For years, safety officials have focused on E. coli; the bacteria has killed hundreds of people and caused the recall of millions of pounds of hamburger meat, spinach and other foods. But officials have typically zeroed in on one particular strain — E. coli O157:H7.  Now, health experts are starting to worry about six other strains of the dangerous bacteria, the New York Times reports.

One of those six strains was found in romaine lettuce and caused an outbreak in April.  At least 26 people in five states became sick and three teenagers suffered kidney failure.

The lettuce from that outbreak tested negative for E. coli O157 but was not tested for the other strains.  Not many food companies test for the six strains and only about 5 percent of medical labs are able to identify them in tests.

E. coli bacteria originate inside cattle and probably travel to farms in manure.  It is illegal to sell ground beef infected with E. coli O157, ever since an outbreak caused by Jack-in-the-Box hamburgers killed four children in 1993. The Department of Agriculture is considering making the other six strains illegal also, but the cattle industry has resisted.

The Centers for Disease Control and Prevention identified 10 outbreaks between 1990 and 2008 involving the six strains, but officials say it is likely that cases are vastly underreported. Agriculture officials expect to have an efficient way to test for the other strains by the end of 2011 and are reluctant to outlaw them as they have E. coli O157 before the test is developed.