(i) Amoebae closely resembling Entamoeba coli (of Man) have been studied in macaques (M. rhesus, M. sinicus, M. nemestrinus) and in a marmoset (Hapale jacchus).
(ii) Pure strains of these amoebae have been isolated and cultivated, and carefully compared with similar pure strains of E. coli from Man (E. coli hominis).
(iii) The complete life-cycles of these various strains—amoebae, cysts, and all stages of precystic and metacystic development—have been studied in vitro.
(iv) The “E. coli” of macaques (E. c. macacorum) was experimentally transmitted to a man, who ingested pure culture-cysts of a strain derived from M. rhesus.
(v) The “E. coli” of the marmoset (E. c. jacchi) was likewise transferred to M. sinicus, and from this host again to M. rhesus and Man.
(vi) The natural infections observed in M. sinicus, M. rhesus, and H. jacchus all died out spontaneously after persisting for various periods (4½ years, 2 years, and 1 year respectively). The experimental infection with E. coli macacorum induced in Man also disappeared spontaneously (after about 6 weeks).
(vii) Attempts to infect kittens experimentally with E. coli jacchi were uniformly unsuccessful.
(viii) All infections with “E. coli” from every source—whether natural or experimental—have proved to be completely innocuous.
(ix) Culture-cysts of various strains of “E. coli” have been found able to live at low temperatures for all times up to about 4½ months (maximum 135 days at 1°–2° C., for a strain from H. jacchus).
(x) The “E. coli” of H. jacchus—and all its experimental derivatives—ingests human red blood-corpuscles in vitro as readily as E. histolytica.
(xi) Several strains of E. coli hominis, isolated directly from naturally infected human beings, have been found to possess the same property.
(xii) Since strains of E. coli thus exist—both in men and in monkeys—which eat human red corpuscles avidly, this faculty can no longer be considered as peculiarly characteristic of E. histolytica; and the importance of this observation, for the diagnosis of amoebic dysentery, is therefore emphasized. The statement that E. histolytica ingests red corpuscles, while E. coli does not, is untrue. It is not possible to distinguish these species by this character alone.
(xiii) As it has been found impossible to distinguish various strains of “E. coli” living in macaques, a marmoset, and men, by any recognizable specific feature—all of them being so closely alike morphologically, physiologically, culturally, and in their ability to inhabit these different hosts—it is concluded that no specific difference actually exists between any of them. The “E. coli” of the Macaques and of the Common Marmoset is probably Entamoeba coli itself—the species which naturally inhabits Man.