gene were found to be the same in five genera of bacteria isolated from 13 widely separated locales.

The researchers propose that efforts be made to spot newly emergent resistance genes so that attempts can be made to block its spread.