The Type VII Secretion System (T7SSb) contributes to interbacterial competition in a murine model of Group BStreptococcus gastrointestinal colonization
Authors: Burghard Anne Claire , Turner Cole , Vaz Michelle , Ratner Adam J 
Published: October 2024

Abstract

Background
Late-onset (LO) disease caused by Group B Streptococcus (GBS) infection is a significant source of morbidity and mortality in neonates. It is believed that gastrointestinal (GI) colonization with GBS plays an important role in the progression to LO disease, although the host-microbial interactions that facilitate GBS GI colonization are poorly understood. The Type VII secretion system (T7SSb) has been identified in several gram-positive bacteria, and its function depends on a membrane-bound ATPase (EssC) which drives secretion of virulence factors. In GBS, the T7SSb has been shown to contribute to vaginal colonization and brain cytotoxicity. We aimed to identify the role of the T7SSb in GBS GI colonization using a murine model and hypothesize that the T7SSb is necessary for sustained GBS GI colonization.

Methods
Two bacterial strains were used, a wild-type (WT) GBS of A909 background (serotype Ia) and an isogenic T7SSb mutant GBS strain (essC::Himar1, transposon inserted). Monocolonization and co-cocolonization experiments were performed by orally inoculating preweaning C57BL/6J mice (12-14 days of life) with one or both strains, respectively. Euthanasia was performed, rectal swabs collected, and the GI tract harvested at day 7 post infection. For co-colonization experiments, competitive indices were calculated at day 7 post infection to determine the contribution of T7SSb to GBS GI colonization. For monocolonization experiments, colonization status and bacterial burden at day 7 post infection were measured.

Results
In cocolonization, WT GBS A909 outcompeted the essC::Himar1 mutant at 7-days post infection with a geometric mean competitive index of 8.8, 95% CI [0.2, 350.1] by rectal swab; 35.7, 95% CI [19.2, 66.3] in the small intestine; 87.6, 95% CI [18.9, 407] in the cecum; and 91.7, 95% CI [33.4, 252] in the colon (Figure 1a). In monocolonization, we noted similar rates of colonization (p=NS, Fisher’s exact test) (Figure 1b) and no difference in bacterial burden (p=NS, 2-way ANOVA test) (Figure 1c) across the GI tract at 7 days post infection.

Conclusion
Our results suggest that T7SSb plays an important role in GBS bacterial competition in the GI tract but is likely not an essential factor for GBS GI colonization. Further exploration of the role of the T7SSb in interbacterial competition may yield relevant information regarding GI colonization dynamics.

Figure 1. Establishing the role of T7SSb in a murine model of GI colonization. (a) Wild type (WT) A909 confers an advantage over an isogenic T7SSb mutant (essC::Himar1) strain in a cocolonization model. Data points represent geometric mean competition indices and error bars represent 95% confidence interval. (b) Colonization status by WT A909 and essC::Himar1 mutant in the GI tract 7-days post infection in monocolonization experiments (p-values represent two-sided Fisher’s exact test). (c) Bacterial burden of monocolonization by WT A909 and essC::Himar1 mutant in the GI tract 7-days post infection (p-values represent 2-way ANOVA test).