The Gram stain was developed more than one hundred years ago by Hans Christian Gram, a Danish microbiologist. This technique is still widely practised in clinical microbiology laboratories today. The Gram stain differentiates bacteria into two large categories on the basis of differences in the structure of their cell wall – Gram positive bacteria appear purple and Gram negative bacteria appear pink/red. Many antibiotics may only have activity against Gram positive bacteria while others only have activity against Gram negative bacteria. The Gram stain can be performed within minutes of the collection of a specimen. Therefore knowledge of the appearance of the bacteria when stained using Gram stain can be used by a physician as rationale for their initial choice of antibiotic.