Morphology: Gram-positive rod, facultative intracellular, β-hemolytic, motile (tumbling motility), grows at 4°C.
Typical drugs
- #1Ampicillin— ± gentamicin synergy for severe infection.
- #2Trimethoprim-sulfamethoxazole— If true severe PCN allergy.
Empiric therapy when resistant
Ampicillin (or amoxicillin) is first-line. Add gentamicin for synergy in severe disease. TMP-SMX is the alternative for true PCN allergy.
Resistance mechanisms
Target alteration
Intrinsic resistance to ALL cephalosporins (low PBP affinity)
Example: Cefepime, ceftriaxone — never adequate for Listeria.
Resistance notes
Intrinsically resistant to all cephalosporins — must add ampicillin to empiric ceftriaxone in any meningitis with Listeria risk factors.
Common syndromes
Pearls
Risk factors: age >50, pregnancy, immunocompromise, alcohol use, hematologic malignancy. Foodborne (deli meats, soft cheeses, raw vegetables). Causes meningoencephalitis, brainstem encephalitis (rhombencephalitis), and bacteremia in pregnancy → neonatal sepsis (listeriosis). MUST add ampicillin if any risk factor for Listeria.
References
- IDSA Bacterial Meningitis (2004)