I have made my own gram controls in the past. I purchase liquid bacterial cultures of gram+ and gram- organisms from a biological supply company (http://www.ctvalleybio.com), spin them down to concentrate the organisms, then inject the concentrated cultures into a fresh piece of lung, gram+ at one end of the tissue, gram- at the other end, or separate pieces for gram+ and gram-. I use lung because there is plenty of empty space to inject into. If you use something like liver, there is no place for the injection to go except by forcing the tissue apart. I put the needle in to about the midpoint of the tissue, then withdraw it slowly as I inject, to distribute the culture through the tissue. Then just drop the tissue into formalin and process as usual. The company has quite a variety of organisms for less than $10 per culture, so you can choose bacilli vs. cocci in various configurations. If you really want to get fancy you can have gram+ bacilli and gram+ cocci at one end of the tissue, gram- bacilli and gram- cocci at the other end.
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