THE TWORT-D'HÉRELLE PHENOMENON
Authors: Gratia André 
Published: June 2004

1. When the few individuals still alive in a dissolved culture of Bacillus coli are transplanted on slanted agar, a culture results which possesses new characteristics. First observed by Bordet and Ciuca, this culture received the temporary name of modified coli.
In the study described above, we found that this modified coli is very heterogeneous and that its three principal characteristics, resistance to lysis, lysogenic properties, and mucoid growth, are shared among different types of organisms that can be isolated when the normal original coli (coli O) is plated together with increasing quantities of the lytic agent: (a) a certain number of bacilli are just resistant enough to survive and grow in the presence of a moderate quantity of lytic agent, but they are still more or less sensitive and produce diseased, irregular, and lysogenic colonies; (b) a few of the organisms are able to resist concentrated lytic agent; they are entirely resistant and give round, healthy, and non-lysogenic colonies (coli O R 2); and (c) among these resistant bacilli only a very few are mucoid (coli 0 R 1). All these types are not motile and not fluorescent.
2. The original coli, when allowed to age, can be dissociated, as we have shown in a preceding paper (1), into two types of organisms, the non-motile coli S and the very motile coli R. Submitted to lysis, coli S gives a very small number, coli R a much greater number of resistant organisms (coli S R and coli R R), but both types never yield any mucoid growth.
3. An old culture of the modified coli obtained by Bordefand Ciuca, when streaked on agar plate, gives two types of colonies: a mucoid and fluorescent type (coli M 1) and a non-mucoid and translucent type (coli M 2). Both types are motile.
Coli M 2, once isolated, keeps its individuality even after several passages in artificial media, but if again submitted to the lytic agent, a great number of mucoid bacilli are found among the organisms which are still alive.
Consequently, different types of Bacillus coli differ greatly in their ability to give a mucoid growth when submitted to the lytic agent. Some, like coli S and coli R, do not possess this property at all. Others, like coli O, possess it to a certain extent, and some, like coli M 2, have it to a very high degree.
4. The mucoid and motile Bacillus coli M 1, when streaked every day on agar plates, remains indefinitely mucoid and motile, but occasionally a mucoid colony shows an indentation made up of non-mucoid growth, which, transplanted, gives a pure culture of non-mucoid and non-motile organisms, coli M 1 a. This new type possesses all the characteristics of the original strain of Bacillus coli, and therefore must be considered as a reversion.
5. The mucoid and motile Bacillus coli M 1, kept growing in synthetic medium, remains perfectly stable; on the other hand, when it is transplanted in broth, Bacillus coli M 1 turns very quickly into a non-mucoid but still very motile organism, or Bacillus coli M 1 b. This last type, which produces translucent colonies on agar and grows granular in broth, never reverts to the mucoid form, even in the presence of lytic agent.
6. A single strain of Bacillus coli has thus been made to yield eleven different forms, all distinguished by striking characteristics, but still possessing the specific properties of Bacillus coli.
Nine of these forms have been submitted to antisera prepared with three different types (Bacillus coli O, Bacillus coli S, and Bacillus coli R). While seven out of these nine strains were agglutinated by any of the three antisera, only the original Bacillus coli (Bacillus coli O) and the reversion to the original type (Bacillus coli M 1 a) were not agglutinable, even by their corresponding antiserum; i.e., the serum obtained from a rabbit immunized with Bacillus coli O, which, however, agglutinated the other types.