Bacilos Gram-positivos

Listeria monocytogenes

Short Gram-positive bacilli, may resemble coccobacilli

Identification & Growth

Identification

Catalase-positive, umbrella-shaped motility at 25 °C, CAMP-positive
Esculin-positive

Growth Conditions

Blood agar: small colonies with a narrow zone of beta-hemolysis
Grows at 4 °C (cold enrichment) and at 35 °C

Clinical Significance

Meningitis in the elderly and immunosuppressed, gestational listeriosis, neonatal sepsis

Intrinsic Resistance

Cephalosporins of all generations (EUCAST Expected Resistant Phenotypes v1.2)
Fosfomycin (may appear susceptible in vitro but is not clinically effective)

Bench Alerts

Do not mistake for a contaminant diphtheroid in CSF
Treatment of choice is ampicillin; a cephalosporin should never be reported as an option

Clinical Notes

Clinical presentations

Causes meningitis/meningoencephalitis and invasive bacteremia, predominantly in neonates, the elderly, and immunosuppressed patients.
In pregnant women, causes mild febrile illness but can lead to chorioamnionitis, miscarriage, stillbirth, or early-onset neonatal sepsis.
Can cause self-limited febrile gastroenteritis in foodborne outbreaks and rhombencephalitis in immunocompetent adults, a rare but severe form.

Specimens and collection

Blood cultures and CSF culture should be obtained in all suspected invasive infection before starting antibiotic therapy.
Placental tissue, amniotic fluid, or meconium should be cultured when gestational/neonatal listeriosis is suspected.
The organism grows at refrigeration temperatures, allowing cold enrichment in low-bacterial-load samples.

Epidemiology and at-risk populations

Transmitted via contaminated food, especially unpasteurized dairy, deli meats, and poorly washed raw vegetables.
Pregnant women, neonates, adults over 65, and immunosuppressed patients (transplant recipients, corticosteroid users) constitute the highest-risk groups.
Outbreaks can be traced to specific food products through public health surveillance and genomic sequencing.

Resistance and therapeutic implications

It is intrinsically resistant to cephalosporins, making this class inadequate even with an in vitro susceptible result.
Ampicillin, often combined with gentamicin for synergy, is the treatment of choice in invasive infection.
Acquired resistance is rare, but empiric use of a cephalosporin alone in meningitis should be avoided due to inadequate Listeria coverage.

Bench and reporting notes

Gram-positive rod with characteristic tumbling motility at 20–25°C aids presumptive identification.
Shows narrow beta-hemolysis on blood agar and may be mistaken for non-pathogenic diphtheroids if not carefully observed.
Isolation from blood, CSF, or obstetric material should be reported urgently given the risk of rapid clinical deterioration.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Listeria monocytogenes in the lab?

Listeria monocytogenes is identified through catalase-positive, umbrella-shaped motility at 25 °c, camp-positive, esculin-positive.

What are the intrinsic resistances of Listeria monocytogenes?

This organism is naturally resistant to cephalosporins of all generations (eucast expected resistant phenotypes v1.2), fosfomycin (may appear susceptible in vitro but is not clinically effective). These drugs should not be reported as susceptible.

Where is Listeria monocytogenes commonly found?

It is typically associated with meningitis in the elderly and immunosuppressed, gestational listeriosis, neonatal sepsis.

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