What is more emblematic of our science than the Gram stain? Since its invention 130 years ago, it has been in frequent and continuous use. It conveniently places most bacteria into one of two groups, the Gram-positives or the Gram-negatives. Gram staining is cheap, effective, quick, and relatively easy to interpret. Its most useful application is in the clinical setting. When examining a smear of, say, pus from an abscess, this stain often allows to include for consideration roughly half the clinically relevant bacterial species while excluding the others. Or consider a patient with meningitis. Here, speed is of the essence because treatment must be initiated right away. A Gram stain of the spinal fluid may reveal within minutes the presence of Gram-positive cocci, (probably pneumococci), Gram-negative cocci (almost certainly meningococci), or Gram-negative slender rods (most likely Haemophilus influenzae). This can make a critical difference in the choice of antibiotics that have to be administered in great haste. However, when it comes to characterizing the bacteria in an environment, its usefulness diminishes, in part because it is not always in step with taxonomy, which I’ll discuss below. And there yet is another side to this story. Quite a few bacterial species that stain positive early in the growth of a culture become Gram-negative later on. Does this detract from the value of Gram’s method? It may, but not in the hands of a person experienced in its use.