The association between the vaginal microbiota and recurrent early spontaneous preterm birth

Goodfellow, Laura ORCID: 0000-0002-8111-5007
(2021) The association between the vaginal microbiota and recurrent early spontaneous preterm birth. Doctor of Medicine thesis, University of Liverpool.

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Preterm birth is the leading cause of death of children under 5 worldwide. Despite over 30 years investment in basic science and clinical research the rate has remained relatively static. This has been attributed to the multifactorial nature of preterm birth. Infection and/or inflammation are estimated to contributing to approximately a third of preterm births, however the exact organism(s) frequently remain elusive and are often attributed to bacteria that are not cultured using standard laboratory techniques. Vaginal bacteria are a presumed source of inflammatory organisms due to the close anatomical relationship with the uterus and cervix. The advent of culture-independent methods of assessment of the microbiota has enabled new interrogation of this theory. In the past seven years over 2000 women have undergone assessment of the vaginal microbiota (VMB) in pregnancy. Most studies have identified components of the VMB that relate to preterm birth. However, poor understanding of the distribution of VMB characteristics in healthy pregnancies, differences in almost every aspect of study design, and reliance on proportional data rather than quantification have hampered efforts to translate these findings into clinically meaningful information. This project advances the field of VMB in preterm birth work in two ways. Firstly we summarised VMB characteristics that have previously been associated with preterm birth and assessed whether these are associated with recurrence of preterm birth under 34 weeks gestation, contextualised by reference to the expected distribution of the VMB characteristics in our locality. Secondly, a criticism of the initial next-generation sequencing studies was over-reliance on data solely describing the proportions of bacteria present in a given sample, without consideration of the overall bacterial load. We were able to address this by complementing our analysis with semi-quantitative analysis using quantitative PCR of the 16S rRNA gene. We found that four VMB characteristics (out of 29 assessed) showed trends towards reproduction of previous work. These associations did not meet traditional statistical significance (<0.05%), most likely because we could only evaluate 22 preterm births. The most striking finding of our analysis was that vaginal bacterial load in the second trimester, irrespective of the bacterial composition, was associated with early spontaneous preterm birth or preterm prelabour rupture of membranes recurrence. Women with a recurrence, compared to those without, had a fivefold higher median vaginal bacterial load. There was a gradient effect of bacterial load on risk of recurrence of preterm birth. Interestingly, domination by lactobacilli other than L. iners protected women from developing high bacterial loads. This could explain the inconsistent association between L. iners and preterm birth in previous studies; if L. iners dominance is only associated with PTB when there is a concurrent high bacterial load, then women with L. iners domination and a normal bacterial load would not show this association. We also found that the women who have high bacterial loads and a preterm birth have a higher rate of early onset chorioamnionitis, strengthening the evidence that the bacterial load is clinically important. This study provides a strong foundation on which to base further studies incorporating assessment of bacterial load alongside traditional 16S rRNA analysis to fully elucidate the contribution of the VMB to preterm birth. Once fully understood the VMB is potentially modifiable and could provide a novel mode of preterm birth prevention.

Item Type: Thesis (Doctor of Medicine)
Divisions: Faculty of Health and Life Sciences
Depositing User: Symplectic Admin
Date Deposited: 27 Jul 2021 13:17
Last Modified: 18 Jan 2023 22:34
DOI: 10.17638/03126373
  • Alfirevic, Zarko
  • Van De Wijgert, Janneke