What is the most common organism causing early-onset neonatal sepsis, and what empiric antibiotic regimen is commonly initiated?

• A. E. coli; vancomycin plus cefotaxime.
• B. Listeria; penicillin alone.
• C. Group B Streptococcus; ampicillin plus gentamicin.
• D. Staphylococcus aureus; nafcillin.

Answer: (D)

Early-onset neonatal sepsis is most commonly caused by Group B Streptococcus, with Escherichia coli as another frequent culprit. To cover both GBS and Gram-negative bacteria while awaiting cultures, the typical initial therapy is ampicillin plus gentamicin. Ampicillin targets GBS and also provides some coverage for Listeria, while gentamicin adds strong activity against Gram-negative organisms like E. coli. Staphylococcus aureus is more often associated with later-onset sepsis and nafcillin would not provide reliable Gram-negative coverage or Listeria coverage, making it less appropriate as the initial empiric choice.