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National Institutes of Health
PLoS Pathogens, reviewed by: Dr. Priya Saxena
“It’s rare for a pregnant woman to get infected, but once she is, she can’t clear the infection unless the placenta is expelled.”
For years, doctors have puzzled over why pregnant women are 20 times more likely than others to be infected by the bacterium Listeria monocytogenes. Researchers at the University of California, Berkeley, now think they have the answer, and it isn’t pretty.
Their research, conducted in guinea pigs, shows that the bacteria can invade the placenta, where – protected from the body’s immune system – they proliferate rapidly before pouring out to infect organs such as the liver and spleen. The illness they cause often results in miscarriage or infection of the fetus.

The study is the first to trace such a pathway of infection, and it dashes the widely-held assumption that immune-system changes during pregnancy are to blame for elevated Listeria infection rates.
“The reason the mother is more susceptible is not necessarily because her immune system is compromised, but because the bacteria that got into her placenta are infecting her,” said Anna Bakardjiev, the study’s lead author and a postdoctoral researcher with Daniel Portnoy, professor of biochemistry and molecular biology at UC Berkeley. “The miscarriages that result from these infections may be a natural defense mechanism to dispel this source of infection.”
The study will be posted on June 30 in the June issue of the online journal PLoS Pathogens.
Listeriosis is a foodborne illness caused by Listeria monocytogenes. Every year in the United States, about 2,500 people fall seriously ill with the disease. About one in three cases occur in pregnant women, and about one in five of all cases results in death, according to the Centers for Disease Control and Prevention (CDC). Apart from pregnant women, the illness primarily affects infants and people with compromised immune systems.
Fever, muscle aches and sometimes gastrointestinal problems are among listeriosis’s most common symptoms. In pregnant women, however, the symptoms are often mild, yet the illness frequently causes miscarriage, stillbirth or premature delivery. Babies that are born to infected mothers are often themselves infected, and many die.
From their earlier work, Portnoy and Bakardjiev knew that Listeria bacteria could not easily infect the placenta but, once there, could not be effectively eliminated. For this study, they wanted to know how the bacteria were able to invade the placenta in the first place: Their hypothesis was that the pathogens first infected organs such as the liver and moved from there to the placenta, an organ that, once infected, provides a protective niche for pathogens.
Bakardjiev, who is a pediatric infectious diseases specialist, chose guinea pigs for these studies because of similarities between the placentas of these rodents and women. Pregnant guinea pigs and women also respond similarly to Listeria infection, exhibiting few symptoms, yet almost invariably miscarrying.
To induce infection, Bakardjiev injected the pregnant guinea pigs with Listeria. When she examined the animals’ organs, she found that for every bacterium present in the placenta, there were 1,000 to 10,000 times as many in the liver and spleen, an indication that the placenta was fairly well protected from infection.
She then infected the animals with a mixture of two distinct strains of Listeria, adjusting the dose so low that placental infections resulted only half the time. When she examined the animals’ placentas 24 hours after the injection, she found, with few exceptions, only one of the two bacterial strains. This told her that it had been a single bacterium that had infected the organ, and that what she was finding were its progeny.
In the liver and spleen, on the other hand, the bacterial strains were present in equal numbers 24 hours after injection.
After 48 hours, the picture changed. At that point, Bakardjiev found a mixture of both strains in the placenta. In the liver and spleen, however, the numbers were now strongly skewed toward whichever strain had originally infected the placenta.
“We reasoned that this meant that a few bacteria had migrated early on from the liver or spleen to the placenta, so now both strains were in the placenta and their populations were burgeoning,” Bakardjiev said. “But there must have been a much larger number that had moved from the placenta back to the liver and spleen. These would have originally been just the single strain, so their numbers skewed the ratio.”
Bakardjiev and Portnoy, who is the study’s principal investigator, called on Julie Theriot, associate professor of biochemistry and of microbiology and immunology at Stanford University School of Medicine, to do the mathematical modeling for the bacterial migrations. Theriot determined that only about one bacterium migrated to the placenta every five hours, while it would have taken a migration of 100,000 bacteria from the placenta to the liver to skew the numbers to the degree they found. Thus, the vast majority of the bacteria in the placenta were a result of bacterial growth there and not from migrations from the liver and spleen.
“It was surprising to find that a single bacterium is sufficient to cause placental infection,” Portnoy said, “but even more surprising to find that they (the bacteria) migrated from the placenta back to the mother’s liver and spleen in such large numbers.”
When Portnoy and Bakardjiev ran the same experiment in non-pregnant guinea pigs, they found that 72 hours after injection, the non-pregnant animals had 1,000-fold lower numbers of Listeria in their livers and spleens than the pregnant animals, and no bacteria in their bloodstreams. In contrast, pregnant animals at 72 hours had the bacteria in their livers, spleens and blood, while the bacterial numbers continued to increase in their placentas, and their fetuses had also become infected.
“I feel that these numbers are an indication that miscarriage is a defense mechanism,” Bakardjiev said. “It’s rare for a pregnant woman to get infected, but once she is, she can’t clear the infection unless the placenta is expelled.”
Portnoy and Bakardjiev are now studying how Listeria moves from the digestive tract to the placenta. “An understanding of these mechanisms,” Portnoy said, “might contribute to designing methods for prevention and therapy of listeriosis in pregnant women.”
The study was supported by funds from the National Institutes of Health. Anna Bakardjiev’s work was also supported by a Career Development Award for physician scientists from the NIH.