It is often discussed whether or not to use commas between keywords in the meta keyword tag. Most search engines will not care as they remove the commas before reading the keywords, but some search engines might still use the comma to find exact keyword and keyword phrase matches.
Some people use commas because it makes it easier to read in the raw code. This can be helpful if you want to edit the META-tags by hand at a later time.
But the meta keyword tag is no longer in use!
In any case, the META-keyword tag does not carry any weight in the overall ranking algorithm for any of the major search engines, as Matt Cutts official stated back in 2009. And you will never get penalized for either using or not using commas.