DES MOINES — A Des Moines hospital is alerting about 2,600 people who had open-heart surgery there from 2012 to 2015 that they might have been exposed to a bacterial infection. Two of the former Mercy Medical Center patients have been diagnosed with an infection with the germs, called nontuberculous mycobacteria. The germ is described as “a cousin to tuberculosis, but it’s much less potent.”