The Antagonism of Coliform Bacteria Against Shigellae

Abstract

A total of 1,243 strains of coliform organisms, isolated by rectal culture from Latin-American children in South Texas, were tested for their antagonistic properties on solid medium against an arbitrarily chosen Shigella paradysenteriae Flexner type III (Z). Of these, 18.3 per cent possessed some antagonistic property. As indicated by identification tests with a random sample of these organisms, most were probably members of the Escherichia coli group. These strains were obtained from 147 individuals. A total of 37, or 25 per cent harbored one or more antagonistic coliform organisms. The antibacterial spectrum of these organisms is remarkably limited. Of a wide range of bacteria tested for susceptibility to sample coliform strains, only 38 of 58 strains of Shigella tested proved to be moderately or greatly affected, and 9 out of 15 strains of E. coli were shown to be moderately affected. Of the Shigella strains studied for susceptibility, a definite tendency was revealed toward a relationship between susceptibility and type specificity. The mechanism of the production of the zones of inhibition was shown to be due to antibiotic production in the instances tested. The zones of inhibition were not affected by rabbit and human blood or serum in concentrations of 5 to 25 per cent. A single case of acute diarrheal disease (of unknown etiology) was studied with reference to the antagonistic coliforms. During the course of the illness, there was a profound change in the coliform flora, in the direction of the development of organisms which showed a great degree of antagonism against Shigella Flexner type III (Z). This shift in the character of the flora paralleled the process of recovery. The possible significance of the findings is discussed.