AbstractIn 1884, Hans Christian Gram, a Danish doctor, developed a differential staining technique that is still the cornerstone of bacterial identification and taxonomic division. This multistep, sequential staining protocol separates bacteria into four groups based on cell morphology and cell wall structure: Gram‐positive cocci, Gram‐negative cocci, Gram‐positive rods, and Gram‐negative rods. The Gram stain is useful for assessing bacterial contamination of tissue culture samples or for examining the Gram stain status and morphological features of bacteria isolated from mixed or isolated bacterial cultures. Curr. Protoc. Microbiol. 15:A.3C.1‐A.3C.8. © 2009 by John Wiley & Sons, Inc.