Which organism has become the most significant pathogen in early-onset pneumonia in preterm infants?

• A. Group B Streptococcus
• B. Staphylococcus aureus
• C. Escherichia coli (correct)
• D. Klebsiella pneumoniae

Answer: (C)

Early-onset pneumonia in preterm infants is most strongly linked to organisms the baby is exposed to during birth, with Gram-negative rods—especially Escherichia coli—being a leading cause in this group. E. coli tends to colonize and ascend or be aspirated around delivery, and its virulence factors promote rapid lung infection and a high risk of bacteremia in the immature neonatal immune system. While Group B Streptococcus is a major pathogen in neonatal infections overall, the burden of early pneumonia in preterm, very low birth weight infants is more frequently driven by E. coli, making it the most significant pathogen in this scenario. Staphylococcus aureus and Klebsiella pneumoniae occur but are less commonly the dominant culprits for early-onset pneumonia in these preterm babies.