In 1884 Hans Christian Gram, a Danish bacteriologist, developed his special stain, which is composed of iodine and potassium iodide-arsenite after preliminary treatment with gentian-violet, whilst working with Carl Friedlander in Berlin. The iodide component of the double stain remains attached only to the bacterial cells after washing, thus serving to distinguish them from any background tissue material.

Gram and others noted that some bacteria, including Typhus, do not retain the distinguishing colour. Cf. Neisser. The Gram stain proved particularly suitable for medically pertinent organisms, which it separated into the two large groups of Gram-positive and Gram-negative. It is less useful for environmental bacteriology. Today, genetic sequencing is the latest technique used to identify bacteria.

Last modified 12 December 2016