Bacterial weaponry that causes stillbirth revealed

In pregnant women, a normally benign bacterium emits tiny toxic balloons that can cause premature labor and stillbirth, a new study finds.

Called Group B Streptococcus, the bacterium lives in the vaginas of 20 to 30 percent of pregnant women worldwide. Strep B doesn’t cause problems in the lower genital tract. But in pregnant mice, Strep B secretes protein-filled balloons that can travel up into the uterus. Those balloons cause inflammation and weaken the amniotic sac, researchers from India report September 1 in PLOS Pathogens.

Scientists already knew that Strep B can be a problem during pregnancy. They didn’t know that it makes tiny long-range weapons. The danger is “not just the bug alone,” says microbiologist Lakshmi Rajagopal of Seattle Children’s Research Institute, “but also something the bug produces.”

Using a scanning electron microscope, researchers from the Indian Institute of Technology-Bombay detected small circular orbs budding off of Strep B bacteria. Inside those little fluid-filled balloons, the researchers found corrosive proteins. The scientists also found that, in mice, the balloons can migrate from the vagina into the uterus. There, the orbs trigger cell death and degrade collagen in the amniotic sac (making it more likely to tear), and can cause inflammation, premature birth and stillbirth. Almost all the pups of pregnant mice with bacterial balloons injected into their amniotic sacs either died in utero or were delivered prematurely. Researchers also found that when the toxic proteins were disabled by inhibitors, the balloons didn’t degrade collagen.

Previous work had implicated a bacterial pigment in Strep B’s ill effects, but the balloons are a potential second mechanism, says study coauthor Anirban Banerjee, a microbiologist at the Indian Institute of Technology.

It’s still unclear why Strep B, which normally keeps a low profile, makes toxins in the first place. They may be used in turf wars, Banerjee suggests, to help Strep B compete against other bacterial species. Strep B isn’t the only bacterium that makes toxic balloons, either, and many microbiologists are working to understand exactly how the bugs secrete them.

In the meantime, these new findings “emphasize the need to develop an approved vaccine” against Strep B, says Rajagopal. Today doctors test pregnant women for the bacterium between 35 and 37 weeks. Strep-positive women take antibiotics during labor to prevent infecting newborns. But the bacteria can quickly return, so antibiotics aren’t a permanent fix. Understanding how and why microbes make these teensy weapons could help doctors discover how to block strep infections in the first place.