Quantitative evaluation of the progress of intracellular infection in vitro: incorporation of 3H-thymidine into deoxyribonucleic acid by Mycobacterium leprae in cultivated blood monocytes.

Growth of intracellular parasites such as Mycobacterium leprae in macrophages derived from human peripheral blood monocytes can be assessed by selective incorporation of [methyl-(3)H]thymidine into deoxyribonucleic acid of the bacterial cells. The radioactive precursor is not taken up by the host cells, and evidence has been presented for its incorporation into bacteria. The procedure is sensitive, reproducible, and highly quantitative.