Antibacterial activity in human amniotic fluid: relationship to zinc and phosphate.

Antibacterial activity, tested with B. subtilis, was present in all 44 human amniotic fluids at between 36 and 42 weeks of pregnancy and was unrelated to gestational age within these limits, or to maternal age or parity. High-molecular-weight (beta-lysin) activity, present in all samples, was directly related to concentration of bound zinc, but not to that of free zinc. Low-molecular-weight antibacterial activity (less than 1000 Daltons) was only present in nine specimens, but sub-threshold concentrations were demonstrable by concentrating the ultrafiltrates in more than half of the remaining specimens. Low-molecular-weight activity in the ultrafiltrates was directly related to the concentration of bound zinc in the original amniotic fluid. Neither high- nor low-molecular-weight antibacterial activities were associated with amniotic fluid phosphate levels.