Catalase-negative listeria monocytogenes causing meningitis in an adult. Clinical and laboratory features

Taxonomy, ecology, and relevance to food safety of the genus Listeria with a particular consideration of new Listeria species described between 2010 and 2022

Since 2010, the genus Listeria has had the addition of 22 new species that more than tripled the number of species identified until 2010. Sixteen of these 22 new species are distantly related to the type species, Listeria monocytogenes, and several of these present phenotypes that distinguish them from classical Listeria species (L. monocytogenes, Listeria innocua, Listeria ivanovii, Listeria seeligeri, Listeria welshimeri, and Listeria grayi). These 22 newly described species also show that Listeria is more genetically diverse than previously estimated. While future studies and surveys are needed to clarify the distribution of these species, at least some of these species may not be widely spread, while other species may be frequently found spread to human-related settings (e.g., farms and processing facilities), and others may be adapted to specific environmental habitats. Here, we review the taxonomic, phylogenetic, and ecological characteristics of these new Listeria species identified since 2010 and re-iterate the suggestion of re-classification of some species into three new genera: Murraya, Mesolisteria, and Paenilisteria. We also provide a review of current detection issues and the relevance to food safety related to the identification of these new species. For example, several new non-pathogenic species could be misidentified as the pathogen L. monocytogenes, based on methods that do not target L. monocytogenes-specific virulence genes/factors, leading to unnecessary product recalls. Moreover, eight species in the proposed new genus Mesolisteria are not good indicators of environmental conditions that could allow L. monocytogenes to grow since Mesolisteria species are unable to grow at low temperatures.

Soil Collected in the Great Smoky Mountains National Park Yielded a Novel Listeria sensu stricto Species, L. swaminathanii

Soil samples collected in the Great Smoky Mountains National Park yielded a Listeria isolate that could not be classified to the species level. Whole-genome sequence-based average nucleotide identity BLAST and in silico DNA-DNA Hybridization analyses confirmed this isolate to be a novel Listeria sensu stricto species with the highest similarity to L. marthii (ANI = 93.9%, isDDH = 55.9%). Additional whole-genome-based analysis using the Genome Taxonomy Database Toolkit further supported delineation as a novel Listeria sensu stricto species, as this tool failed to assign a species identification. Phenotypic and genotypic characterization results indicate that this species is nonpathogenic. Specifically, the novel Listeria species described here is phenotypically (i) nonhemolytic and (ii) negative for phosphatidylinositol-specific phospholipase C activity; the draft genome lacks all virulence genes found in the Listeria pathogenicity islands 1, 2, 3, and 4 as well as the internalin genes inlA and inlB. While the type strain contains an apparently intact catalase gene (kat), this strain is phenotypically catalase-negative (an unusual characteristic for Listeria sensu stricto species). Additional analyses identified a nonsynonymous mutation in a conserved codon of kat that is likely linked to the catalase-negative phenotype. Rapid species identification systems, including two biochemical and one matrix-assisted laser desorption/ionization, misidentified this novel species as either L. monocytogenes, L. innocua, or L. marthii. We propose the name L. swaminathanii, and the type strain is FSL L7-0020T (=ATCC TSD-239T). IMPORTANCEL. swaminathanii is a novel sensu stricto species that originated from a US National Park and it will be the first Listeria identified to date without official standing in the nomenclature. Validation was impeded by the National Park's requirements for strain access, ultimately deemed too restrictive by the International Committee on Systematics of Prokaryotes. However, lack of valid status should not detract from the significance of adding a novel species to the Listeria sensu stricto clade. Notably, detection of non-monocytogenes sensu stricto species in a food processing environment indicate conditions that could facilitate the presence of the pathogen L. monocytogenes. If isolated, our data show a potential for L. swaminathanii to be misidentified as another sensu stricto, notably L. monocytogenes. Therefore, developers of Listeria spp. detection and identification methods, who historically only include validly published species in their validation studies, should include L. swaminathanii to ensure accurate results.

Listeria costaricensis sp. nov

A bacterial strain isolated from a food processing drainage system in Costa Rica fulfilled the criteria as belonging to the genus Listeria, but could not be assigned to any of the known species. Phylogenetic analysis based on the 16S rRNA gene revealed highest sequence similarity with the type strain of Listeria floridensis (98.7 %). Phylogenetic analysis based on Listeria core genomes placed the novel taxon within the Listeria fleishmannii, L. floridensis and Listeria aquatica clade (Listeria sensu lato). Whole-genome sequence analyses based on the average nucleotide blast identity (ANI<80 %) indicated that this isolate belonged to a novel species. Results of pairwise amino acid identity (AAI>70 %) and percentage of conserved proteins (POCP>68 %) with currently known Listeria species, as well as of biochemical characterization, confirmed that the strain constituted a novel species within the genus Listeria. The name Listeria costaricensis sp. nov. is proposed for the novel species, and is represented by the type strain CLIP 2016/00682^T (=CIP 111400^T=DSM 105474^T).

Temperature-dependent requirement for catalase in aerobic growth of Listeria monocytogenes F2365

Listeria monocytogenes is a Gram-positive, psychrotrophic, facultative intracellular food-borne pathogen responsible for severe illness (listeriosis). The bacteria can grow in a wide range of temperatures (1 to 45°C), and low-temperature growth contributes to the food safety hazards associated with contamination of ready-to-eat foods with this pathogen. To assess the impact of oxidative stress responses on the ability of L. monocytogenes to grow at low temperatures and to tolerate repeated freeze-thaw stress (cryotolerance), we generated and characterized a catalase-deficient mutant of L. monocytogenes F2365 harboring a mariner-based transposon insertion in the catalase gene (kat). When grown aerobically on blood-free solid medium, the kat mutant exhibited impaired growth, with the extent of impairment increasing with decreasing temperature, and no growth was detected at 4°C. Aerobic growth in liquid was impaired at 4°C, especially under aeration, but not at higher temperatures (10, 25, or 37°C). Genetic complementation of the mutant with the intact kat restored normal growth, confirming that inactivation of this gene was responsible for the growth impairment. In spite of the expected impact of oxidative stress responses on cryotolerance, cryotolerance of the kat mutant was not affected.

Listeriosis due to infection with a catalase-negative strain of Listeria monocytogenes

A strain of Listeria monocytogenes recovered from blood and cerebrospinal fluid had no detectable catalase activity, a characteristic used for primary identification. The sporadic occurrence of pathogenic catalase-negative strains highlights the need for a reconsideration of diagnostic criteria and questions the role of catalase in the pathogenesis of listeria infection.

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.

Central nervous system infection with Listeria monocytogenes. 33 years' experience at a general hospital and review of 776 episodes from the literature

We reviewed 776 previously reported and 44 new cases of CNS listeriosis outside of pregnancy and the neonatal period, and evaluated the epidemiologic, diagnostic, and therapeutic characteristics of this infection. Among patients with Listeria meningitis/meningoencephalitis, hematologic malignancy and kidney transplantation were the leading predisposing factors, but 36% of patients had no underlying diseases recognized. The infection occurred throughout life, with a higher incidence before the age of 3 and after the age of 45-50 years. Fever, altered sensorium, and headache were the most common symptoms, but 42% of patients had no meningeal signs on admission. Compared with patients with acute meningitis due to other bacterial pathogens, patients with Listeria infection had a significantly lower incidence of meningeal signs, and the CSF profile was significantly less likely to have a high WBC count or a high protein concentration. Gram stain of CSF was negative in two-thirds of cases of CNS listeriosis. One-third of patients had focal neurologic findings, and approximately one-fourth developed seizures over their course. Mortality was 26% overall, and was higher among patients with seizures and those older than 65 years of age. Relapse occurred in 7% of episodes. Ampicillin for a minimum of 15-21 days (with an aminoglycoside for at least the first 7-10 days) remains the treatment of choice. Cerebritis/abscess due to L. monocytogenes, without meningeal involvement, is less common but may be diagnosed by blood cultures and CNS imaging, or by stereotactic biopsy. Longer antibiotic therapy (at least 5-6 weeks) is needed in the presence of localized CNS involvement.

Isolation of catalase-negative Listeria monocytogenes strains from listeriosis patients and their rapid identification by anti-p60 antibodies and/or PCR

Two catalase-negative Listeria monocytogenes serovar 1/2b strains were isolated from listeriosis patients in 1995 in Germany. The infections appeared in individuals from different cities at different seasons and were caused by L. monocytogenes strains of different clonal types. In particular, the catalase reaction of one strain isolated from blood was consistently negative, whereas this reaction was only reversibly blocked when the strain was freshly isolated from ascitic fluid. After subculturing, the catalase-positive reaction was restored. Initially, identification of these isolates was difficult to achieve not only because of the lack of a catalase reaction, which generally distinguishes L. monocytogenes from other morphologically similar pathogenic gram-positive bacteria, but also because other routinely used biochemical tests such as CAMP and the commercial API test gave unclear results. However, rapid and unequivocal identification of these strains was possible by analyzing secretions of the p60 protein in culture supernatants by enzyme-linked immunosorbent assay or Western blot (immunoblot) analysis with our recently developed Listeria- and L. monocytogenes-specific anti-p60 antibodies. Additionally, the identifications were confirmed by Listeria- and L. monocytogenes-specific PCR analyses with primers derived from the iap, hly, and prfA genes. Immunoanalyses also allowed for the differentiation of these two strains, whereas no differentiation was possible by PCR when the internal, variable repetitive iap gene portion was analyzed. However, size variations of the PCR products comprising these gene portions which were obtained from a number of L. monocytogenes strains belonging to the same serotypes indicated that this type of PCR is not only useful for specific identifications but may be used in parallel as an additional marker for epidemiological studies. In conclusion, the data suggest that catalase production should not be taken as a strict criterion for the identification of listeriae. Furthermore, at least the infection caused by the stably catalase-negative strain supports the notion that catalase does not seem to be necessary for the intracellular growth of L. monocytogenes.