We certainly know that multidrug-resistent bacteria cause harm. Plenty of case studies there. I'm no biologist, so I don't know whether or not we can show, for

We certainly know that multidrug-resistent bacteria cause harm. Plenty of case studies there. I'm no biologist, so I don't know whether or not we can show, for any given strain of multidrug-resistent bacteria found in a sick patient, whether its ancestors most likely acquired its resistance genes while living in a farm animal, while living in an adult human who was taking antibiotics for some other condition, or someplace else. My guess is that such a demonstration might be possible -- cheap gene sequencing, plus extensive statistics, yields a lot of deductive power -- but I don't know if it has been done already or not, and maybe I'm wrong and it's going to be impossible to do conclusively. Bacterial genomes aren't always that big, and bacteria are quite promiscuous with genetic material, so their line of family descent gets fuzzy much faster than it does with multicellular creatures like ourselves. One thing is certain: There will be a lot more work attempting to answer these questions.