For nearly two decades, Public Enemy No. 1 for the food industry and its government regulators has been a virulent strain of E. coli bacteria that has killed hundreds of people, sickened thousands and prompted the recall of millions of pounds of hamburger, spinach and other foods.

But as everyone focused on controlling that particular bacterium, known as E. coli O157:H7, the six rarer strains of toxic E. coli were largely ignored.

Collectively, those other strains are now emerging as a serious threat to food safety. In April, romaine lettuce tainted with one of them sickened at least 26 people in five states, including three teenagers who suffered kidney failure.

Although the federal government and the beef and produce industries have known about the risk posed by these other dangerous bacteria for years, regulators have taken few concrete steps to directly address it or even measure the scope of the problem.

For three years, the United States Department of Agriculture has been considering whether to make it illegal to sell ground beef tainted with the six lesser-known E. coli strains, which would give them the same outlaw status as their more famous cousin. The meat industry has resisted the idea, arguing that it takes other steps to keep E. coli out of the beef supply and that no outbreak involving the rarer strains has been definitively tied to beef.

The severity of the April outbreak is spurring a reassessment.

“This is something that we really have to look at,” said Senator Kirsten Gillibrand, Democrat of New York, who plans to introduce a bill that would pre-empt the Agriculture Department by declaring a broad range of disease-causing E. coli to be illegal in ground beef and requiring the meat industry to begin testing for the microbes. “How many people do we have to see die or become seriously ill because of food poisoning?”