# Neonatal Sepsis Microbiology

> NEET-PG Microbiology notes on neonatal sepsis covering early and late onset organisms, Group B streptococcus, Listeria, E. coli K1 and the sepsis screen.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/neonatal-sepsis-microbiology
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-15
- Last updated: 2026-10-15
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Jawetz, Melnick and Adelberg's Medical Microbiology
- How to cite: "Neonatal Sepsis Microbiology", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/neonatal-sepsis-microbiology

## Direct answer

Neonatal sepsis is systemic infection in the first 28 days of life, and its microbiology divides by the time of onset. Early-onset disease, within 72 hours of birth, is led by Group B streptococcus and Escherichia coli acquired from the maternal genital tract. Late-onset disease, from 72 hours to 28 days, is led by coagulase-negative staphylococci, Staphylococcus aureus and Klebsiella acquired from the hospital or community.

## What you must remember

- **Early onset:** Within 72 hours of birth, usually from the maternal genital tract, with Group B streptococcus leading in Western series, followed by E. coli and Listeria monocytogenes; Indian NICUs report mostly gram-negative bacilli.
- **Late onset:** From 72 hours to 28 days, usually from hospital or community contact, with coagulase-negative staphylococci, S. aureus, Klebsiella and Candida as common causes.
- **Group B streptococcus screening:** A rectovaginal swab culture at about 35 to 37 weeks of gestation guides intrapartum antibiotic prophylaxis for colonised women.
- **Listeria:** A gram-positive bacillus that causes early-onset sepsis and meningitis after maternal infection that may be mild or unrecognised, and ampicillin is the drug of choice.
- **E. coli K1:** Its capsular K1 antigen is linked to neonatal meningitis and helps the organism resist complement.
- **Candida:** Causes late-onset invasive disease in preterm infants on broad-spectrum antibiotics, central lines or parenteral nutrition.
- **Gram-negative resistance:** Klebsiella and other nosocomial gram-negative bacilli cause NICU outbreaks, so local antibiogram data shape empirical therapy.
- **Sepsis screen:** Blood culture before antibiotics, CSF examination when meningitis is suspected, and serial CRP help confirm and monitor disease.

## Early versus late onset at a glance

Neonatal sepsis is a clinical syndrome with nonspecific signs, so the microbiology sets the empirical treatment. Timing of onset is the first clue to the likely organism and its source.

| Feature | Early onset (under 72 hours) | Late onset (72 hours to 28 days) |
|---|---|---|
| Source | Maternal genital tract | Hospital environment or community |
| Leading organisms | Group B streptococcus, E. coli, Listeria | Coagulase-negative staphylococci, S. aureus, Klebsiella, Candida |
| Typical picture | Respiratory distress and rapid deterioration | Meningitis, focal infection or central line sepsis |
| Key tests | Blood culture and maternal history | Blood and CSF cultures, line swabs |
| Empirical focus | Ampicillin with an aminoglycoside pending culture | Choice guided by local antibiogram and MRSA risk |

The timing points to the likely source, whereas culture confirms the species and its susceptibility. Early-onset organisms are largely maternal, which is why antenatal screening for Group B streptococcus matters. A baby who looks well at birth but deteriorates within hours still needs a full sepsis screen, because early Group B streptococcal disease can progress quickly.

## Where the exam tries to catch you

Neonatal sepsis questions usually test whether the candidate uses the time of onset to choose the organism.

- **Trap:** Treating all neonatal sepsis as early-onset Group B streptococcus. — **Correct:** Late-onset disease in NICU infants is commonly due to coagulase-negative staphylococci or Klebsiella, so the organism changes after 72 hours.
- **Trap:** Discarding a negative blood culture as proof of no sepsis. — **Correct:** Small neonatal volumes and prior antibiotics reduce the yield, so clinical signs and repeat sampling still matter.
- **Trap:** Ignoring Listeria because the mother had no fever. — **Correct:** Maternal infection can be mild or silent, so Listeria stays on the differential, and ampicillin with gentamicin covers it empirically.
- **Trap:** Assuming every gram-negative bacillus in a neonate is E. coli. — **Correct:** Klebsiella, Pseudomonas and Enterobacter also cause late-onset disease, so culture identification is needed.

## Frequently asked questions

### Which organism causes most early-onset neonatal sepsis?
Group B streptococcus is the leading cause of early-onset neonatal sepsis, acquired from the maternal genital tract during labour. Escherichia coli, especially K1 strains, is the second major cause, and Listeria monocytogenes is also important.

### Why does Listeria matter in neonatal sepsis?
Listeria monocytogenes causes early and late neonatal sepsis and meningitis, often after maternal infection that is mild or unrecognised. It is a gram-positive bacillus, and ampicillin is the drug of choice, usually combined with gentamicin in neonates.

### How is Group B streptococcus carriage detected in pregnancy?
A rectovaginal swab culture taken at about 35 to 37 weeks of gestation identifies women colonised with Group B streptococcus. Positive women receive intrapartum antibiotic prophylaxis, usually penicillin, to prevent early-onset disease in the newborn.

### What is the onset window for late-onset neonatal sepsis?
Late-onset neonatal sepsis begins after 72 hours of life and up to 28 days, usually from hospital or community contact. Coagulase-negative staphylococci, Staphylococcus aureus and Klebsiella are common causes, and central lines are a key risk factor.

### Which fungus causes late-onset neonatal infection in preterm infants?
Candida species, including C. albicans and C. parapsilosis, cause late-onset invasive infection in preterm infants receiving broad-spectrum antibiotics, central lines or parenteral nutrition. Diagnosis is by blood culture, and antifungal treatment is started promptly when invasive candidiasis is suspected.