Hugh Hefner was battling E. coli and blood poisoning when he died of cardiac arrest and respiratory failure, the New York Post reports.

The 91-year-old Playboy founder’s death certificate revealed he had been battling the bacterial infection and septicemia (blood poisoning) for six days prior to his passing on Sept. 27, the Post said.

Hefner’s infection was “highly resistant to antibiotics,” according to the report.

He passed away in his famed mansion in Los Angeles, and laid to rest in a private memorial Saturday.