Mechanism of the Desoxycholate‐citrate Medium for the Isolation of Dysentery Bacilli

Abstract

In the preceding paper(1), the author described the selection mechanism played by desoxycholate for the isolation of dysentery organis. When colon and dysentery bacilli were suspended in a liquid medium which contained desoxycholate and citrate and were incubated at 37°C for a few hours, a marked difference was found between number of surviving colon bacilli and dysentery organisms. The number of viable coli decreased to one-tenth or to one-twentieth of the initial cell count, but that of dysentery bacilli decreased only to one half of the initial count. The oxygen consumption of the cell suspensions was also suppressed proportionately to the cell count. In accord with these observations it was noted that the stainability of the incubated cells with nucleic acid staining, basic dyes such as methyl green or thionine dyes decreased far more in the case of coli than in dysentery bacilli, and that nucleic acids were actually released into the medium in which the cells were incubated. From these findings, the mechanism of bacterial selection observed with desoxycholate is considered, at least in its principal features, to depend on its nucleic acid-extracting action and . the resulting bactericidal effect of desoxycholate, and on the difference of these effects among species and strains of enteric bacteria.In the present paper the author reports studies on the chemical determination of nucleic acids released into the medium. Some features on the extracting action of desoxycholate and related substances, and the relation of this bile salt action to bacterial activity are discussed in connection with their possible contribution to the selective effect. MATERIALS AND METHODSStrains used. The strains described in the previous papers(1, 2) were used also for the present studies.Media. An improved synthetic medium was used. It consisted of (except otherwise indicated, expressed in terms of M/l) : L-sodium glutamate, 0.01 ; L-aspartic acid, 0.005 ; I,-cystine, 0.0005 ; DL-tryptophan, 0.001 (0.0005 as Lisomer) ; DL-methionine, 0.001 (0.0005 as L-isomer) ; DL-isoleucine, 0.001 (0.0005