Antibiotic interrelationships among the enteric group of bacteria.

Instances of antagonism among the enteric group of bacteria have been frequently reported in the literature, but these are concerned primarily with mixed cultures in which many different factors, difficult to analyze, are involved. In 1925 Gratia observed a strain of Escherichia coli which produced a diffusible and thermolabile antibiotic substance. He called this substance "principe V." This "principe V" seemed to be specific and at the time appeared to be active against only one other strain of Escherichia coli and Shigella dysenteriae. Later, Gratia and Fredericq (1947) found that antibiotic strains of E. coli are not at all infrequent, but that the antibiotic spectrum of the different strains was characteristically different and distinct. The antibiotic substances produced, to which was assigned the term "colicin" (Gratia and Fredericq, 1947), differ not only in their antibiotic spectra but also in their physiochemical properties (Gratia and Fredericq, 1946). It has been observed that some strains may produce several colicins with characteristically different antibiotic spectra. It was furthermore observed that practically every member of the Enterobactericceae studied, including the strains which themselves produced antibiotic colicins, was sensitive to one or more colicins produced by some other member of the Enterobacteriaccae. In the present study of antibiotic interrelationships among members of the enteric group of bacteria we employed the following technique (Fredericq, 1946): A small (about 1-mm) loop of a broth culture of a strain to be tested for production of antibiotics was stabbed on previously poured and dried peptone agar plates. After incubation for 48 hours at 37 C the culture was killed by exposure to chloroform vapors for a period of about 1 hour, and the chloroform was then allowed to evaporate. The entire surface of this agar plate was then inoculated with a strain being observed for sensitivity. A convenient procedure for this purpose consisted of covering the surface of the medium with a sterile filter paper and then inoculating this filter paper wvith a ml or two of a culture of the organism being studied for sensitivity. After contact for several minutes the filter paper was removed and the plate incubated for 24 hours at 37 C. If an antibiotic was produced, growth of a sensitive strain wN-as inhibited in a circular zone in the vicinity of the stab but not restricted on the rest of the plate, as may be seen in figure 1. It will be observed that twro strains were markedly antibiotic, one only very slightly, and the fourth showed no evidence of antibiosis against a strain of Escherichia being tested for sensitivity. The antagonistic action under consideration appears to be quite different