Three antibiotics previously thought to be useless against MRSA infections may be able to kill the pathogen when used together in humans, according to new research.

Investigators from Washington University in St. Louis say the drugs — meropenem, piperacillin and tazobactam — belong to a class of antibiotics that hasn’t been effective against the staph infection for decades. But when used together, the three proved effective at killing MRSA in cultures and in mice.

“MRSA infections kill 11,000 people each year in the United States, and the pathogen is considered one of the world’s worst drug-resistant microbes,” said lead researcher Gautam Dantas, Ph.D., an associate professor of pathology and immunology.

Researchers also found that the three-drug combo did not produce antibiotic resistance. They added that they have plans to study whether other antibiotics thought to be ineffective against bacterial pathogens can be combined to create effective treatments.

“We know all bacteria eventually develop resistance to antibiotics, but this trio buys us some time, potentially a significant amount of time,” Dantas said.