Papillary variation includes all heritable changes of character which take place in the papillae of bacterial colonies and the corresponding variations in liquid media.

The following is a short summary of what is known about these variations. Papillary variation was first observed in B. coli-mutabile, and other coliform bacteria show the same variations. This race forms white colonies on lactose neutral-red plates. After 1–5 days’ incubation minute papillae, which become red and increase in size, appear on their surfaces. Subcultures from thesepapillae give colonies of two kinds, white and red. The white colonies continue to throw red, the reds breed true (Neisser, 1906; Massim, 1907). Similar variation to different sugars occurs in other related bacteria: B. dysentenae Sonne varies to lactose, B. dysentenae Mexner to maltose, B. paracolon to saccharose, B. coh-mutabile, B. acidi-lactici and B. typhosus to dulcite (Twort, 1907; Reiner Miiller, 1909; Burn, 1910; Stewart, 1926, 1927). Vacation takes place only on the sugar concerned, but, once it has taken place, the character remains fixed, even after long growth on other media. Variation is not determined by length of exposure to the sugar. It occurs at (or after) the end of the logarithmic phase. If this phase is kept going by daily subculture (in the appropriate sugar) the bacteria of the chain carried on from culture to culture do not vary, however long they are exposed (Stewart, 1927; Deere, Dulaney & Michelson, 1939). On the other hand, if the logarithmic phase is curtailed by heavy inoculation of a culture, variation occurs early (Stewart, 1927). If the bacterium is grown on a plate without sugar until the colonies are well formed and the sugar is then added, with care not to disturb the colonies, variants appear after very short exposure (Stewart, 1927, 1928). A variation 33–2 may be completed in, two distinct steps; thus a race of B. typhosus formed white colonies on dulcite with pink papillae, subcultures from which gave white and pink colonies; the pink colonies formed dark red papillae from which on subculture came pink and dark red colonies, the latter bred true.