MIICROBIOLOGICAL DETER1\IINATION OF DEOXYRIBoNUCLEIC ACID TITRIMETRIC ESTIMATION OF THYMIDINE SQREN

One of the most sensitive and specific methods for quantitative estimation of deoxyribonucleic acid (DNA) is the microbiological one developed by Hoff-J0rgensen (1951, 1954). This method is l)ased on the fact that Thermobacterium acidophilum (Lactobacillus acidophilus) strain R 26 Orla Jensen is unable to grow in the absence of deoxyribosides. The DNA containing sample is digested with deoxyribonuclease, and a portion is adldled to a medium which is complete, except for deoxyribosides. The chemically defined medium of L0vtrup and Roos (1957) may be used. After inoculation and incubation the growth response is estimated turbidimetrically. About 1 j,g of DNA is needed for this method. If further sensitivity is required, the response must be determined in a different way. It seems that the method worked out by Lowry and Bessey (1944) for microdetermination of riboflavin could be useful. In this method, the growth of the bacteria is estimated by microtitration of the lactic acid produced by the cells. The present paper contains a description of a method for determination of thymidine (which is taken to represent all deoxyribosides) in which the principles of Lowry and Bessey have been adapted for use with our bacterial strain. It will be shown that it has been possible in this modification to increase the sensitivity of the method about 500 times. The treatment of the DNA samples in such small quantities also requires a special techniique, but this question will be dealt with in a sep)arate paper.