I have made gram controls by injecting tissue a number of times.  I like to use lung because it is full of small voids where the injected material can collect without damaging the tissue.  If you inject into a more solid tissue like liver or kidney or muscle, the injection necessarily forces the tissue apart and creates one large void.  I purchased my cell cultures from a biological supply company like www.ctvalleybio.com . They offer a good variety of forms (cocci, bacilli) in both gram+ and gram- types, so you can pretty much custom design your control.  Make sure you get cultures grown on broth, not plates.  Spin the culture down to concentrate the organisms for injection.  Inject the tissue fresh (unfixed).  Push the needle almost all the way through, then slowly inject as you withdraw the needle. Then just drop the tissue into formalin.  Works very well.

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