Endocarditis due to Citrobacter diversus developing resistance to cephalothin.

A 43-year-old man was admitted with acute bacterial endocarditis. Citrobacter diversus susceptible to cephalothin was isolated from blood cultures. Citrobacter diversus was later isolated from the aortic valve cusps at surgery, but this isolate was resistant to cephalothin. Laboratory testing showed that the Citrobacter diversus recovered from blood cultures was capable of producing mutants highly resistant to cephalothin.