Gram stain The Gram stain does the following: Classifies bacteria according to whether they retain crystal violet stain (gram-positive—blue) or not (gram-negative—red) Highlights cell morphology (eg, bacilli, cocci) and cell arrangement (eg, clumps, chains, diploids) Identifies polymorphonuclear leukocytes, indicating bacterial infection rather than colonization Such characteristics can direct antibiotic therapy pending definitive identification.