Studies on the growth-inhibiting property of amniotic fluids from two United States population groups.

This study attempted to determine if lack of antimicrobial activity in amniotic fluid is the reason United States blacks have more amniotic fluid bacterial infection than whites. No significant interracial differences were found in a study of 111 fluids from whites and 56 fluids from blacks. It has been claimed that the phosphate:zinc ratio in amniotic fluid is an accurate predictor of antimicrobial activity. The present study found this often to be untrue. Amniotic fluids that lacked antimicrobial activity gained such activity when zinc was added, but the in vitro zinc levels required were usually higher than observed physiologic concentrations. Previous studies have claimed that antimicrobial activity first appears in the amniotic fluid in the third trimester of gestation. We found such activity in the majority of fluids by the end of the first trimester, with a Staphylococcus aureus indicator; in contrast, with Escherichia coli and Streptococcus agalactiae as test organisms, greater inhibition was observed after 35 weeks.