Well, lots of researchers manipulate microbes in various ways in the lab. They delete genes. They make mutants They insert genes. Sometimes, they insert antibiotic resistance genes to help with the genetic manipulations they are doing.
Do researchers always think about the potential risks of what they are doing? Well, probably not. Most of the time that is OK as the risks are negligible. But some of the time, there are real risks to consider. One example of a real risk is the introduction into some pathogen of genes encoding a form of antibiotic resistance not seen normally in that pathogen. If that strain escapes from the lab, it could, in theory, spread into the real world and make treating infections by that pathogen more difficult.
All Things Considered had a very interesting story on "Making Drug-Resistant Germs In The Lab" about exactly this issue a few days ago where they discussed how one researcher submitted to an NIH oversight panel a request to carry out this type of experiment. It seems as though very few researchers actually submit requests to carry out these experiments, even though many are doing it. NPR also discussed how the CDC reviews requests to manipulate certain really nasty pathogens and that most of the requests have been granted. Unfortunately, I cannot find a transcript for this story to quote, but it is really worth listening to.