Subscribe to The Chicago Manual of Style Online
Q. When one makes an adjective out of a proper name, does one retain the capital letter? For example, should “gram-negative,” the adjective describing a bacterium showing a certain result on Gram’s test, actually be written “Gram-negative” (as my spellchecker seems to “think”)?
A. Many such terms do take initial upper case. CMOS notes, however, that “personal, national, or geographical names, and words derived from such names, are often lowercased when used with a nonliteral meaning,” as in “dutch oven” or “french fries.” In the case of “gram-negative,” Merriam-Webster supports you by lowercasing, although a brief online survey suggests that uppercasing is also accepted by authorities.