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, and geographic names, and words derived from them, are often lowercased when used in a nonliteral sense or when they enter common usage,” as in “dutch oven” or “french fries” (see CMOS 8.62). 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.