Adaptation of Salmonella enterica to antibiotics and innate immunity effectors

Abstract

Salmonella enterica is a bacterial pathogen causing major morbidity and mortality in low- and middle-income countries. The bacteria can cause a wide range of disease, ranging from the severe systemic disease typhoid fever to localized gastroenteritis. Characteristics of typhoid fever, caused by Salmonella enterica serovar Typhi (S. Typhi), include the bacteria’s ability to proliferate within host cells, intrinsic resistance to selected antibiotics, and emerging acquired antibiotic resistance. As S. Typhi is strictly human adapted and highly pathogenic one often uses Salmonella enterica serovar Typhimurium (S. Typhimurium) as a model organism for understanding details of typhoid fever.

Using S. Typhimurium this thesis reveals genetic details governing intrinsic antibiotic resistance and virulence in in vitro and in vivo infection models, as well as details the interplay between the pathogen and phagocytic cells. In this we have discovered a new genetic determinant for intrinsic vancomycin resistance coding for muramyl endopeptidase MepS (also known as Spr), an enzyme tasked with cleaving cell wall, and that MepS is functionally connected to the periplasmic protease Prc (also known as Tsp) in this matter. Vancomycin is an antibiotic that inhibits cell wall synthesis, but is not effective against Gram-negative enteric bacteria. This has been thought to be due to the relative impermeability of the outer membrane resulting in vancomycin not being able to access its target the cell wall (due to its large size). However, we present results that adds to this in showing that the outer membrane is not the only factor resulting in intrinsic vancomycin resistance.

With regard to intracellular pathogenesis of S. Typhimurium in mouse infection models we show in this thesis that the periplasmic protease Prc is required for the full fitness of the bacterium when in macrophages and mice. This requirement is dependent on the cell wall synthesizing enzyme PBP3SAL highlighting the possible role of Prc in regulation of bacterial proliferation during intracellular phases of infection. As for further aspects of intracellular pathogenesis of S. Typhimurium in macrophages we show that the presence of S. Typhimurium in single cells correlates with hypoxia and lack of iNOS, an innate immunity effector tasked with killing invading organisms by producing reactive nitrogen species. We suggest this correlation to be a result of general shut-off of protein synthesis due to hypoxia generated by the presence of S. Typhimurium proliferating within the macrophage.

All these result add to the basic knowledge of both determinants for intrinsic antibiotic resistance and aspects governing intracellular pathogenesis of S. Typhimurium with regards to both genes involved and effect on innate immunity effectors. We believe the results presented in this thesis is a good starting point for further studies regarding further mechanistical studies into the phenomena described.