Somehow, despite extensive searching, I cannot find an answer....

Does O stand for oxygen and H signify hydrogen?


The "H" and "O" identifiers first appeared in German-language publications around 1920 to describe different forms of disease-causing bacilli. Thankfully, Arkwright and Goyle provide an English explanation of the terms in their 1924 review.1

These writers called the two forms, which they described, the "H" and the "O" form, because in the case of B. proteus, the first bacillus investigated, the "H" form was represented by the normal, which grew with a spreading film (Hauch) on the surface of agar, whilst the "O" form, which occurred as a variant, grew in isolated colonies without any film (ohne Hauch).

The literal translation of Hauch is "breath", and ohne Hauch is "without breath", though some sources translate Hauch as "thin layer", which is probably the intended meaning.


  1. Arkwright JA, Goyle AN. The Relation of the “Smooth” and “Rough” Forms of Intestinal Bacteria to the “O” and “H” Forms of Weil and Felix. Br J Exp Pathol. 1924;5(2):104-114.
  • 2
    $\begingroup$ As a German speaker, "Hauch" idiomatically means something like "a touch of ...", so "thin layer" is indeed an appropriate translation in this context. $\endgroup$ Dec 4 '21 at 15:32

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