Diagnostic ultrastructure of Listeria monocytogenes in human central nervous tissue

Human Listeriosis

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen that can cause severe invasive infections upon ingestion with contaminated food. Clinically, listerial disease, or listeriosis, most often presents as bacteremia, meningitis or meningoencephalitis, and pregnancy-associated infections manifesting as miscarriage or neonatal sepsis. Invasive listeriosis is life-threatening and a main cause of foodborne illness leading to hospital admissions in Western countries. Sources of contamination can be identified through international surveillance systems for foodborne bacteria and strains' genetic data sharing. Large-scale whole genome studies have increased our knowledge on the diversity and evolution of L. monocytogenes, while recent pathophysiological investigations have improved our mechanistic understanding of listeriosis. In this article, we present an overview of human listeriosis with particular focus on relevant features of the causative bacterium, epidemiology, risk groups, pathogenesis, clinical manifestations, and treatment and prevention.

Listeria monocytogenes at the interface between ruminants and humans: A comparative pathology and pathogenesis review

The bacterium Listeria monocytogenes (Lm) is widely distributed in the environment as a saprophyte, but may turn into a lethal intracellular pathogen upon ingestion. Invasive infections occur in numerous species worldwide, but most commonly in humans and farmed ruminants, and manifest as distinct forms. Of those, neuroinfection is remarkably threatening due to its high mortality. Lm is widely studied not only as a pathogen but also as an essential model for intracellular infections and host-pathogen interactions. Many aspects of its ecology and pathogenesis, however, remain unclear and are rarely addressed in its natural hosts. This review highlights the heterogeneity and adaptability of Lm by summarizing its association with the environment, farm animals, and disease. It also provides current knowledge on key features of the pathology and (molecular) pathogenesis of various listeriosis forms in naturally susceptible species with a special focus on ruminants and on the neuroinvasive form of the disease. Moreover, knowledge gaps on pathomechanisms of listerial infections and relevant unexplored topics in Lm pathogenesis research are highlighted.

Imaging of Human Neurolisteriosis: A Prospective Study of 71 Cases

Background

Neurolisteriosis ranks among the most severe neurological infections. Its radiological features have not been thoroughly studied. We describe here the neuroradiological features of neurolisteriosis and assess their prognostic value.

Methods

Patients with microbiologically proven neurolisteriosis were enrolled from November 2009 to October 2013 in MONALISA study. Magnetic resonance and computed tomography images were studied by 2 independent neuroradiologists. Predictors of 3-month mortality were determined using logistic regression.

Results

Seventy-one patients were included; 42 were men (59%). Mean age was 64 years. Sixty patients (85%) reported signs of encephalitis, with clinical brainstem involvement in 16 (23%). Images were abnormal in 87% of cases (62/71). Main neuroradiological images were meningeal enhancement (25/71, 35%), abscess(es), or nodular image(s) evocative of abscess (10/71, 14%), hemorrhages (11/71, 15%), contrast-enhancing ventricles, or hydrocephalus (7/71, 10%). White-matter images (42/71, 59%), dilated Virchow-Robin spaces (22/71, 31%), and cerebral atrophy were also reported (34/71, 48%). Brainstem involvement (meningeal enhancement, abscess) was reported in only 7/71 cases (10%). Three-month survival was lower in patients with hydrocephalus or contrast-enhancing ventricles (1/7 [14%] than without [47/64, 73%], P = .005) and in patients with parenchymal images (abscess[es], nodule[s]\, or white matter images; 25/46 [54%] vs 23/25 without [92%], P = .004). Parenchymal images were associated with lower 3-month survival in the multivariable model (odds ratio 5.60, 95% confidence interval [1.42-29.6], P = .02).

Conclusions

Neurolisteriosis presents as a combination of neuroradiological images, none being specific. Radiological signs of rhombencephalitis are uncommon, whereas, unexpectedly, hemorrhagic images are frequent. The negative prognostic value of parenchymal neuroradiological images was evidenced.

Clinical Trials Registration

NCT01520597.

Histopathology of Listeria Meningitis

Listeria monocytogenes meningitis is the third most common cause of bacterial meningitis in adults and has high mortality and morbidity rates. We describe the clinical course and score brain pathology of 5 patients who died of listeria meningitis. All patients were immunocompromised and ages ranged between 48 and 76 years. Three cases were confirmed by cerebrospinal fluid culture; one was confirmed by brain culture; and one diagnosis was based on a positive blood culture and neuropathological findings. Mild inflammation of meningeal arteries was found in 3 of 5 cases (60%). Moderate/severe ventriculitis was seen in 4 of 4 cases (100%), abscesses in 3 of 4 cases (75%), mild vascular inflammation in 4 of 5 cases (80%), mild/moderate hemorrhage in 2 of 4 cases (50%), mild/moderate thrombosis of meningeal artery in 3 of 5 cases (60%), and 1 case (25%) showed a moderate infarct. The inflammatory cells present in the meninges were characterized by a mix of monocytes, macrophages, and neutrophils and removal of apoptotic inflammatory cells by macrophages (efferocytosis). Gram stain showed intra- and extracellular presence of rod-shaped bacteria in 3 cases. Pathological examination was characterized by moderate to severe ventriculitis, abscesses and abundant efferocytosis which has been suggested to be exploited by L. monocytogenes for cell-to-cell spread.

To Be Cytosolic or Vacuolar: The Double Life of Listeria monocytogenes

Intracellular bacterial pathogens are generally classified into two types: those that exploit host membrane trafficking to construct specific niches in vacuoles (i.e., "vacuolar pathogens"), and those that escape from vacuoles into the cytosol, where they proliferate and often spread to neighboring cells (i.e., "cytosolic pathogens"). However, the boundary between these distinct intracellular phenotypes is tenuous and may depend on the timing of infection and on the host cell type. Here, we discuss recent progress highlighting this phenotypic duality in Listeria monocytogenes, which has long been a model for cytosolic pathogens, but now emerges as a bacterium also capable of residing in vacuoles, in a slow/non-growing state. The ability of L. monocytogenes to enter a persistence stage in vacuoles might play a role during the asymptomatic incubation period of listeriosis and/or the carriage of this pathogen in asymptomatic hosts. Moreover, persistent vacuolar Listeria could be less susceptible to antibiotics and more difficult to detect by routine techniques of clinical biology. These hypotheses deserve to be explored in order to better manage the risks related to this food-borne pathogen.

Endothelial Cells Use a Formin-Dependent Phagocytosis-Like Process to Internalize the Bacterium Listeria monocytogenes

Vascular endothelial cells act as gatekeepers that protect underlying tissue from blood-borne toxins and pathogens. Nevertheless, endothelial cells are able to internalize large fibrin clots and apoptotic debris from the bloodstream, although the precise mechanism of such phagocytosis-like uptake is unknown. We show that cultured primary human endothelial cells (HUVEC) internalize both pathogenic and non-pathogenic Listeria bacteria comparably, in a phagocytosis-like process. In contrast with previously studied host cell types, including intestinal epithelial cells and hepatocytes, we find that endothelial internalization of Listeria is independent of all known pathogenic bacterial surface proteins. Consequently, we exploited the internalization and intracellular replication of L. monocytogenes to identify distinct host cell factors that regulate phagocytosis-like uptake in HUVEC. Using siRNA screening and subsequent genetic and pharmacologic perturbations, we determined that endothelial infectivity was modulated by cytoskeletal proteins that normally modulate global architectural changes, including phosphoinositide-3-kinase, focal adhesions, and the small GTPase Rho. We found that Rho kinase (ROCK) is acutely necessary for adhesion of Listeria to endothelial cells, whereas the actin-nucleating formins FHOD1 and FMNL3 specifically regulate internalization of bacteria as well as inert beads, demonstrating that formins regulate endothelial phagocytosis-like uptake independent of the specific cargo. Finally, we found that neither ROCK nor formins were required for macrophage phagocytosis of L. monocytogenes, suggesting that endothelial cells have distinct requirements for bacterial internalization from those of classical professional phagocytes. Our results identify a novel pathway for L. monocytogenes uptake by human host cells, indicating that this wily pathogen can invade a variety of tissues by using a surprisingly diverse suite of distinct uptake mechanisms that operate differentially in different host cell types.

Listeriolysin O mediates cytotoxicity against human brain microvascular endothelial cells

Penetration of the brain microvascular endothelial layer is one of the routes Listeria monocytogenes use to breach the blood-brain barrier. Because host factors in the blood severely limit direct invasion of human brain microvascular endothelial cells (HBMECs) by L. monocytogenes, alternative mechanisms might be used by this bacterium to penetrate the endothelial cell layer. In this study, we evaluated the cytotoxicity of proteins secreted by L. monocytogenes against HBEMCs using a live/dead staining method. Interestingly, the integrity of the plasma membrane of HBMECs was impaired by proteins secreted by the EGD wild-type strain but not proteins secreted by the isogenic ΔprfA strain. Therefore, we investigated the cytotoxicity of proteins secreted by several isogenic mutant strains (ΔplcA, Δmpl and Δhly) incapable of producing the prfA-regulated bacterial products PlcA, Mpl and LLO, respectively. Results from both fluorescent microscopy and flow cytometry analyses showed that proteins secreted by the Δhly strain were not cytotoxic to HBMECs, whereas those secreted by the ΔplcA and Δmpl strains were cytotoxic. These results suggest that LLO-mediated cytotoxicity against brain microvascular endothelial cells enables L. monocytogenes to effectively penetrate the brain microvascular endothelial layer.

The endothelium as a target for infections

The endothelial cells lining vascular and lymphatic vessels are targets of several infectious agents, including viruses and bacteria, that lead to dramatic changes in their functions. Understanding the pathophysiological mechanisms that cause the clinical manifestations of those infections has been advanced through the use of animal models and in vitro systems; however, there are also abundant studies that explore the consequences of endothelial infection in vitro without supporting evidence that endothelial cells are actual in vivo targets of infection in human diseases. This article defines criteria for considering an infection as truly endothelium-targeted and reviews the literature that offers insights into the pathogenesis of human endothelial-target infections.

Invasion of the central nervous system by intracellular bacteria

Infection of the central nervous system (CNS) is a severe and frequently fatal event during the course of many diseases caused by microbes with predominantly intracellular life cycles. Examples of these include the facultative intracellular bacteria Listeria monocytogenes, Mycobacterium tuberculosis, and Brucella and Salmonella spp. and obligate intracellular microbes of the Rickettsiaceae family and Tropheryma whipplei. Unfortunately, the mechanisms used by intracellular bacterial pathogens to enter the CNS are less well known than those used by bacterial pathogens with an extracellular life cycle. The goal of this review is to elaborate on the means by which intracellular bacterial pathogens establish infection within the CNS. This review encompasses the clinical and pathological findings that pertain to the CNS infection in humans and includes experimental data from animal models that illuminate how these microbes enter the CNS. Recent experimental data showing that L. monocytogenes can invade the CNS by more than one mechanism make it a useful model for discussing the various routes for neuroinvasion used by intracellular bacterial pathogens.

Multiple cerebral abscesses because of Listeria monocytogenes: three case reports and a literature review of supratentorial listerial brain abscess(es)

BACKGROUND

Central nervous system involvement often follows bacteremia because of Listeria monocytogenes. Meningitis is clinically the most common manifestation, while brain abscess occurs in about 1% of patients. Brain abscess is usually solitary but in recent years, probably in part because of the availability of computerized tomography and magnetic resonance imaging, several reports have described two or more separate supratentorial abscesses.

METHODS

We have described three patients with listerial brain abscesses and reviewed the North American and European literature of brain abscess(es) because of L. monocytogenes through December 2001. We have evaluated the role of underlying diseases and therapeutic immunosuppression on the development of solitary or greater than one brain abscess.

RESULTS

In contrast to meningitis, where immunosuppression does not predispose either to disease incidence or to higher mortality, patients with solitary and particularly those with more than one supratentorial abscess usually are immunosuppressed either by disease or by therapy. Corticosteroids in particular are significant predisposing factors, especially in those patients with two or more brain abscesses. Mortality resulting from listerial brain abscess, whether solitary or multiple, is nearly three times higher than nonlisterial brain abscess, probably in part because of both underlying diseases and immunosuppressive therapy.

CONCLUSIONS

Therapy with high-dose ampicillin in combination with gentamicin appear to be the drugs of choice, followed by trimethoprim/sufamethoxazole and vancomycin. In general, antimicrobial therapy appears to be satisfactory treatment without surgical intervention.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Antibodies to Listeria monocytogenes

Listeria monocytogenes is one of the leading foodborne pathogens and has been implicated in numerous outbreaks in the last 2 decades. Immunocompromised populations are usually the most susceptible to Listeria infections. Although the pathogenic mechanism is a complex process, significant progress has been made in unravelling the mechanism in recent years. It is now clear that numerous extracellular and cell-associated proteins, such as internalin, listeriolysin, actin polymerization protein, phospholipase, metalloprotease, and possibly p60 proteins, are essential for L. monocytogenes entry into mammalian cells, survival inside the phagosome, escape into the cytoplasm, and cell-to-cell spread. Other proteins may be responsible for growth and physiology or to maintain the structural integrity of the bacteria. Monoclonal and polyclonal antibodies have been developed against many of those antigens or their synthetic derivatives that have helped greatly to determine the structure and function of these antigens. The antibodies were also used for the diagnosis and detection, immunocytochemical staining, and serotyping of Listeria. Humoral immune response to live L. monocytogenes cells was examined in naturally or experimentally infected hosts. Studies revealed that only extracellular antigens induced the humoral response, whereas cell-associated antigens had apparently no response. It is speculated that during the occasional bacteremic phase, L. monocytogenes releases extracellular antigens that are then processed by the immune system for antibody production. As L. monocytogenes is an intracellular pathogen, the cell-associated antigens are not persistent in the blood circulation and thus fail to stimulate the humoral immune response.