Erwinia, a genus of bacterial plant pathogens, has long been recognized as closely related to the Escherichia-Aerobacter group. This view, although readily accepted, has been the object of little comparative study; the relationship is usually taken for granted, the result being a relatively poor understanding of the similarities of the two groups. On the basis of its disease-producing ability in plants, Bergey et al. (1939) gives the group tribal ranking, Erwineae, in the family Enterobacteriaceae. Other than the obvious facts that the species of Erwinia are peritrichously flagellated, gram negative, do not form spores and ferment many carbohydrates, including lactose, our knowledge of the relationship is meager. The possibility that the Erwinial might be coliform organisms has been given comparatively little thought. Stuart et al. (1938, 1940) recognized the possibility of Erwinia cultures being confused with coliform organisms, or vice versa, and included some plant cultures in their studies. Stanley (1939) likewise declared the soft-rot group to belong to the colon-typhoid-dysentery group, and suggested that organisms identified as Erwinia might be Escherichia coli, the latter interpreted in a broad sense. According to Parr (1939), F. D. Chester stated that the genus Erwinia was established on a purely utilitarian basis and had no genetic standing. Dowson (1939), following the usual British practice, advocated the inclusion of the soft-rot pathogens in the genus Bacterium Lehmann and Neumann. By this procedure he recognized the apparent close relationship of the coliform and Erwinia groups, as the former are likewise placed in this genus. Unfortunately, the biochemical characteristics of the soft-rot organisms are as variable as those of coliform isolates (Stanley 1939, Elrod 1941). It has been shown, especially by Stanley, that fermentative shifts are common, tending to throw a culture from one species to another. These changes usually concern a change from an aerogenic to an anaerogenic state, although some variation has been noted in the methyl-red, Voges-Proskauer and citrate tests. Elrod (1941) found that the soft-rot group is as antigenicaily heterogeneous as the coliform group. In the light of our abundant knowledge of the coliform group, it was decided first to employ "coliform" methods in an attempt to arrive at a better understanding of the Erwinia-Escherichia-Aerobacter relationship. The recent work of Stuart, Mickle and Borman (1940) on aberrant coliforms offers a way of 1 The term Erwinia in this paper is taken to mean the so-called soft-rot group. This group includes: Erwinia carotovora (Jones) Holland, Erwinia aroideae (Townsend) Holland, Erwinia solanisapra (Harrison) Holland and Erwinia phytophthora (Appel) Bergey et al.
