Neonatal Meningitis: Most Likely Infectious Agent
The most likely infectious agent is Gram-positive bacilli, β-hemolytic, motile, grows at 4°C—describing Listeria monocytogenes.
Clinical Context and Risk Factors
This neonate presents with classic early-onset bacterial meningitis occurring soon after birth, with critical risk factors that point toward Listeria monocytogenes:
- Prolonged labor with premature rupture of membranes creates the opportunity for vertical transmission from mother to infant 1
- No prenatal care means the mother likely did not receive appropriate screening or prophylaxis 1
- CSF findings (increased protein, decreased glucose, neutrophils, elevated opening pressure) confirm bacterial meningitis 1
Epidemiology of Neonatal Meningitis
While Group B Streptococcus (Streptococcus agalactiae) and Escherichia coli cause approximately two-thirds of all neonatal meningitis cases 1, the specific microbiologic characteristics described in the answer choices point definitively to Listeria monocytogenes.
According to ESCMID guidelines, the distribution of neonatal meningitis pathogens across multiple European countries (2006-2012) shows 1:
- Streptococcus agalactiae (GBS): 58%
- Escherichia coli: 21%
- Listeria monocytogenes: 2%
- Streptococcus pneumoniae: 4%
Why Listeria monocytogenes is the Answer
The microbiologic description "Gram-positive bacilli, β-hemolytic, motile, grows at 4°C" is pathognomonic for Listeria monocytogenes 1. These unique characteristics distinguish it from all other neonatal meningitis pathogens:
- Gram-positive bacilli: Rules out E. coli (Gram-negative) and Streptococcus species (cocci, not bacilli)
- β-hemolytic: Characteristic hemolysis pattern on blood agar
- Motile: Exhibits characteristic tumbling motility
- Growth at 4°C (cold enrichment): Unique diagnostic feature of Listeria
Analysis of Other Answer Choices
Gram-negative cocci in pairs, grows on chocolate agar: Describes Neisseria meningitidis, which is rare in neonates and typically causes meningitis in adolescents and adults 1
Gram-negative pleomorphic bacilli, requires heme and NAD: Describes Haemophilus influenzae, which has virtually disappeared as a cause of meningitis in vaccinated populations and is extremely rare in neonates 1
Gram-positive cocci, β-hemolytic, catalase-negative, bacitracin-resistant: Describes Group B Streptococcus (S. agalactiae), which is the MOST common cause of neonatal meningitis overall but is described as cocci, not bacilli 1
Spirochete that cannot be cultured in vitro: Describes Treponema pallidum (syphilis), which causes chronic meningitis (symptoms ≥4 weeks), not acute neonatal presentation 2
Clinical Significance and Mortality Risk
Despite being less common than GBS or E. coli, Listeria remains an important cause of neonatal meningitis, particularly in settings with inadequate prenatal care 1. The lack of prenatal care in this case is critical—preventive measures like dietary counseling to avoid unpasteurized dairy products and deli meats would have been missed 1.
Neonates with bacterial meningitis present with nonspecific symptoms including irritability, poor feeding, respiratory distress, and seizures (9-34% of cases), exactly as described in this clinical scenario 1.
Common Pitfalls
- Don't assume GBS just because it's most common: The microbiologic characteristics must match the organism 1
- Recognize that Listeria is specifically associated with inadequate prenatal care and certain maternal risk factors (immunocompromised state, dietary exposures) 1
- The unique growth at 4°C is a classic board examination clue for Listeria monocytogenes that distinguishes it from all other bacterial pathogens