Host inflammatory dynamics reveal placental immune modulation by Group B Streptococcus during pregnancy.

Group B Streptococcus (GBS) is a pathobiont that can ascend to the placenta and cause adverse pregnancy outcomes, in part through production of the toxin ß-hemolysin/cytolysin (ß-h/c). Innate immune cells have been implicated in the response to GBS infection, but the impact of ß-h/c on their response is poorly defined. We show that GBS modulates innate immune cell states by subversion of host inflammation through ß-h/c, allowing worse outcomes. We used an ascending mouse model of GBS infection to measure placental cell state changes over time following infection with a ß-h/c-deficient and isogenic wild type GBS strain. Transcriptomic analysis suggests that ß-h/c-producing GBS elicit a worse phenotype through suppression of host inflammatory signaling in placental macrophages and neutrophils, and comparison of human placental macrophages infected with the same strains recapitulates these results. Our findings have implications for identification of new targets in GBS disease to support host defense against pathogenic challenge.

Group B Streptococcus Capsular Serotype Alters Vaginal Colonization Fitness.

BACKGROUND: Group B Streptococcus (GBS) remains a leading cause of infant morbidity and mortality. A candidate vaccine targets 6 GBS serotypes, offering a potential alternative to intrapartum antibiotic prophylaxis to reduce disease burden. However, our understanding of the contributions of specific capsule types to GBS colonization and disease remains limited. METHODS: Using allelic exchange, we generated isogenic GBS strains differing only in the serotype-determining region in 2 genetic backgrounds, including the hypervirulent clonal complex (CC) 17. Using a murine model of vaginal cocolonization, we evaluated the roles of the presence of capsule and of expression of specific capsular types in GBS vaginal colonization fitness independent of other genetic factors. RESULTS: Encapsulated wild-type strains COH1 (CC17, serotype III) and A909 (non-CC17, serotype Ia) outcompeted isogenic acapsular mutants in murine vaginal cocolonization. COH1 wild type outcompeted A909. Notably, expression of type Ia capsule conferred an advantage over type III capsule in both genetic backgrounds. CONCLUSIONS: Specific capsule types may provide an advantage in GBS vaginal colonization in vivo. However, success of certain GBS lineages, including CC17, likely involves both capsule and noncapsule genetic elements. Capsule switching in GBS, a potential outcome of conjugate vaccine programs, may alter colonization fitness or pathogenesis.

Genome-Wide fitness analysis of group B Streptococcus in human amniotic fluid reveals a transcription factor that controls multiple virulence traits.

Streptococcus agalactiae (group B Streptococcus; GBS) remains a dominant cause of serious neonatal infections. One aspect of GBS that renders it particularly virulent during the perinatal period is its ability to invade the chorioamniotic membranes and persist in amniotic fluid, which is nutritionally deplete and rich in fetal immunologic factors such as antimicrobial peptides. We used next-generation sequencing of transposon-genome junctions (Tn-seq) to identify five GBS genes that promote survival in the presence of human amniotic fluid. We confirmed our Tn-seq findings using a novel CRISPR inhibition (CRISPRi) gene expression knockdown system. This analysis showed that one gene, which encodes a GntR-class transcription factor that we named MrvR, conferred a significant fitness benefit to GBS in amniotic fluid. We generated an isogenic targeted deletion of the mrvR gene, which had a growth defect in amniotic fluid relative to the wild type parent strain. The mrvR deletion strain also showed a significant biofilm defect in vitro. Subsequent in vivo studies showed that while the mutant was able to cause persistent murine vaginal colonization, pregnant mice colonized with the mrvR deletion strain did not develop preterm labor despite consistent GBS invasion of the uterus and the fetoplacental units. In contrast, pregnant mice colonized with wild type GBS consistently deliver prematurely. In a sepsis model the mrvR deletion strain showed significantly decreased lethality. In order to better understand the mechanism by which this newly identified transcription factor controls GBS virulence, we performed RNA-seq on wild type and mrvR deletion GBS strains, which revealed that the transcription factor affects expression of a wide range of genes across the GBS chromosome. Nucleotide biosynthesis and salvage pathways were highly represented among the set of differentially expressed genes, suggesting that MrvR may be involved in regulating nucleotide availability.