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?”

The issue will be one of the first faced by President Obama’s nominee to head the department’s food safety division, Dr. Elisabeth Hagen, who is scheduled to testify Thursday in her Senate confirmation hearing.

Part of the problem is that so little is known about the rarer E. coli strains, which have been called the “big six” by public health experts. (The term refers to the fact that, after the O157 strain, these six strains are the most virulent of a group of related E. coli.) Few food companies test their products for the six strains, many doctors do not look for them and only about 5 percent of medical labs are equipped to diagnose them in sick patients.

A physiological quirk of E. coli O157 makes it easy to test for in the lab, and many types of food are screened for it. The other E. coli strains are much harder to identify and testing can be time-consuming. The Agriculture Department has been working to develop tests that could be used in meat plants to rapidly detect the pathogens.