For years, scientists have noticed a particular oral bacterium can be found in the amniotic fluid of about 20 percent of women with premature babies. The debate has always been whether this bacterium, called Fusobacterium nucleatum ( F. nucleatum ), is merely “an innocent bystander” in the womb or indeed a culprit in either directly or indirectly causing early labor, a major public health problem in the United States.
In the April issue of the journal Infection and Immunity , researchers report they may be closer to an answer. Based on a series of laboratory experiments, they show F. nucleatum can directly infect the placenta and adversely affect pregnancies in mice. The scientists say the micro-organism may act as an opportunistic pathogen that circulates to the uterus, colonizes the placenta, and takes advantage of the reduced immune response found there during pregnancy.
Although the strain of F. nucleatum used in the study typically colonizes the mouth and plays a contributory role in periodontal, or gum, disease, the scientists say their study did not evaluate the hotly debated topic of whether an oral bacterium can shed from the mouth and infect sites elsewhere in the body.
“We were interested specifically in the question of whether F. nucleatum could cause adverse pregnancy outcomes,” said Dr. Yiping Han, a scientist at Case Western Reserve University in Cleveland , Ohio and the senior author on the study. “We used a simplified model in which we injected the bacteria directly into the bloodstream. It is quite difficult to mimic human periodontal disease as the initiating step of infection in a mouse model, but this line of research certainly would be well worth pursuing in future studies.” If the particular subspecies of F. nucleatum in both the mouth and uterus are studied, the researchers say they believe the causal relation between the two events can be made even more evident.
Of the more than 500 bacterial species that inhabit the mouth, F. nucleatum stands out as one of the dominant species. Yet, as scientists have pursued the possibility that oral bacteria can cause preterm birth, most of the research has focused on other pathogens, such as Porphyromonas gingivalis . This left unanswered whether F. nucleatum , which has been found in the amniotic fluid of some women who gave birth prematurely, also might play a role.
To pursue this question, Han et al. first showed that two types of the bacterium – one taken from the womb, the other from the throat – could indeed attach to and invade epithelial and endothelial cells grown in a culture dish. Epithelial cells function as a physical and biological barrier against pathogens in many organs, including the uterus, while endothelial cells form the inner lining of blood vessels. Another Fusobacterium strain found in vaginal infections was not detected on or inside these cells, and the authors said it was not associated with premature birth.
Next, the group injected oral strains of F. nucleatum into the blood vessels of pregnant mice at a point that corresponds to women in their third trimester, when they are most at risk for premature births. This led to stillbirths of many of the fetuses, comparable to human premature delivery. Unlike humans, mice usually have multiple fetuses in one uterus, and, since early delivery is unfeasible for the remaining healthy pups, they carry out a problematic pregnancy.
The scientists wondered if the fetal death was due to a general infection or, as in humans, an infection localized only in the uterus. To test this, they measured the amount of bacteria present in the liver, spleen, placenta, fetus and amniotic fluid for several days after injection. The group found that bacteria were quickly eliminated from other organs, but multiplied rapidly in the uterus, suggesting the bacteria specifically targeted this area. According to Han, without a placenta present, infection with F. nucleatum in organs other than the gums, would not take place in their model. Normally, the body's immune response recognizes and destroys foreign molecules, such as bacteria and viruses, but in pregnancy this surveillance system is turned down to facilitate growth of the fetus. Exploiting a less-vigilant immune system, the bacteria can grow and spread more readily in the uterus.
Of great interest to the scientists was whether they could actually observe the bacteria moving from blood vessels, through the uterine tissue, and into the amniotic fluid. To visualize this process, they used an electron microscope, which can magnify small tissue samples thousands of times. “Others have isolated F. nucleatum from the uteriof women in premature labor, but we believe we are the first to actually show live bacteria invading through the endothelial lining into the placenta and eventually to amniotic fluid, where they can be detected with standard hospital tests,” explained Han.
Han said her group observed that the membrane surrounding the fetus also was infected, mimicking human fetal membrane infections called chorioamnioitis, which is a known cause of premature birth. As cells there or elsewhere in the placenta respond to the spread of the infection, the scientists suspect they release molecules that can trigger early contractions of the uterus.
Han, a microbiologist, intends to further explore exactly how F. nucleatum finds its way through the uterine cells, entering one side and exiting the other, and why the bacteria preferentially choose the uterus tissue layers. Ultimately, should the group establish that the bacterium causes preterm births in people, Han said she hopes to develop a vaccine to prevent an infection in pregnant women. “We believe our results strengthen the link between periodontal disease and premature birth and would like to look further into the specific mechanisms involved in uterine infection. Hopefully, one day we will be able to prevent it. For now, we subscribe to the advocated importance of good dental care, especially during pregnancy,” added Han.
Collaborating with Dr. Han were Drs. Raymond W. Redline, Gale B. Hill and Thomas S. McCormick. The study is titled " Fusobacterium nucleatum Induces Premature and Term Stillbirths in Pregnant Mice: Implication of Oral Bacteria in Preterm Birth" and was published in the journal Infection and Immunity in April, 2004. The work was supported by the NIDCR and University Hospitals of Cleveland