May 28 (HealthDay News) -- The number of Americans hospitalized with the dangerous intestinal superbug Clostridium difficile has been increasing by more than 10,000 a year, and the germ was a factor in nearly 300,000 hospitalizations in 2005, more than double the number in 2000, a new study says.

Lead author Dr. Marya Zilberberg, of the University of Massachusetts, and colleagues looked at more than 36 million annual discharges from non-governmental U.S. hospitals to create national estimates for C. difficile cases, the Associated Press reported.

The researchers also concluded that 2.3 percent of C. difficile cases in 2004 were fatal (about 5,500 deaths), nearly double the percentage of C. difficile cases that were fatal in 2000.

C. difficile has developed resistance to some antibiotics and has become a common threat in hospitals and nursing homes.

"The nature of this infection is changing. It's more severe," Dr. L. Clifford McDonald, of the U.S. Centers for Disease Control and Prevention, told the AP. He was not involved in the study, which is published in the June issue of the journal Emerging Infectious Diseases.