A sludgy substance seen on the ultrasound images of about 15 percent of pregnancies is in fact a bacterial biofilm in the amniotic fluid, according to the USC School of Dentistry’s resident expert on the slimy structures.

The findings appear in the January cover story of the American Journal of Obstetrics and Gynecology, of which J. William Costerton, professor of research in the School of Dentistry and director of the Center for Biofilms, is an author.

“Doctors kept seeing this stuff on the ultrasound and we wanted to figure out what it was,” Costerton said. “We suspected it was bacteria but didn’t know for sure.”

Researchers with the National Institute of Child Health and Human Development in Detroit followed hundreds of pregnancies and aspirated the unknown sludge from the amniotic fluid of one patient at 28 weeks of pregnancy. Costerton said scanning electron and confocal microscopy captured incredible images of bacterial cells in the sludge, while bacteria-specific probes stained the cells and the surrounding matrix, proving the sludge’s status as a biofilm.

“It was originally thought that pregnancy was a sterile process until the baby was born,” Costerton said. “But it turns out that the baby is encountering bacteria in the amniotic fluid as early as a couple months of gestation.”

The National Institutes of Health estimate that nearly 80 percent of microbial infections in the human body are due to biofilms, which can be remarkably resistant to typical antibiotic treatment due to the matrix formed by the secretions of the bacterial cells.

While the presence of a biofilm near a developing fetus can be a worrisome thought to prospective parents, Costerton said that less than one percent of babies encounter a problem related to bacteria, while bacteria is seen in the membranes of 80 percent of healthy pregnancies.

Beyond concerns regarding prenatal health, the findings are remarkable in terms of the study of biofilms themselves, he added.

“Biofilms are very difficult to culture,” Costerton said. “So finding and capturing images of intact biofilms like this is incredibly useful.”