The largest U.S. grocery chain, Kroger Co., is voluntarily recalling all ground beef products sold in Michigan and certain Ohio stores between May 21 and June 8 after the meat was linked to an outbreak of illness from E. coli bacteria.

Testing has confirmed beef samples contain a strain of the bacteria that has sickened at least 19 people in Ohio. Another 15 have been sickened in Michigan.

The recall comes after the U.S. Centers for Disease Control and Prevention (CDC) said on Tuesday that outbreaks of ground beef-related E. coli bacteria in Michigan and Ohio were linked.

The recalled products include all varieties and weights of ground beef products bearing a Kroger label sold between May 21st and June 8th at Michigan and Columbus and Toledo, OH, Kroger retail establishments.

Consumers are urged to check their refrigerators and freezers and discard or return the ground beef products for a refund, officials warned. The officials also urge people to thoroughly cook hamburger and, if possible, use a digital thermometer to see that the meat is cooked to at least 160 degrees.

E.coli is a potentially deadly bacterium that can cause bloody diarrhea, dehydration, and in the most severe cases, kidney failure. The very young, seniors and persons with weak immune systems are the most susceptible to foodborne illness.

Symptoms can include stomach cramps, diarrhea and vomiting. Most people get better within 15 days, but some cases can lead to kidney failure.