Olschewski, K. | bioRxiv Science

bioRxiv2020

The protected physiological status of intracellular Salmonella enterica persisters reduces host cell-imposed stress

Today, we are faced with increasingly occurring bacterial infections that are hard to treat and often tend to relapse. These recurrent infections can occur possibly due to antibiotic-tolerant persister cells. Antibiotic persistent bacteria represent a small part of a bacterial population that enters a non-replicating (NR) state arising from phenotypic switching. Intracellularly, after uptake by phagocytic cells, Salmonella enterica serovar Typhimurium (STM) forms persister cells that are able to subvert immune defenses of the host. However, the clear physiological state and perceptual properties are still poorly understood and many questions remain unanswered. Here we describe further development of fluorescent protein-based reporter plasmids that were used to detect intracellular NR persister cells and monitor the expression of stress response genes via extensive flow cytometric analyses. Moreover, we performed extensive measurements of the metabolic properties of NR STM at the early course of infection. Our studies demonstrate that NR STM persister cells perceive their environment and are capable respond to stress factors. Since persisters showed a lower stress response compared to replicating (R) STM, which was not a consequence of a lower metabolic capacity, the persistent status of STM serves as protective niche. Furthermore, up to 95% of NR STM were metabolically active at the beginning of infection additionally showing no difference in the metabolic capacity compared to R STM. The accessory capability of NR STM persisters to sense and to react to stress with constant metabolic activity may supports the pathogen to create a more permissive environment for recurrent infections.

microbiology↗

bioRxiv2020

Fluorescent protein-based reporters reveal stress response of intracellular Salmonella enterica on single cell level

Intracellular bacteria such as Salmonella enterica are confronted with a broad array of defense mechanisms of their mammalian host cells. The ability to sense host cell-imposed damages, and to mount efficient stress responses are crucial for survival and proliferation of intracellular pathogens. The various combinations of host defense mechanisms acting on intracellular bacteria and their individual response also explain the occurrence of distinct subpopulations of intracellular S. enterica such as dormant or persisting, slowly or rapidly replicating cells. Here we describe a set of fluorescence protein (FP)-based reporter strains that were used to monitor the expression of cytoplasmic or periplasmic stress response systems on a single cell level. This is mediated by a fast maturing FP as reporter for induction of stress response genes. We evaluated slower maturing FPs for a second function, i.e. the analyses of the status of intracellular proliferation of pathogens. The combination of two FPs allows, on a single cell level, the interrogation of stress response and intracellular proliferation. Application of these reporters to S. enterica allowed us to detect and quantify distinct intracellular subpopulations with different levels of stress response and proliferation. ImportanceSensing of, and responding to host-mediated damages are important defensive virulence traits of bacterial pathogens. Intracellular pathogens such as Salmonella enterica are exposed to various types of antimicrobial host cell defenses that impose, among other, periplasmic and cytosolic stresses. Intracellular S. enterica form distinct subpopulations that differ in proliferation rate, metabolic activity and persister formation. Here we deploy fluorescence protein-based reporter strains to monitor, on a single cell level, the response of intracellular S. enterica to periplasmic or cytoplasmic stress. A second fluorescent protein reports the biosynthetic capacity of individual intracellular S. enterica. The dual fluorescence reporters can be deployed to characterize by flow cytometry phenotypically diverse subpopulations and stress responses in intracellular bacteria.

microbiology↗