Amniotic fluid activity against Bacteroides fragilis and group B streptococci.

The antimicrobial capacity of amniotic fluid (AF) against B. fragilis, group B streptococci and Escherichia coli, the latter as a reference indicator, was investigated in samples from 30 patients. In order to study trimester as well as individual variations in the antibacterial activity, 10 individual samples from each trimester of pregnancy (AF1, AF2, AF3), were separately tested. With B. fragilis all trimester samples showed initial inhibition lasting for 8 h, the bacteria then regaining growth capacity in all AF1, and the majority of AF2 and AF3 specimens. However, 4 AF2 and 3 AF3 samples showed continued bacteriostatic activity for up to 24 h. Group B streptococci exhibited uninhibited growth in all trimester samples. The growth of E. coli in AF1 and AF2 was not inhibited, whereas in AF3 a temporary arrest was observed at 8 h, this being followed by growth to control levels at 24 h. The findings may explain the frequent isolation of B. fragilis from cases of septic abortions during the first trimester. Individual variation in the effectiveness of the antibacterial activity against B. fragilis in AF2 and AF3 may explain why this microorganism, which is often implicated in severe maternal and neonatal infections, is occasionally recovered from the mother's endocervix without signs of disease. The inefficacy of AF in defence against group B streptococci correlates well with the frequent isolation of these bacteria from cases of severe congenital infections associated with premature delivery.