1 to 6 polar flagellae with tumbling motility at 25ºC
Demonstrates "umbrella" motility in the tube
Can grow at refrigerator temperatures, as low as 4ºC
Doesn't grow on usual stool culture plates
13 serovars based on cellular O and flagellar H antigens
Most disease from 4b, 1/2a, and 1/2b
Epidemiology
In newborns, transmitted vertically from the mother
In others, it is acquired from ingestion of contaminated food followed by mucosal invasion and bacteremia
The bacterium can grow at fridge temperatures
Both sporadic cases as well as foodborne outbreaks
Largest outbreak in Canada was associated with deli meats in 2008
Other outbreaks include ice cream (2015), frozen foods (2016), deli meat at a Druxy's in Princess Margaret Hospital (2018), prepackaged salads and kale (2018)
In gut, it is phagocytosed into endothelial cells, then disseminates hematogenously
Endocytosis is aided by LPXTG on the mucosal cell surface and helps it to adhere, as well as E-cadherin, which activates phagocytosis
Some hypothesize that uptake of Listeria is increased by concurrent GI infections
Listeriolysin O, the major virulence factor, is a pore-forming toxin that prevents T-cell response
Listeriolysin O and phspholipases help is escape the phagosome
Once in the cytoplasm, they activate actin to carry them to the cell membrane, where they push out and form filopods which can be phagocytosed by nearby cells
Moves from cell to cell without time spent outside
In the CNS, it can invade the blood-brain barrier endothelial cells directly, or it can be transported across by cirtculating white blood cells ("Trojan horse" mechanism), or it can be inoculated into oral tissues followed by macrophage phagocytosis and invasion of cranial nerves
The latter may be most important for rhomboencephalitis
Immune response primarily cell-mediated rather than antibody-mediated, since the bacterium doesn't spend any time outside of cells
Risk Factors
Mostly around impaired cellular immunity
Lymphoma, pregnancy (especially third trimester), advanced HIV, and corticosteroid immunosuppression, especially in transplant recipients
In HIV and transplantation, may be prevented by Pneumocystis prophylaxis
Incubation period for invasive disease is 30 days (range 11 to 90 days)
Bacteremia
Most common presentation outside of neonatal period is as a non-specific febrile illness
Can cause non-specific febrile illness, often with a prodrome of diarrhea and nausea, myalgias
Febrile Gastroenteritis
Diarrhea, nausea, and vomiting, often with fever
Associated with foodborne outbreaks, including chocolate milk, cold corn and tuna salad, cold smoked trout, deli meat
Non-invasive in healthy patients
Needs high innoculum
CNS Infections
Causes a spectrum of CNS disease from meningitis to encephalitis to rhombencephalitis to abscesses
Meningitis
A bacterial meningitis with high mortality, usually in very young, old >50 years, pregnant, or immunocompromised
Malignancy is the most common risk factor, followed by transplantation, alcohol use disorder, immunosuppression, diabetes, and HIV
Symptoms include subacute presention of fevers, nausea, and headache, with meningismus, and altered and fluctuating mental status
Focal neurologic deficits
Can sometimes have movement disorders, including ataxia, termors, and myoclonus, as well as seizures
Two thirds have neutrophil-predominant CSF
Mortality 15-30%
Even if cured, can be left with permanent focal neurologic deficits
Encephalitis/Cerebritis
Localized parenchymal infection that can lead to abscess formation
Altered consciousness and cognitive dysfunction
CSF cultures only positive about half the time
Can mimic herpes encephalitis
Rhombencephalitis
Brainstem encephalitis
Can occur in health adults
Diphasic illness, with prodrome of fever, headache, nausea, and vomiting lasting 4 days, followed by asymmetric cranial nerve deficits, cerebellar dysfunction, and focal neurological deficits
Half of patients have respiratory failure
CSF often only mildly abnormal
Brain Abscess
Often bacteremic and concurrent meningitis
Can have abscesses in unusual locations, including subcortical abscesses of the thalamus, pons, and medulla
Spinal Cord Infection
Rare cases
Endocarditis
Can affect both native and prosthetic valves
Mortality 50%
Neonatal Listeriosis
Infection in first and second trimester frequently result in intrauterine fetal death
Later in pregnancy, stillbirth or spontaneous abortion is common
Presents in prematurity or neonatal period as early-onset sepsis, likely acquired in utero during third trimester
Disseminated listeriosis called granulomatosis infantiseptica
Widespread abscesses and granulomas, especially in lungs, liver, and spleen
Can have a miliary pattern on chest x-ray
Listeria can be isolated from conjunctiva, ear, nose, throat, meconium, amniotic fluid, placenta, blood, and sometimes CSF
Can also present in term births as late-onset meningitis (~2 weeks post-partum)
Probably acquired during delivery, since it is far less common with cesarean section
Mortality in neonatal infections is high at around 45%
