In the present paper and the preceding two reports
(1, 2) the author has clarified and described the main features of the selective effects of the desoxycholate-citrate medium. The following characteristics were shown by coli and dysentery strains:
1) the decrease in the number of viable enteric bacteria grown in the desoxycholate-citrate medium was most remarkable in coli strains,
2) the decrease in the oxygen consumption paralleled the decrease of viable cells,
3) the decrease of the stainability of bacterial cells with basic, nucleic acidstaining dyes (or the decrease of the number of normally staining cells) was proportional to the above two decreases,
4) nucleic acids (probably accompanied with other cell constituents) were actually released into the medium,
5) citrate promoted the above effects of desoxycholate, and
6) the effect of citrate was counteracted by magnesium ion (or by manganese ion) which restored the growth suppressed by the desoxycholate-citrate medium.
Among these, details of nucleic acids released into the desoxycholate-citrate medium, and observations on the nutritional requirements of enteric organisms grown on desoxycholate-containing media are reported in this paper. The action of desoxycholate and related substances and its relation to bacterial activity are discussed in connection with a possible mechanism of the selective effect of this medium.
In conclusion, the principal feature of the selection mechanism of the desoxychorate-citrate medium is considered to depend on or to have a close relationship to the nucleic acid-extracting or the nucleoprotein-splitting action of desoxycholate, which strips off the superficial cell constituents, forcing bacteria to survive under critical conditions. The difference among species and strains is presumed to result in the selective growth of shigellas in combination with the action of citrate and other nutritional condition of basal media.
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