🧫 Group D Antigen Confusion & Vancomycin-Resistant Gram-Positive Cocci: Master the Lookalikes

FRCPath Prep Medical Microbiology Consultants
Jul 26
2 min read

Published on: July 26, 2025

Author: Team FRCPathPrep

Category: Gram-Positive Cocci, OSPE Essentials, FRCPath Microbiology

🧬 Introduction

When you encounter Gram-positive cocci in chains, especially from blood or device cultures, the differential is broad — and confusing. Organisms that:

React with Group D antigen
Appear similar on Gram stain
Show variable vancomycin resistance
Are PYR +/-, bile esculin positive

...can trick even experienced microbiologists.

This guide clarifies who’s who — including Enterococcus, Streptococcus gallolyticus, Pediococcus, Leuconostoc, and others.

🔍 Part 1: Group D Antigen Reactors

Organism	Group D Reaction	Key Features
Enterococcus spp.	✅ Positive	PYR +, 6.5% NaCl +, variable vancomycin resistance
Streptococcus gallolyticus (S. bovis group)	✅ Positive	PYR –, 6.5% NaCl –, vanc-sensitive, CRC association
Listeria monocytogenes	✅ Cross-reactive	Gram-positive rod, catalase +, tumbling motility
Lactobacillus spp.	⚠️ Occasionally	GPR, catalase –, rare clinical relevance

🧪 Differentiation Table (FRCPath Essential)

Test	Enterococcus	S. gallolyticus	Listeria	Lactobacillus
Gram stain	GPC	GPC	GPR	GPR
Catalase	❌ Pseudo +/–	❌	✅	❌
PYR	✅ Positive	❌ Negative	✅ Positive	❌
6.5% NaCl growth	✅ Yes	❌ No	❌ No	❌ No
Motility	❌	❌	✅ (25°C)	❌
Vancomycin susceptibility	Variable	✅ Sensitive	✅ Sensitive	Intrinsically resistant
Clinical cue	UTI, IE	Endocarditis + CRC	Pregnancy/neonatal sepsis	Rare infections

🔎 Part 2: Vancomycin-Resistant Group D Lookalikes

Now enter the intrinsically vancomycin-resistant Gram-positive cocci that look like Enterococcus but aren’t:

Organism	PYR	Vancomycin	Morphology	Key Points
Enterococcus spp.	✅	R/S	GPC in pairs/short chains	HLAR test needed
Pediococcus spp.	❌	❌ Resistant	GPC in tetrads	Misidentified as enterococci
Leuconostoc spp.	❌	❌ Resistant	GPC in chains	Intrinsically Vanc-R, not VRE
S. gallolyticus	❌	✅ Sensitive	GPC in chains	CRC-associated

🧠 OSPE-Focused Differentiation

Test	Enterococcus	Pediococcus	Leuconostoc	S. gallolyticus
Gram stain	GPC	GPC in tetrads	GPC in chains	GPC in chains
PYR	✅ Positive	❌ Negative	❌ Negative	❌ Negative
Bile esculin	✅ Positive	✅ Positive	✅ Positive	✅ Positive
6.5% NaCl	✅ Growth	❌ No growth	❌ No growth	❌ No growth
Vancomycin	✅/❌ R/S	❌ Resistant	❌ Resistant	✅ Sensitive
Clinical Pearl	IE, UTI	Rare/prosthetic	Mislabelled as VRE	IE + bowel cancer

🧠 Clinical Mnemonics

“Enterococcus enters salt” → Grows in 6.5% NaCl
“Gallolyticus guards the colon” → CRC red flag
“Pediococcus plays with tetrads”
“Leuconostoc lies about resistance” → Vanc-R but not VRE

🏥 FRCPath Clinical Scenario

💉 A patient with a prosthetic joint grows Gram-positive cocci in chains, PYR negative, bile esculin positive, and vancomycin-resistant. 📌 Diagnosis?👉 Likely Leuconostoc or Pediococcus — not VRE.