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

Abstract

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.

Keywords: Listeriosis; food contamination; intracellular pathogen; pathogenesis; stress response.

Graphical abstract

Figure 1.

L. monocytogenes contamination sources. Transmission scenarios for L. monocytogenes between soil/water, animals, crop/vegetables/fruits, industries, food products, humans, and environment. Potential transmission pathways indicated by arrows

Figure 2.

L. monocytogenes response to stress encountered in the environment and within host. Different types of stress encountered in food and food-processing environments (blue) and within host (beige) indicated with green arrows. Low pH and high osmolality are relevant stresses both in host and food. L. monocytogenes responses depicted as sectors. Genes participating in responses to both low temperature and high osmolality (helicases, ribosomal proteins, transporters) written on boundary of respective sectors. L. monocytogenes that tolerates quaternary ammonium compound (QACs) exposure overexpresses efflux pumps, like bcrABC operon. Two representative groups of enzymes induced specifically upon cold exposure: helicases, e.g. lmo0866, an RNA helicase homologue to DEAD-box protein A; and RNases, e.g. lmo1027, protein similar to Ribonuclease J1. Sodium/proton antiporter, encoded by operon mrpABCDEFG, induced exclusively under high osmolality conditions, as in salt-preserved food. Large overlap exists in response to high osmolality and low temperature: helicases, ribosomal proteins, and well-characterized osmolyte transporters like oppBCE, betL, gbu and opuC. cspA and cspD induced in both conditions: cspA predominant in response to cold shock; cspD appears in high osmotic conditions. Acidic environments increase transcription of exclusion systems as GAD and ADI, intending to raise intracellular pH. In gut, L. monocytogenes competes for nutrients with host microbiota by induction of secondary metabolic pathways like ethanolamine catabolism, or production of bacteriocins like listeriolysin S (LLS). Bile acids secreted to intestine promote induction of bsh and bilE that allow L. monocytogenes survival, and prfA which prepares L. monocytogenes for internalization and intracellular lifestyle. Regulation of these responses not yet fully elucidated (unknown factors), but some regulators shown to play role in control of stress responses are: SigB, two-component system LisRK, and transcriptional regulators HrcA and CtsR

Figure 3.

Model of SigB regulation in response to stress in Listeria monocytogenes. The stressosome is composed of RsbR, RsbR paralogues, RsbS and RsbT proteins. In unstressed conditions, RsbT is predicted as sequestrated within core of complex. Different stress types lead to phosphorylation of RsbR and RsbS by kinase RsbT through unknown mechanisms. This phosphorylation event results in release of RsbT from core of stressosome, initiating downstream SigB activation cascade. Stress-induced release of RsbT activates RsbU phosphatase. RsbU dephosphorylates RsbV, which then binds with RsbW, thus facilitating SigB release. L. monocytogenes triggers SigB-dependent general stress response after exposure to diverse environmental conditions. This model highlights the mechanisms of stressosome activation with the most characterized stress conditions reported to date

Figure 4.

Trespassing the intestinal barrier. A) L. monocytogenes can breach intestinal barrier through three different cellular types: M-cells, goblet cells and enterocytes. L. monocytogenes will reach Peyer’s patches after M-Cell-mediated phagocytosis followed by transcytosis, where it can infect macrophages and dendritic cells, surviving intracellularly. L. monocytogenes invades secretory goblet cells through E-cadherin displayed on its junctions. Trespassing through enterocytes can be mediated in two ways: (i) via E-cadherin exposure during natural cellular extrusion, for instance, as a result of apoptosis of an enterocyte, and (ii) via LAP-HSP60 interaction. This interaction promotes redistribution of claudin-1, occludin and E-cadherin, which perturbs cell junctions, allowing translocation between enterocytes. Also, LAP-HSP60 interaction promotes E-cadherin exposure on lateral face of enterocytes, allowing Listeria cellular invasion and transcytosis. B) Infections of trophoblasts by free-circulating bacteria in blood. C) Species specificities of InlA, InlA^m and InlB

Figure 5.

L. monocytogenes intracellular lifecycle. Invasion of non-phagocytic cells mediated by InlA and InlB interaction with host receptors E-cadherin (E-cad) and C-Met respectively, enhances actin polymerization and leads to bacterial internalization. Once inside a primary vacuole in host cytoplasm, L. monocytogenes can follow different pathways. Bacterium remains in vacuole, leading to transcytosis, as in goblet cells. In some macrophages, L. monocytogenes can replicate inside this vacuole, developing spacious Listeria containing phagosomes (SLAPs), whose formation is associated with autophagy and low LLO secretion. The vacuole can be lysed by virulence factors LLO, PlcA, PlcB, Mpl and PplA. Release of LLO into the cytoplasm has different effects on host cell, like histone modifications, mitochondrial fission, etc. In the cytoplasm of trophoblasts and hepatocytes, L. monocytogenes can be engulfed into an acidic vacuole known as Listeria containing vacuole (LisCV). Formation of LisCV’s may be due to xenophagy process in host cell and loss of ActA in L. monocytogenes. Listeria in the cytoplasm induces ActA, which interacts with host Arp 2/3 complex and formins. This promotes actin polymerization, which propels Listeria throughout the cytoplasm and leads to protrusion formation on adjacent cell. Internalin C (InlC) secretion in host cell cytoplasm perturbs apical junctions, facilitating cell-to-cell spread. LLO, also secreted in the protrusion, damages host cell membrane, exposing inner phosphatidylserine in exoplasmic layer of protrusion membrane. Exofacial exposure of phosphatidylserine is recognized as an eat-me signal that promotes Listeria-containing vesicle engulfment by macrophages. Therefore, L. monocytogenes also exploits efferocytosis for cell-to-cell spread. Bacterium will be hosted in new cell within double membrane vacuole that can be lysed again, repeating its infectious lifecycle

Figure 6.

Schematic representation of transmission, pathophysiology (right side) and clinical signs (left side) of listeriosis in humans (a) and ruminants (b). Listeria, via contaminated food products, reaches intestine. In immunocompetent humans, Listeria produces febrile gastroenteritis; in immunocompromised individuals it traverses intestinal barrier, produces septicemia, can cross blood–brain barrier and cause meningoencephalitis. Newborn infection occurs as consequence of maternal chorioamnitis (“early-onset” sepsis) or by contamination from birth canal colonized with Listeria from digestive tract (“late-onset” meningitis). Listeria localized infections occur in multiple organs. In ruminants, Listeria vehiculated through contaminated silage crosses oral epithelium (facilitated by small breaches of the oral mucosa), ascends to brain stem via trigeminal nerve, leading to unilateral cranial nerve paralysis and circling disease syndrome. In ruminants, Listeria also causes septicemia, abortion and, less frequently reported, mastitis and eye infections