Many bacterial species are mitogenic for human blood B lymphocytes.

Thirty bacterial species were tested for their ability to stimulate to increased DNA synthesis in human blood lymphocytes. A definite stimulation was obtained with eighteen bacterial species. For three of these species ten different strains of each were tested, and all increased DNA synthesis. The maximum response was after 3--4 days of culture, suggesting a mitogenic effect. This was confirmed by the induction of polyclonal antibody production shown by a plague assay, which was positive for nine of eleven species tested. Most bacterial species increased the DNA synthesis in B-lymphocyte-enriched and unseparated lymphocytes but had negligible activity on T-lymphocyte-enriched cultures. Among bacteria with a mitogenic effect and ability to induce polyclonal antibody production are Staphylococcus aureus strain Cowan I with a high content of protein A and many common human pathogens such as Haemophilus influenzae, Mycobacterium tuberculosis, Neisseria gonorrhoeae, Streptococcus group A and Streptococcus pneumoniae.