One of Koch's first photographs of Bacillus anthracis

(x700) Bacillus anthracis, from infected spleen. A thin layer of tissue was allowed to dry on a coverslip; then the preparation was stained with aniline brown and immersed in glycerol. The technique causes the red blood cells to lose their color, whereas the bacilli and the nuclei of the white blood corpuscles were colored brown. The blood corpuscles appear in the photograph as pale circles, the nuclei of the white blood corpuscles appear rather dark, and the bacilli, because they are colored mostly brown, are extremely strong and dark. The bacilli are clearly articulated and appear truncated rather than rounded at the end. "This truncated and punctured appearance, as assumed by the dried and stained anthrax bacterium, is so characteristic of it that it may be used in the diagnosis of anthrax with perfect certainty. Indeed, a few months ago, in a man who had two days earlier from anthrax in the form of a diffuse swelling on the left side of the neck, the correct diagnosis could be made by the discovery of some bacilli which had this characteristic mark, which latter was confirmed by successful inoculation of the anthrax substance on animals." (Source: Koch R. Verfahren zur Untersuchung, zum Conservieren und Photographiren der Bakterien. Beiträge zur Biologie der Pflanzen 1877;2:399-434.)