Gram-positive bacteria
Bacteria with thick peptidoglycan retaining crystal violet stain.
Gram-positive bacteria are bacteria that give a positive result in the Gram stain test, a traditional method used to quickly classify bacteria into two broad categories based on cell wall type. They have a thick layer of peptidoglycan in their cell wall that retains the crystal violet stain, causing them to appear purple under an optical microscope. This characteristic distinguishes them from gram-negative bacteria, which have a thin peptidoglycan layer and do not retain the stain. Gram-positive bacteria are significant in medicine and microbiology because their thick peptidoglycan layer and lack of an outer membrane make them more receptive to certain cell wall–targeting antibiotics, such as penicillin, which inhibits cell wall synthesis.
- classification
- Historically classified as Firmicutes (gram-positive), later split into high G+C phylum (Actinobacteria) and low G+C phylum (Firmicutes) based on 16S rRNA studies
Lore & Background
Gram-positive bacteria are defined by their response to the Gram stain test, which relies on the thick peptidoglycan layer in their cell wall. This layer retains the crystal violet stain after iodine fixation and decolorization, unlike gram-negative bacteria. Their cell wall contains teichoic acids and lipoteichoic acids, which serve as chelating agents and aid in adherence. Peptidoglycan chains are cross-linked by the enzyme DD-transpeptidase, a target for beta-lactam antibiotics like penicillin. Only some species have a capsule or flagella; those with flagella have two basal body rings, compared to four in gram-negative bacteria.
Reader's Guide
Gram-positive bacteria are a major group in bacterial classification, historically divided into divisions based on Gram staining, such as Bacillota (positive) and Gracilicutes (negative). However, molecular studies by Carl Woese and colleagues using 16S ribosomal RNA revealed that gram-positive bacteria are not monophyletic, leading to their division into high G+C (Actinobacteria) and low G+C (Firmicutes) phyla. Their significance lies in their structural simplicity—a single lipid membrane and thick peptidoglycan—which makes them vulnerable to antibiotics that target cell wall synthesis, unlike gram-negative bacteria with an outer membrane. The distinction between monoderm (single membrane) and diderm (two membranes) bacteria is supported by conserved signature indels in proteins like DnaK and GroEL. Some bacteria, such as Deinococcus, stain gram-positive but have an outer membrane, suggesting an evolutionary intermediate. Exceptions include Negativicutes, which are diderm and stain gram-negative, and Mollicutes, which lack peptidoglycan and are gram-indeterminate.
Did You Know?
- Gram-positive bacteria retain the crystal violet stain due to a thick peptidoglycan layer (20–80 nm) in their cell wall.
- Penicillin contains a beta-lactam ring that binds to DD-transpeptidase, inhibiting cross-linking of peptidoglycan and causing cell rupture.
- Based on 16S rRNA studies, gram-positive bacteria were divided into high G+C (Actinobacteria) and low G+C (Firmicutes) phyla.
- Some bacteria, like Deinococcus, stain gram-positive but belong to the Deinococcus-Thermus phylum and have atypical cell walls, so they are not standard examples of gram-positive bacteria.
Frequently Asked Questions
What are Gram-positive bacteria?
Gram-positive bacteria are a broad group of prokaryotes defined by their cell-wall architecture: a thick peptidoglycan layer that traps crystal violet dye during the classic Gram stain, causing them to appear purple under a light microscope. This single staining trait is what separates them from the Gram-negative camp.
How are Gram-positive bacteria classified in modern taxonomy?
Before molecular phylogenetics, nearly all Gram-positive organisms were lumped under one phylum called Firmicutes. 16S rRNA sequencing later revealed a deep split, and the group was formally divided into two phyla—Actinobacteria (high G+C) and Firmicutes (low G+C)—reflecting their true evolutionary distance.
What's the structural difference between Gram-positive and Gram-negative bacteria?
Gram-positive species have a single cytoplasmic membrane wrapped in a massive, exposed peptidoglycan sheet, while Gram-negative species sandwich a thin peptidoglycan layer between an inner membrane and an additional outer membrane. That extra outer membrane is what prevents the crystal violet from sticking, so Gram-negatives take up the counterstain instead.
Why does the Gram-positive distinction matter in medicine?
The thick, externally exposed peptidoglycan makes these organisms especially vulnerable to antibiotics like penicillin that block cell-wall synthesis, and their surface teichoic acids are key ligands in immune recognition and virulence. Clinicians rely on the Gram stain as a rapid first step to narrow down which antibiotic class to start.
How did the old 'Firmicutes' name get reworked?
The original Firmicutes phylum was essentially a 'Gram-positive catch-all' assigned before sequencing tools existed. Once 16S rRNA data showed that Actinobacteria were as phylogenetically distant from true Firmicutes as either group is from Gram-negatives, taxonomists split the phylum, and the name Firmicutes was restricted to the low G+C lineage.
More in Higher-Level Taxonomy & Evolution 1-24
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