(CNN) Nearly a dozen Americans who had surgery in Tijuana, Mexico, have returned home with dangerous antibiotic-resistant infections, the US Centers for Disease Control and Prevention said Wednesday.

All of the travelers diagnosed with the life-threatening bacteria, carbapenem-resistant Pseudomonas aeruginosa or CRPA, had undergone invasive medical procedures.

"Infections caused by this particular drug-resistant Pseudomonas are rare in the United States and difficult to treat," the CDC said. Patients typically require long and "complex antibacterial drug combinations and courses" to defeat the infection.

The CDC has confirmed 11 cases, said lead investigator Dr. David Ham, medical officer for the CDC's Division of Healthcare Quality Promotion. Nine occurred between August and November; a case from 2015 was retrospectively identified, and one case is still under investigation.

Nearly half of the infected patients had undergone surgery at Grand View Hospital in Tijuana; the rest had undergone surgeries at other hospitals and clinics in the area. Ten of the procedures were weight-loss surgeries, Ham said.

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