Secondary structure of the mRNA encoding listeriolysin O is essential to establish the replicative niche of L. monocytogenes

PoreGlow: A split green fluorescent protein-based system for rapid detection of Listeria monocytogenes

Listeria monocytogenes, a severe foodborne pathogen causing severe diseases underscores the necessity for the development of a detection system with high specificity, sensitivity and utility. Herein, the PoreGlow system, based on split green fluorescent protein (GFP), was developed and assessed for the fast and accurate detection of L. monocytogenes. Split GFP-encapsulated liposomes were optimized for targeted analysis. The system utilizes listeriolysin O (LLO), a toxin produced by L. monocytogenes that enlarges the pores split GFP-encapsulated liposomes, to detect L. monocytogenes by measuring the fluorescent signal generated when the encapsulated GFP is released and reacted with the externally added fragment of the split GFP. The system exhibited a limit of detection of 0.17 μg/ml for LLO toxin and 10 CFU/mL for L. monocytogenes with high sensitivity and specificity and no cross-reactivity with other bacteria. The PoreGlow system is practical, rapid, and does not require sample pre-treatment, making it a promising tool for the early detection of L. monocytogenes in food products, which is crucial for preventing outbreaks and protecting public health.

Eosinophils promote CD8+ T cell memory generation to potentiate anti-bacterial immunity

Memory CD8+ T cell generation is crucial for pathogen elimination and effective vaccination against infection. The cellular and molecular circuitry that underlies the generation of memory CD8+ T cells remains elusive. Eosinophils can modulate inflammatory allergic responses and interact with lymphocytes to regulate their functions in immune defense. Here we report that eosinophils are required for the generation of memory CD8+ T cells by inhibiting CD8+ T cell apoptosis. Eosinophil-deficient mice display significantly impaired memory CD8+ T cell response and weakened resistance against Listeria monocytogenes (L.m.) infection. Mechanistically, eosinophils secrete interleukin-4 (IL-4) to inhibit JNK/Caspase-3 dependent apoptosis of CD8+ T cells upon L.m. infection in vitro. Furthermore, active eosinophils are recruited into the spleen and secrete more IL-4 to suppress CD8+ T cell apoptosis during early stage of L.m. infection in vivo. Adoptive transfer of wild-type (WT) eosinophils but not IL-4-deficient eosinophils into eosinophil-deficient mice could rescue the impaired CD8+ T cell memory responses. Together, our findings suggest that eosinophil-derived IL-4 promotes the generation of CD8+ T cell memory and enhances immune defense against L.m. infection. Our study reveals a new adjuvant role of eosinophils in memory T cell generation and provides clues for enhancing the vaccine potency via targeting eosinophils and related cytokines.

RibU is an essential determinant of Listeria pathogenesis that mediates acquisition of FMN and FAD during intracellular growth

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are essential riboflavin-derived cofactors involved in a myriad of redox reactions across all forms of life. Nevertheless, the basis of flavin acquisition strategies by riboflavin auxotrophic pathogens remains poorly defined. In this study, we examined how the facultative intracellular pathogen Listeria monocytogenes, a riboflavin auxotroph, acquires flavins during infection. A L. monocytogenes mutant lacking the putative riboflavin transporter (RibU) was completely avirulent in mice but had no detectable growth defect in nutrient-rich media. However, unlike wild type, the RibU mutant was unable to grow in defined media supplemented with FMN or FAD or to replicate in macrophages starved for riboflavin. Consistent with RibU functioning to scavenge FMN and FAD inside host cells, a mutant unable to convert riboflavin to FMN or FAD retained virulence and grew in cultured macrophages and in spleens and livers of infected mice. However, this FMN- and FAD-requiring strain was unable to grow in the gallbladder or intestines, where L. monocytogenes normally grows extracellularly, suggesting that these sites do not contain sufficient flavin cofactors to promote replication. Thus, by deleting genes required to synthesize FMN and FAD, we converted L. monocytogenes from a facultative to an obligate intracellular pathogen. Collectively, these data indicate that L. monocytogenes requires riboflavin to grow extracellularly in vivo but scavenges FMN and FAD to grow in host cells.

Pathogenicity and virulence of Listeria monocytogenes: A trip from environmental to medical microbiology

Listeria monocytogenes is a saprophytic gram-positive bacterium, and an opportunistic foodborne pathogen that can produce listeriosis in humans and animals. It has evolved an exceptional ability to adapt to stress conditions encountered in different environments, resulting in a ubiquitous distribution. Because some food preservation methods and disinfection protocols in food-processing environments cannot efficiently prevent contaminations, L. monocytogenes constitutes a threat to human health and a challenge to food safety. In the host, Listeria colonizes the gastrointestinal tract, crosses the intestinal barrier, and disseminates through the blood to target organs. In immunocompromised individuals, the elderly, and pregnant women, the pathogen can cross the blood-brain and placental barriers, leading to neurolisteriosis and materno-fetal listeriosis. Molecular and cell biology studies of infection have proven L. monocytogenes to be a versatile pathogen that deploys unique strategies to invade different cell types, survive and move inside the eukaryotic host cell, and spread from cell to cell. Here, we present the multifaceted Listeria life cycle from a comprehensive perspective. We discuss genetic features of pathogenic Listeria species, analyze factors involved in food contamination, and review bacterial strategies to tolerate stresses encountered both during food processing and along the host's gastrointestinal tract. Then we dissect host-pathogen interactions underlying listerial pathogenesis in mammals from a cell biology and systemic point of view. Finally, we summarize the epidemiology, pathophysiology, and clinical features of listeriosis in humans and animals. This work aims to gather information from different fields crucial for a comprehensive understanding of the pathogenesis of L. monocytogenes.

RNA-Mediated Control in Listeria monocytogenes: Insights Into Regulatory Mechanisms and Roles in Metabolism and Virulence

Listeria monocytogenes is an intracellular pathogen that is well known for its adaptability to life in a broad spectrum of different niches. RNA-mediated regulatory mechanisms in L. monocytogenes play important roles in successful adaptation providing fast and versatile responses to a changing environment. Recent findings indicate that non-coding RNAs (ncRNAs) regulate a variety of processes in this bacterium, such as environmental sensing, metabolism and virulence, as well as immune responses in eukaryotic cells. In this review, the current knowledge on RNA-mediated regulation in L. monocytogenes is presented, with special focus on the roles and mechanisms underlying modulation of metabolism and virulence. Collectively, these findings point to ncRNAs as important gene regulatory elements in L. monocytogenes, both outside and inside an infected host. However, the involvement of regulatory ncRNAs in bacterial physiology and virulence is still underestimated and probably will be better assessed in the coming years, especially in relation to discovering the regulatory functions of 5′ and 3′ untranslated regions and excludons, and by exploring the role of ncRNAs in interaction with both bacterial and host proteins.