孕妇李斯特氏菌病发病率提高20倍的详细机制

【DavidShi译文】美国加州大学伯克利分校的研究人员发现，李斯特氏菌先影响胎盘，然后将胎盘当作安全港和滋生地，从而进一步影响其他器官

【伯克利报道】多年以来，医生们一直苦苦思考着这一问题：为何怀孕期妇女感染单核细胞增生李斯特氏菌是其他人的20倍？美国加州大学伯克利分校的研究人员认为他们已经找到答案，未臻完美的答案。

他们用豚鼠做试验，结果显示该细菌能侵入胎盘。胎盘保护它们免收母体免疫系统的攻击。它们在胎盘里迅速增生之后扩散出去，再感染其他器官如肝脏和脾脏。

该研究首次跟踪了这一感染途径，并动摇了一个被广泛认可的假设：孕妇免疫系统的改变提高了她们感染李斯特氏菌的比率。

安娜·巴克雅伍是该项研究的首要作者，她跟随加州大学伯克利分校生化和分子生物学教授丹尼尔·波特诺做博士后研究。安娜·巴克雅伍说：“孕妇更易感染该菌的原因，不是她的免疫系统对该菌妥协了，而是该菌进入了她的胎盘，再感染了她。由此类感染引起的流产可能是一种自然防御机制，其目的是消灭感染源。”

六月版的网络杂志《PLoS Pathogens》将于6月30日发表这项研究。

李斯特氏菌病是由单核细胞增生李斯特氏菌引起的，经食物传播的疾病。美国每年约有2500人感染此病。根据疾病控制和预防中心的说法，约三分之一的病人为孕妇，约五分之一的病人死亡。除孕妇外，该病主要感染婴儿和免疫系统妥协者。

李斯特病最常见的症状是发烧、肌肉疼痛和间或发作的肠胃病等。然而，孕妇中的症状常常表现温和，但常引起流产、死产或早产。感染该病的孕妇产出的婴儿常感染上该病，很多这样的婴儿死亡。

根据他们早先的研究，波特诺和巴克雅伍认识到，李斯特氏菌不能轻易感染胎盘，但一旦感染上，就很难有效去除。现在这项研究中，他们希望知晓该菌首先如何能够入侵胎盘的：他们假设，病原体先感染肝脏这样的器官，再转移至胎盘。一旦感染上胎盘，该器官就能为病原菌提供保护性的小环境。

巴克雅伍是一个小儿科传染病专家，她选择豚鼠作为研究材料的原因是，豚鼠胎盘和孕妇胎盘之间存在诸多相似之处。怀孕豚鼠和孕妇对李斯特氏菌感染的反应也相似，表现出症状不明显，然而往往流产。

为了引发感染，巴克雅伍往孕豚鼠里注射了李斯特氏菌。当她检查孕鼠器官的时候，发现每一种感染胎盘的细菌在肝脏和脾脏中数量是胎盘中的1,000 到10,000倍。这提示胎盘得到相当好的保护，以免于感染。

接着她将两种远缘的李斯特氏菌株的混合物注射到孕豚鼠中，并降低剂量，以致仅有半数胎盘中受感染。她在注射24后检查孕豚鼠胎盘时发现，除极个别情况外，只有一种菌。这提示她，只有一种细菌感染了胎盘，而她所观察到的是该菌的后代。

在另一方面，肝脏和脾脏中细菌数量在注射24小时之后无变化。48小时后情形发生了变化。此时，巴克雅伍发现胎盘中都出现了两种菌株。在肝脏和脾脏，细菌数发生强烈的倾斜，倾斜到最初传染胎盘的这种细菌，无论这种细菌是上述两种中的哪一种。

“我们推测，这意味着一些细菌早先已经从肝脏和脾脏迁移至胎盘。于是现在两种细菌都出现在胎盘，并且它们的数量开始增长。”巴克雅伍说，“但是肯定有更多细菌从胎盘迁移回肝脏和脾脏。这些细菌应该是最初的那种单一菌株，于是这种细菌的数量远高于另一种。”

巴克雅伍和该研究的主要负责人波特诺，邀请了斯坦福大学医学院生化和微生物学和免疫学副教授朱莉·特立特，来建立细菌迁移的数学模型。特立特确定，每5个小时里只有一个细菌迁移至胎盘，而从胎盘迁移至肝脏，从而引起他们观察到的数量倾斜的细菌数量可能高达100,000。这样，胎盘绝大多数细菌的存在是细菌在胎盘内生长的结果，而非肝脏和脾脏迁移所致。

“令人惊异的是，单个细菌足以引起胎盘感染，”波特诺说，“但更让人惊奇的是，它们（细菌）迁移回母体肝脏和脾脏的数量如此巨大。”

波特诺和巴克雅伍在非孕豚鼠身上重复这一试验时，发现注射后72小时非孕豚鼠肝脏和脾脏中李斯特氏菌数量低于孕豚鼠中的1000倍，并且非孕鼠的血液中无细菌。相反，注射72小时后孕鼠的肝脏、脾脏和血液中有该菌，在胎盘中的细菌数持续上升，且它们的胎儿也受到感染。

“我感觉到，这些数据提示了流产是一种防御机制，”巴克雅伍说。“孕妇受感染的几率很小，但一旦感染上，除了排出胎盘，她没法清除感染。”

波特诺和巴克雅伍现在正在研究李斯特氏菌如何从消化道迁移至胎盘。“对这些机制的理解，”波特诺说，“可能有助于设计方法来预防和治疗孕妇中的李斯特病。”该研究由美国国家卫生研究院提供资助。安妮·巴克雅伍的工作同时也受国家卫生研究院为内科医师研究人员提供的职业培训奖学金的资助。

【下面为英文原文】

Finally: A study that details why Listeriosis rates are 20-fold higher during pregnancy

UC Berkeley researchers discover that Listeria bacteria first infect the placenta, then use it as a safe harbor and breeding ground before pouring out to infect other organs

BERKELEY ?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.