The addition of deoxyribonucleic acid (DNA) to living cells, thereby changing their genetic composition and properties. Such transformation was first reported by Fred Griffith in 1928 as occurring in pneumonia bacteria (pneumococci, or Streptococcus pneumoniae) growing in animals injected in the laboratory. Other investigators produced the same phenomenon in bacteria of the same species growing in the test tube. The process is looked upon as the transfer of a transforming principle, or substance, from donor bacteria to suitable recipient bacteria. The recipient bacteria are usually closely related to the donor strain. It is realized that the process may occur in natural conditions, for example, in a host animal infected with two parasitic strains, and indeed it might play a part in the rapid evolution of pathogenic bacteria.