A week after grilling hamburgers in his backyard in November 2011, business consultant Kenneth Koehler became violently ill. He suffered stomach pains, diarrhea and nausea – and rushed to the hospital emergency room in Biddeford, Maine.

Days later, his doctors told him that his burger was contaminated with Salmonella Typhirium, a strain commonly found in ground beef.

But Koehler’s salmonella was more dangerous than he realized. Records provided to Reuters by Koehler showed that the salmonella strain in the ground beef was resistant to nine types of antibiotics. Three of the antibiotics that didn’t work were cephalosporins, including ceftriaxone, according to the U.S. Centers for Disease Control and Prevention records.

Neither Koehler, 56, nor his doctors know for certain whether ceftiofur had been administered to any of the cows whose meat was contaminated. But they knew the drugs that his salmonella strain were resisting. That’s because he was among the last of 19 people from seven states sickened in the outbreak, according to Koehler and the CDC records.

Based on the CDC’s testing of the salmonella strain, Koehler said, his doctors already determined ceftriaxone wouldn’t work. Instead, they prescribed ciprofloxacin, a powerful antibiotic in a different drug category. In humans, about 3 percent of all salmonella samples tested in 2012 by the CDC were resistant to ceftriaxone.

“They went directly to cipro,” Koehler said of his doctors. “To put it bluntly, this salmonella really kicked my butt.”

Investigators for the Maine branch of the CDC tested the leftover beef in Koehler’s freezer. The tests showed the source of his salmonella was ground beef bought at a Hannaford Bros. supermarket in Saco, Maine. Hannaford declined to comment.

Koehler was fortunate. He was treated and released the same day. Eight other people with ceftriaxone-resistant salmonella were hospitalized in the same outbreak, according to the CDC records.