Listeria monocytogenes
Gram-positive, facultative intracellular, short rod; one of the most elegant bacterial models of cytoskeletal exploitation and intercellular spread. Psychrotrophic growth at 4°C (refrigerator temperature), salt tolerance (up to 10% NaCl), and pH tolerance (pH 4.4–9.4) make cold-chain contamination of ready-to-eat foods (deli meats, soft cheeses, smoked salmon) a persistent public-health hazard. All major virulence genes are controlled by the PrfA transcriptional activator (Crp/Fnr family; mRNA thermosensor active at 37°C). InlA (Internalin A) binds human E-cadherin (CDH1) at Glu16 — a residue unique to human/guinea pig E-cadherin — triggering clathrin-mediated endocytosis into intestinal epithelial cells and syncytiotrophoblasts. LLO (listeriolysin O) — a cholesterol-dependent cytolysin optimally active at phagosomal pH (5.5) and inactivated at cytoplasmic pH (7.2) — forms pores in the phagosomal membrane, releasing bacteria into the nutrient-rich cytosol. ActA nucleates host Arp2/3 complex to form branched actin comet tails (0.1–1.4 µm/min) that propel bacteria into adjacent cells via double-membrane vacuoles, never exposing them to extracellular antibodies or complement. CNS disease (meningoencephalitis, rhombencephalitis) and pregnancy-associated listeriosis carry 20–50% case fatality rates. Intrinsic cephalosporin resistance — critical prescribing point: add ampicillin to cephalosporin empirical meningitis regimens in at-risk patients.
Classification & Structure
| Gram stain | Gram-positive; thick peptidoglycan cell wall; no outer membrane; lipoteichoic acids on surface; beta-haemolysis on blood agar (LLO-mediated) — distinguishes pathogenic from non-pathogenic Listeria species |
| Morphology | Short rod, 0.4–0.5 µm × 0.5–2.0 µm; occurs singly or in pairs; no capsule; no spores; catalase-positive, oxidase-negative; produces acetoin (VP+); hydrolyses aesculin (esculin agar) — used in selective media (Oxford, PALCAM) |
| Motility | Peritrichous flagella expressed at <37°C (characteristic tumbling motility at room temperature on semisolid agar — "umbrella motility"); flagella downregulated at 37°C inside the host (PrfA regulation — avoids TLR5 innate sensing) |
| Toxins | LLO (listeriolysin O; cholesterol-dependent cytolysin; pore-forming; pH-optimised for phagosome lysis); PI-PLC (PlcA; phosphatidylinositol-specific phospholipase C; cooperates with LLO for primary phagosome disruption); PC-PLC (PlcB; broad-range phospholipase C; activated by metalloprotease Mpl; lyses double-membrane secondary phagosome) |
| Spores | None formed — no sporulation in Listeria. Environmental persistence relies on biofilm formation in food processing environments, psychrotrophic growth, and acid/salt/osmotic stress tolerance mediated by sigma factor σB. |
Pathogenesis
1 · PrfA thermoswitch — virulence gene master regulator
PrfA (Positive regulatory factor A) is a Crp/Fnr transcription factor family member that activates all major virulence genes at body temperature. A 5' UTR RNA thermosensor (mRNA thermometer) in the prfA mRNA hairpin melts at 37°C, exposing the Shine-Dalgarno sequence and allowing ribosome access. This elegant thermosensing mechanism ensures virulence genes — hly (LLO), plcA, plcB, actA, inlA, inlB, mpl — are expressed in warm-blooded hosts but silenced in the cold food environment, preventing premature expression that would waste metabolic resources.
2 · InlA/E-cadherin invasion — intestinal epithelium and placenta
InlA (Internalin A) is an LPXTG-anchored surface protein that binds E-cadherin (CDH1) at position Glu16 — a residue unique to human and guinea pig E-cadherin (rodent E-cadherin has Pro16, explaining poor mouse model fidelity for human listeriosis). E-cadherin is normally basolateral but is accessible apically at villous tips where cells are shed and in M cells of Peyer's patches. InlA binding co-opts the E-cadherin endocytosis machinery (clathrin-mediated; α-catenin/β-catenin/α-actinin recruitment) to internalise the bacterium. InlA also binds E-cadherin on human syncytiotrophoblasts and extravillous trophoblasts → placental invasion → fetal bacteraemia. InlB (Internalin B) binds Met (hepatocyte growth factor receptor) and gC1qR on hepatocytes and brain microvascular endothelial cells → hepatocyte and BBB invasion.
3 · LLO phagosomal escape — the cytosol breakthrough
After internalisation, L. monocytogenes faces destruction in the phagosomal compartment. LLO (listeriolysin O) is a cholesterol-dependent cytolysin (CDC family) that inserts into the phagosomal membrane as oligomeric pores. LLO is optimally active at phagosomal pH (5.5) and is denatured/inactivated at cytoplasmic pH (7.2) — a critical safety mechanism preventing LLO from lysing the plasma membrane after phagosomal escape. PI-PLC (PlcA) cooperates with LLO in disrupting the primary single-membrane phagosome. The bacterium escapes into the nutrient-rich cytosol, where it replicates rapidly (generation time ~1 h at 37°C). Once in the cytosol, the bacterium activates host cGAS-STING (sensing cytosolic bacterial DNA) and NOD1/NOD2 (sensing peptidoglycan fragments), but these innate responses are not sufficient to clear infection in immunocompromised hosts.
4 · ActA actin comet tail — intracellular motility and cell-to-cell spread
Once in the cytosol, L. monocytogenes exploits host cytoskeletal machinery for intracellular motility. ActA (expressed asymmetrically on one bacterial pole) functions as a WASP-family protein mimic, directly activating the host Arp2/3 complex to nucleate branched F-actin networks at the bacterial surface. Host VASP, profilin, cofilin, and ADF are recruited — the same molecular ensemble driving lamellipodium extension in motile cells. The resulting actin comet tail propels the bacterium at 0.1–1.4 µm/min through the cytoplasm toward the cell periphery. The bacterium pushes against the plasma membrane, forming a protrusion into the adjacent cell. This protrusion is engulfed by the neighbour to form a double-membrane vacuole (secondary phagosome). PC-PLC (PlcB) and LLO cooperate to lyse both membranes → bacteria released into the next cell's cytosol → cycle repeats. This mechanism allows dissemination through tissues entirely intracellularly, never exposed to extracellular antibodies, complement, or neutrophils.
5 · Blood-brain barrier and feto-placental barrier crossing
Three complementary mechanisms enable CNS invasion: (i) Trojan horse via infected monocytes: circulating infected monocytes (which acquired Listeria from the liver/spleen) cross the inflamed BBB endothelium; (ii) Direct InlB/Met-dependent invasion of brain microvascular endothelial cells (BMEC) → intracellular spread through the BBB; (iii) Choroid plexus: InlA-dependent invasion of choroid plexus epithelial cells (which express apical E-cadherin) → direct entry into CSF. Feto-placental crossing: InlA binds E-cadherin on human syncytiotrophoblasts; InlB binds Met on extravillous trophoblasts → placental invasion → fetal bacteraemia → chorioamnionitis → miscarriage, stillbirth, or neonatal sepsis (granulomatosis infantiseptica).
6 · Cytosolic innate sensing — paradoxical cGAS-STING response
Once in the cytosol, L. monocytogenes triggers multiple innate sensors: NOD1/NOD2 (peptidoglycan fragments → NF-κB → IL-6, IL-12, TNF-α); NLRP3 inflammasome (LLO-induced K⁺ efflux → IL-1β/IL-18 + pyroptosis); AIM2 inflammasome (cytosolic dsDNA → IL-1β + pyroptosis); and critically, cGAS-STING (bacterial DNA + mitochondrial DNA released by LLO pore-forming activity → type I IFN production). Paradoxically, type I IFN promotes listeriosis by inducing lymphocyte apoptosis and downregulating IL-17 responses, creating an immunosuppressive niche — explaining why IFN-β treatment can worsen listeriosis experimentally.
Host Immune Response
Disease Spectrum
| Presentation | Severity | At-Risk Groups | Mortality |
|---|---|---|---|
| Febrile gastroenteritis | Self-limited; watery diarrhoea, fever, myalgia, headache; 6–48h after ingestion of heavily contaminated food (outbreak setting); no bacteraemia; resolves 1–3 days | Any person ingesting large inoculum (>10⁷–10⁸ CFU); outbreak food vehicles: coleslaw, corn salad, cheese | <1% |
| Invasive listeriosis — primary bacteraemia | Fever, rigors, bacteraemia; without CNS involvement; may present as septic shock in severe cases; blood cultures positive for Gram-positive rods | Immunocompromised (>65 years, corticosteroids, TNF-α inhibitors, calcineurin inhibitors, haematological malignancy, HIV); diabetes | 20–30% |
| Meningoencephalitis | Fever, stiff neck, altered consciousness; CSF: mononuclear or mixed pleocytosis (unique among bacterial meningitis — most bacterial causes are neutrophilic), elevated protein; Listeria often misidentified as "Gram-positive rod contaminant" or diphtheroids in CSF — clinical vigilance essential | Immunocompromised; elderly (>65); organ transplant; haematological malignancy; pregnant (rare CNS in mother; more common in neonate) | 30–40% |
| Rhombencephalitis | Brainstem involvement: cranial nerve palsies (CN VI, VII, IX, X); ataxia, nystagmus, dysarthria; altered consciousness; MRI: T2 hyperintensity in pons, medulla, cerebellum; may occur in previously healthy adults | Adults (can occur in immunocompetent); distinct clinical presentation; misdiagnosed as viral encephalitis/stroke | ~50% |
| Maternal-neonatal listeriosis | Maternal: flu-like febrile illness, bacteraemia → placental infection → chorioamnionitis → fetal infection. Neonatal early-onset (<5 days): septicaemia, pneumonia, granulomatosis infantiseptica (miliary abscesses in liver/spleen). Neonatal late-onset (5–28 days): meningitis (birth canal exposure, different from transplacental) | Pregnant women (3rd trimester highest risk; 17× higher risk than general population); neonates; uncooked soft cheeses, deli meats, smoked fish | Maternal ~1%; neonatal early-onset 20–40%; neonatal late-onset meningitis 30–50% |
Treatment & Prophylaxis
| Ampicillin IV (drug of choice) | 2 g IV every 4 hours; bactericidal against Listeria; first-line for all forms of invasive listeriosis. Critical prescribing note: Listeria is intrinsically resistant to all cephalosporins (including 3rd-generation ceftriaxone, cefotaxime) — add ampicillin to any empirical meningitis regimen in at-risk patients (elderly, pregnant, immunocompromised) even when cephalosporin is already given. |
| Gentamicin (synergy) | Added to ampicillin for bacteraemia and meningitis; synergistic killing in vitro and in animal models (aminoglycoside crosses bacterial cell wall after ampicillin-mediated permeabilisation); dose 1–1.5 mg/kg IV every 8h (renal dose adjustment); standard for non-pregnant adults with invasive listeriosis. |
| TMP-SMX (penicillin allergy) | Trimethoprim-sulfamethoxazole (TMP-SMX; co-trimoxazole) 5/25 mg/kg IV or PO every 6–8h; best-validated alternative; bactericidal against Listeria; avoid in 1st trimester (folate antagonism) and near term (kernicterus risk). |
| Duration | Bacteraemia: 14 days; meningitis: 21 days; brain abscess or rhombencephalitis: 42–56 days (4–8 weeks); neonatal meningitis: 21 days. Prolonged courses required because of intracellular sanctuary sites. |
| Dexamethasone (NOT recommended) | Dexamethasone is standard adjunctive therapy for pneumococcal meningitis but is contraindicated in Listeria meningitis — it reduces ampicillin penetration into the CSF and may worsen outcomes. This distinction is critical when empirical treatment covers both pathogens. |
| No vaccine | No approved human vaccine against listeriosis. Prevention is based on food safety: HACCP (hazard analysis critical control points) protocols, refrigeration temperature monitoring (≤4°C), and dietary advice for high-risk groups during pregnancy (avoid unpasteurised soft cheeses, deli meats, ready-to-eat smoked fish, raw sprouts). |
| Resistance | Resistance to ampicillin is extremely rare in L. monocytogenes; no validated plasmid-mediated ampicillin resistance reported. Intrinsic resistance to all cephalosporins, fosfomycin, and fluoroquinolones (reduced target affinity). Linezolid has activity in vitro and is used in compassionate cases of refractory CNS listeriosis when ampicillin cannot be used. |
Connections
References
- Cossart P. Illuminating the landscape of host-pathogen interactions with the bacterium Listeria monocytogenes. Proc Natl Acad Sci USA. 2011;108(49):19484-91. doi:10.1073/pnas.1112371108 · PubMed 22114192
- Vazquez-Boland JA, Kuhn M, Berche P, et al. Listeria pathogenesis and molecular virulence determinants. Clin Microbiol Rev. 2001;14(3):584-640. doi:10.1128/CMR.14.3.584-640.2001 · PubMed 11432815
- Lecuit M, Dramsi S, Gottardi C, et al. A single amino acid in E-cadherin responsible for host specificity towards Listeria monocytogenes. EMBO J. 1999;18(14):3956-63. doi:10.1093/emboj/18.14.3956 · PubMed 10406798
- Bennett JE, Dolin R, Blaser MJ. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Elsevier; 2020. elsevier.com
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021. elsevier.com
- World Health Organization. Listeriosis. WHO Fact Sheet. 2018. who.int/news-room/fact-sheets/detail/listeriosis
Contribute to the Pathogen Atlas
This entry covers L. monocytogenes biology, InlA/LLO/ActA mechanisms, innate cytosolic sensing, and clinical management. Planned expansions include whole-genome sequencing outbreak tracing methodology, BBB invasion structural biology, and food safety HACCP frameworks. Every entry follows the same schema: structured frontmatter, peer-reviewed citations, and cross-atlas links.