Photographic gelatin is one the world's most purified products. Eastman Gelatine has been able to transition to supplying the pharmaceutical industry because the purity requirements for photography meet or exceed medical and food standards. A bioassay is provided to prove that a batch of gelatin meets stringent industry standards -- i.e. no contaminants found.
Give it a moment's thought. If gelatin grew a bunch of evil organisms when it got wet, how could gelatin be used as a culture medium in a biology lab? If there were biological contaminants, they would rapidly overgrow the desired culture. But no, if you practice sterile inoculation technique, you are almost guaranteed a pure culture. No tuberculosis, staph, strep, or E.coli. I've grown things in a petri dish of gelatin that you don't want to know about, but it wasn't by accident. We don't have to fear gelatin.