gram-negative [Hans C.J. Gram, Danish physician, 1853–1938; L, negare, to say no] , having the pink color of the counterstain used in Gram’s method of staining microorganisms. This property is a primary method of characterizing organisms in microbiology. Some of the most common gram-negative pathogenic bacteria are Bacteroides fragilis, Salmonella typhi, Shigella dysenteriae, and Yersinia pestis.
