# Group B Streptococcus and Neonatal Infection

> Group B Streptococcus for NEET-PG Microbiology: CAMP test, early and late onset neonatal disease, 35 to 37 week screening and intrapartum prophylaxis.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/group-b-streptococcus-and-neonatal-infection
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Cowan and Steel's Manual for the Identification of Medical Bacteria; Mandell, Douglas and Bennett's Principles and Practice of Infectious Diseases
- How to cite: "Group B Streptococcus and Neonatal Infection", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/group-b-streptococcus-and-neonatal-infection

## Direct answer

Group B Streptococcus, Streptococcus agalactiae, is a Lancefield group B, CAMP-positive streptococcus that colonises the maternal rectovaginal tract and passes to the newborn during labour. It causes early-onset sepsis, usually within the first day of life, and late-onset disease, including meningitis, from seven days to three months. Intrapartum antibiotic prophylaxis in women with a positive screen or risk factors is the main preventive measure.

## What you must remember

- **CAMP test:** group B streptococci enhance the beta-lysin of Staphylococcus aureus, producing an arrowhead zone of haemolysis where the two streaks meet.
- **Biochemistry:** hippurate hydrolysis positive and bacitracin resistant, while PYR and catalase are negative, which separates it from group A streptococci.
- **Early-onset disease:** presents within the first week, most often in the first day, with sepsis, pneumonia or shock; preterm labour, rupture of membranes beyond 18 hours and intrapartum fever raise the risk.
- **Late-onset disease:** arises between seven days and three months, and meningitis is its characteristic presentation, so CSF examination is essential.
- **Antenatal screen:** a combined vaginal and rectal swab taken at 35 to 37 weeks in older texts (current ACOG advice is 36 0/7 to 37 6/7 weeks), cultured on selective enrichment broth.
- **Intrapartum prophylaxis:** intravenous penicillin G, with ampicillin as an alternative, given in labour to colonised women and to those with risk factors.
- **Adult disease:** causes urinary tract infection, bacteraemia and skin infection, particularly in pregnant women, older adults and people with diabetes.

## A pathway from antenatal screen to the newborn

A mother with preterm labour and a positive culture from 36 weeks presents a clear case for prophylaxis. Each step in the pathway builds on the last, and the newborn is the final point of control.

1. **Antenatal screen:** a rectovaginal swab is taken at about 36 to 37 weeks (35 to 37 in older texts), and the result is recorded in the labour notes so the team knows the colonisation status.
2. **Risk review in labour:** a positive screen, a previous infant with invasive disease, preterm labour or fever triggers prophylaxis even when the culture result is unknown.
3. **Intrapartum antibiotics:** intravenous penicillin G is started in labour and continued until delivery, with clindamycin or vancomycin used only when penicillin allergy and susceptibility testing require it.
4. **Newborn assessment:** an infant with early-onset risk is observed closely, and where the risk is high, blood culture is taken with empirical ampicillin and gentamicin.
5. **Laboratory confirmation:** GBS grows as small grey colonies with narrow beta-haemolysis on blood agar, and CAMP and hippurate tests confirm the species.
6. **Late-onset review:** an infant aged seven days to three months with fever or irritability needs a lumbar puncture, because meningitis is the usual presentation.

## Where the exam tries to catch you

Group B streptococcus is easy to confuse with group A on the bench, and its neonatal timing is a frequent viva point.

- **Trap:** treating GBS as group A streptococcus because both are beta-haemolytic — **Correct:** GBS is bacitracin resistant, hippurate positive and CAMP positive, whereas group A is bacitracin sensitive and PYR positive.
- **Trap:** assuming a negative antenatal swab at 35 weeks protects the baby — **Correct:** colonisation can change before delivery, so intrapartum risk factors still trigger prophylaxis.
- **Trap:** linking meningitis with early-onset disease — **Correct:** meningitis is the classic presentation of late-onset disease, after seven days of life.

## Frequently asked questions

### Which test identifies Streptococcus agalactiae in the laboratory?
The CAMP test identifies Streptococcus agalactiae, showing an arrowhead zone of enhanced haemolysis where a streak of group B streptococci meets a streak of Staphylococcus aureus. Hippurate hydrolysis and bacitracin resistance support the same identification.

### When is rectovaginal screening for group B streptococcus performed?
Rectovaginal screening for group B streptococcus is performed at 35 to 37 weeks in older texts and 36 0/7 to 37 6/7 weeks in current ACOG guidance, using culture on a selective enrichment medium. The result guides intrapartum antibiotic prophylaxis during labour and delivery.

### How does early-onset group B streptococcal disease differ from late-onset disease?
Early-onset group B streptococcal disease appears within the first week of life, most often in the first day, and is usually acquired during labour. Late-onset disease appears between seven days and three months of age and often presents as meningitis.

### Which antibiotics are used for neonatal group B streptococcal sepsis?
Penicillin G or ampicillin, usually combined with gentamicin for empirical cover, is used for neonatal group B streptococcal sepsis. Therapy is then adjusted to the culture and susceptibility result, and meningitis needs a longer course guided by the CSF response.

### Why is intrapartum penicillin given even when the mother looks healthy?
Intrapartum penicillin is given because a colonised mother can transmit group B streptococcus to her baby during delivery without showing any sign of infection. Prophylaxis prevents early-onset neonatal sepsis, which can progress rapidly even when the baby looks normal at birth.