San Juan Bautista, Calif. ? A produce processing company at the center of the E. coli outbreak in spinach that has sickened at least 183 people, killing one, is starting to test a sample from each lot of greens it packages, hoping to prevent future outbreaks, the company’s chief executive officer said Thursday.

The ultimate source of the bacteria still is unknown. But government officials have found nine bags of Dole brand baby spinach that tested positive for the bacteria were packed by Natural Selection Foods LLC, and that the tainted greens were grown in Monterey, San Benito or Santa Clara counties.

Recent testing of Natural Selection Food’s facilities by the U.S. Food and Drug Administration and the California Department of Health Services showed the company’s processing plants are clean, said Natural Selection Foods CEO Charles Sweat.

Still, he announced the company already is implementing more stringent testing procedures, adopting a sampling system much like the one that helped reduce the number of human E. coli infections caused by beef.

“While we had food safety practices in place, we need to move to a new level of food safety,” he said.

In the meantime, the company is continuing to collaborate with federal and state investigators in their search for the source of the bacteria, Sweat said.

Results from tests being conducted in farms are not yet available. Tracing the tainted greens back to the individual fields is difficult because the product of different growers is mixed before being packaged, Sweat said.