Listeria monocytogenes
The causative organism is Listeria monocytogenes, definitively identified by the pathognomonic microbiologic description of β-hemolytic, Gram-positive rods with tumbling motility. 1
Microbiologic Identification
The microbiology report provides the diagnostic key:
- Gram-positive rods (bacilli) with β-hemolytic properties and tumbling motility are pathognomonic for Listeria monocytogenes 1
- This characteristic tumbling motility distinguishes Listeria from all other organisms in the differential 1
- The other answer choices are eliminated by Gram stain alone: E. coli and N. meningitidis are Gram-negative, while S. agalactiae is a Gram-positive coccus (not a rod) 2
- HSV would not appear on Gram stain and requires different diagnostic methods 1
Epidemiologic Context in Neonatal Meningitis
While Listeria represents only 2% of neonatal meningitis cases compared to the dominant pathogens (S. agalactiae at 58% and E. coli at 21%), it remains clinically significant 2, 1:
- Group B Streptococcus (S. agalactiae) and E. coli cause approximately two-thirds of all neonatal meningitis cases 2, 1
- Recent cohort studies and surveillance data show L. monocytogenes accounts for only a minority of cases, though it was historically considered more important 2
- At 1 month of age, this infant falls into the "late neonatal" period (second to sixth weeks of life), where transmission is typically nosocomial or horizontal rather than vertical 2
Clinical Significance and Mortality Risk
Despite lower incidence, Listeria meningitis carries substantial morbidity and mortality:
- The CSF findings confirm bacterial meningitis: elevated opening pressure (240 mm H₂O), elevated WBC count (1200/mm³), elevated protein (200 mg/dL), and low glucose (30 mg/dL) 1
- Neonates with bacterial meningitis present with nonspecific symptoms including irritability, poor feeding, respiratory distress, and seizures occur in 9-34% of cases 2, 1
- Listeria remains an important cause of neonatal meningitis, particularly in settings with inadequate prenatal care 1
Critical Clinical Pitfall
Do not be misled by epidemiologic frequency alone—while S. agalactiae and E. coli are statistically more common, the microbiologic characteristics provided in this case are diagnostic for Listeria monocytogenes and override epidemiologic probabilities 1. The Gram stain and culture characteristics must guide the diagnosis, not just age-based pathogen distribution 2.