Meselson and Stahl cultured E. coli in a 15N medium to get the DNA of heavy density. Then bacteria were shifted to a 14N medium, DNA of replication cycles 0, 1, and 2 was isolated. The DNA was all of intermediate density after one replication cycle and ruled out the conservative replication model, which predicts that both heavy density DNA and light density DNA will be present. This result proved the semi-conservative replication model, according to which all DNA molecules will consist of one 15N-labeled DNA strand and one 14N-labeled DNA strand. The result of second replication cycle were also in consistent with semi-conservative replication model. So, the correct answer is 'Escherichia coli'.