Identification of conditionally essential genes for Streptococcus suis infection in pigs

Jesús Arenas,Aldert Zomer,Jose Harders-Westerveen,Hester J Bootsma,Marien I De Jonge,Norbert Stockhofe-Zurwieden,Hilde E Smith,Astrid De Greeff

doi:10.1080/21505594.2020.1764173

Abstract

ABSTRACT Streptococcus suis is a Gram-positive bacterium and zoonotic pathogen that causes meningitis and sepsis in pigs and humans. The aim of this study was to identify genes required for S. suis infection. We created Tn-Seq libraries in a virulent S. suis strain 10, which was used to inoculate pigs in an intrathecal experimental infection. Comparative analysis of the relative abundance of mutants recovered from different sites of infection (blood, cerebrospinal fluid, and meninges of the brain) identified 361 conditionally essential genes, i.e. required for infection, which is about 18% of the genome. The conditionally essential genes were primarily involved in metabolic and transport processes, regulation, ribosomal structure and biogenesis, transcription, and cell wall membrane and envelope biogenesis, stress defenses, and immune evasion. Directed mutants were created in a set of 10 genes of different genetic ontologies and their role was determined in ex vivo models. Mutants showed different levels of sensitivity to survival in whole blood, serum, cerebrospinal fluid, thermic shock, and stress conditions, as compared to the wild type. Additionally, the role of three selected mutants was validated in co-infection experiments in which pigs were infected with both wild type and isogenic mutant strains. The genetic determinants of infection identified in this work contribute to novel insights in S. suis pathogenesis and could serve as targets for novel vaccines or antimicrobial drugs.

Highlights

Streptococcus suis is the major etiological agent of streptococcal disease in pigs, a systemic infection that is the major cause of morbidity and mortality in young pigs leading to significant economic losses to the swine industry worldwide
In regular experimental infections using the intravenous inoculation route, it is challenging to retrieve sufficient amounts of bacteria, since they are rapidly dispersed through the whole body via the bloodstream
The amounts of bacteria in blood and cerebrospinal fluid (CSF) fluctuated over time, in blood (Figure 1(a))

Summary

Introduction

Streptococcus suis is the major etiological agent of streptococcal disease in pigs, a systemic infection that is the major cause of morbidity and mortality in young pigs leading to significant economic losses to the swine industry worldwide. Current immunization strategies focus on commercial and autologous whole-cell bacterins These vaccine formulations only induce protection against the serotype present in the bacterin and are not efficacious for all serotypes [4]. Several live-attenuated vaccine strategies were described; including those based on auxotrophic mutants for aromatic amino acids [5], or mutants lacking virulence-associated factors [6,7]. These vaccine preparations show a better efficacy than bacterins; there are sometimes concerns regarding safety when the mutant is not fully attenuated [8,9]. Several promising subunit vaccine candidates have been described, there is not yet a commercial cross-protecting vaccine on the market

Objectives

Methods

Results