A natural germ killer in the sex organ of male rats may protect their testes from infection with sexually transmitted pathogens. Rats produce this peptide only in the epididymis, the place where sperm cells mature after they have left the testes. The molecule belongs to the densin family of antimicrobial peptides, which are positively charged and act against a variety of microbes.

Researchers from China and the U.S. report in today's Science that they blocked the production of the peptide in a culture of epididymal cells. As a result the cells became more susceptible to bacterial infection. Since the peptide only occurs in a certain part of the epididymis, it might also be involved in sperm maturation, protection or immobilization, although this is somewhat speculative.