Gram-Positive Bacteria

Gram-positive bacteria are a major structural and phylogenetic grouping of prokaryotes characterized by a monoderm cell envelope consisting of a single cytoplasmic phospholipid bilayer surrounded by a thick, multilayered peptidoglycan (murein) cell wall measuring 20 to 80 nanometers (nm) in thickness and lacking the outer lipopolysaccharide-containing membrane found in diderm Gram-negative bacteria. First differentiated in 1884 by the Danish physician Hans Christian Gram while examining lung tissue from pneumonia patients in Berlin, Gram-positive bacteria retain the primary crystal violet–iodine (CV–I) dye complex when washed with an organic decolorizer (95% ethanol or acetone) and therefore appear deep purple-violet under bright-field microscopy, whereas Gram-negative organisms lose the dye complex during lipid extraction and take up the pink/red safranin or carbol fuchsin counterstain.

The physical mechanism of the Gram stain derives directly from cell-wall ultrastructure. In both bacterial groups, basic crystal violet cations penetrate the cell wall and react with aqueous potassium triiodide (Lugol's mordant) to form bulky, water-insoluble crystal violet–iodine precipitates within the cytoplasm and cell envelope. When ethanol or acetone is applied to a Gram-positive cell, the organic solvent dehydrates the thick, up-to-40-layer-deep peptidoglycan meshwork, shrinking its effective pore radius and trapping the large CV–I complexes inside the cell. In contrast, in Gram-negative cells, ethanol dissolves the outer lipid membrane and passes through the thin (2–7 nm, 1–3 layer) periplasmic peptidoglycan sheet, washing out the dye.

Peptidoglycan Synthesis and Cross-Linking

Accounting for 40% to 80% of the dry weight of the Gram-positive cell wall, peptidoglycan forms a continuous, covalently cross-linked macromolecular sacculus that withstands internal cytoplasmic turgor pressures of 15 to 25 atmospheres (1.5 to 2.5 MPa), preventing osmotic lysis in hypotonic environments. The glycan backbone consists of linear chains of alternating N-acetylglucosamine (GlcNAc or NAG) and N-acetylmuramic acid (MurNAc or NAM) monosaccharides linked by β-(1→4) glycosidic bonds. Attached via an amide bond to the D-lactoyl carboxyl group at C-3 of each MurNAc residue is a short tetrapeptide or pentapeptide stem synthesized in the cytoplasm by Mur ligases as L-Ala–γ-D-Glu–L-Lys–D-Ala–D-Ala (or containing meso-diaminopimelic acid [meso-DAP] at position 3 in Bacillus, Listeria, and Clostridium).

Cytoplasmic UDP-MurNAc-pentapeptide is coupled to the C₅₅ isoprenoid carrier undecaprenyl phosphate (bactoprenol) to form Lipid I, and addition of GlcNAc yields Lipid II, which is flipped across the cytoplasmic membrane by MurJ/Amj flippases. On the extracytoplasmic surface, bifunctional penicillin-binding proteins (PBPs) and SEDS-family glycosyltransferases (RodA, FtsW) catalyze two reactions: transglycosylation, which polymerizes the GlcNAc–MurNAc disaccharide units into nascent glycan strands, and DD-transpeptidation, which cleaves the terminal D-Ala–D-Ala bond and links the carboxyl group of D-Ala at position 4 on a donor stem to the ε-amino group at position 3 on an adjacent acceptor stem. In Staphylococcus aureus, this cross-link spans a flexible pentaglycine (Gly₅) interpeptide bridge assembled by FemA/FemB/FemX transferases, resulting in 70% to 90% cross-linking density across the sacculus.

Wall Teichoic Acids, Lipoteichoic Acids, and Surface Proteins

Interwoven throughout the Gram-positive peptidoglycan matrix are abundant polyanionic glycopolymers termed teichoic acids, which can constitute up to 50% of the total wall mass and confer a strong net negative surface charge:

• Wall Teichoic Acids (WTAs): Covalently attached via a phosphodiester bond to the C-6 hydroxyl group of MurNAc residues, WTAs consist of 20 to 40 repeating units of poly(ribitol-phosphate) or poly(glycerol-phosphate) synthesized by the tar or tag gene clusters. Tailor-made modifications of the ribitol or glycerol hydroxyl groups with D-alanine esters (catalyzed by the dltABCD operon) introduce positively charged amino groups that partially neutralize the phosphate backbone, conferring resistance to cationic antimicrobial peptides (CAMPs) such as human β-defensins and cathelicidin LL-37.
• Lipoteichoic Acids (LTAs): Anchored non-covalently into the outer leaflet of the cytoplasmic membrane via a diacylglycerol glycolipid moiety, poly(glycerol-phosphate) LTAs extend upward through the peptidoglycan layers. Together, WTAs and LTAs chelate divalent cations (Mg²⁺ and Ca²⁺) required for membrane stability, regulate endogenous autolysins (peptidoglycan hydrolases required for cell division), and act as potent pathogen-associated molecular patterns recognized by host Toll-like receptor heterodimers (TLR2/TLR6) and macrophage scavenger receptors.

Gram-positive bacteria also anchor specialized virulence adhesins—such as Staphylococcus aureus Protein A (which binds the Fcγ region of host IgG antibodies in reverse orientation to block opsonophagocytosis) and fibronecting-binding proteins—directly to the peptidoglycan cell wall via the cysteine transpeptidase Sortase A. Sortase A recognizes a C-terminal LPXTG sorting motif on secreted surface proteins, cleaves between the threonine and glycine residues, and forms an amide bond linking the threonine carboxyl group to the pentaglycine cross-bridge of Lipid II.

Modern phylogenetics based on 16S rRNA sequence analysis and core genome alignments divides Gram-positive bacteria into two principal evolutionary phyla distinguished by the guanine-plus-cytosine (G+C) molar percentage of their genomic DNA:

Phylum Bacillota (Firmicutes): Low G+C Gram-Positive Bacteria

Organisms in the phylum Bacillota (formerly Firmicutes) typically possess genomic G+C contents below 50% (often 30% to 45%) and encompass both non-spore-forming cocci/bacilli and endospore-forming rods:

• Non-Spore-Forming Cocci and Rods: The order Caryophanales includes Staphylococcus (catalase-positive, grape-like clusters; S. aureus causes skin abscesses, osteomyelitis, endocarditis, and toxic shock syndrome via superantigen TSST-1, while coagulase-negative S. epidermidis colonizes indwelling catheters). The order Lactobacillales (lactic acid bacteria) includes Streptococcus (catalase-negative chains; β-hemolytic Group A S. pyogenes causes pharyngitis, necrotizing fasciitis, and rheumatic fever, while α-hemolytic S. pneumoniae is the leading cause of community-acquired pneumonia and meningitis), Enterococcus (E. faecalis and E. faecium), and Lactobacillus. Listeria monocytogenes is a facultative intracellular, psychrotolerant rod that escapes the macrophage phagolysosome using the pore-forming toxin listeriolysin O (LLO) and propels itself from cell to cell via ActA-mediated host actin polymerization.
• Endospore-Forming Rods (Bacillus and Clostridium): In response to nutrient starvation, members of the aerobic/facultative genus Bacillus (B. anthracis, B. cereus, B. subtilis) and the obligate anaerobic genus Clostridium (C. tetani, C. botulinum, C. perfringens, and Clostridioides difficile) execute an asymmetric cell division program governed by the master regulator Spo0A and a cascade of compartment-specific RNA polymerase sigma factors (σ^F, σ^E, σ^G, σ^K) to form a metabolically dormant endospore. The endospore core is dehydrated and mineralized with calcium dipicolinate (up to 15% of spore dry weight) and small acid-soluble spore proteins (SASPs) that bind DNA in an A-like conformation, encased within a specialized cortex peptidoglycan and keratin-like protein coat that withstands boiling (100 °C), desiccation, UV radiation, and chemical disinfectants for decades.

Phylum Actinomycetota (Actinobacteria): High G+C Gram-Positive Bacteria

Members of the phylum Actinomycetota possess high genomic G+C contents (>50% to 74%) and exhibit diverse morphologies ranging from coryneform rods to branching, fungal-like filamentous mycelia. Soil-dwelling Streptomyces species reproduce via aerial spores and synthesize more than two-thirds of all naturally derived clinical antibiotics (including streptomycin, tetracycline, erythromycin, chloramphenicol, and vancomycin). Medically significant pathogens include Corynebacterium diphtheriae (which secretes bacteriophage-encoded diphtheria AB exotoxin that ADP-ribosylates eukaryotic elongation factor 2 [eEF-2] to halt host translation) and the order Mycobacteriales (Mycobacterium tuberculosis and Mycobacterium leprae). Although phylogenetically Gram-positive, mycobacteria covalently link their peptidoglycan to an arabinogalactan polysaccharide esterified with long-chain (C₆₀–C₉₀) α-alkyl, β-hydroxy fatty acids called mycolic acids. This waxy outer mycomembrane renders mycobacteria impermeable to crystal violet, requiring the heated carbol fuchsin Ziehl–Neelsen acid-fast stain for visualization.

Because the Gram-positive peptidoglycan sacculus is directly exposed to the extracellular environment without a protective outer membrane porin barrier, Gram-positive bacteria are intrinsically sensitive to host lysozyme (muramidase)—present in tears, saliva, mucus, and phagocyte granules—which hydrolyzes the β-(1→4) glycosidic bond between MurNAc and GlcNAc. Similarly, they are highly susceptible to cell-wall-active antibiotics: β-lactams (penicillins, cephalosporins, carbapenems), which act as structural analogues of D-Ala–D-Ala to acylate the catalytic serine of PBPs, and high-molecular-weight (~1,450 Da) glycopeptides (vancomycin, teicoplanin), which cannot cross the Gram-negative outer membrane porins (<600 Da cutoff) but readily diffuse through the Gram-positive wall to form five hydrogen bonds with terminal D-Ala–D-Ala residues on Lipid II, sterically blocking transglycosylation and transpeptidation.

To survive this selective pressure, clinical Gram-positive pathogens have evolved major mechanisms of antimicrobial resistance. Methicillin-resistant Staphylococcus aureus (MRSA) carries the mobile staphylococcal cassette chromosome mec (SCC*mec*) encoding PBP2a (mecA), an alternative transpeptidase whose narrow active-site cleft exhibits low binding affinity for almost all β-lactams except fifth-generation cephalosporins (ceftaroline). Vancomycin-resistant enterococci (VRE) and S. aureus (VRSA) acquire the vanA operon (VanS/VanR two-component sensor, VanH dehydrogenase, VanA ligase, and VanX D,D-dipeptidase), which replaces the terminal D-Ala–D-Ala dipeptide with D-Ala–D-lactate, eliminating a single hydrogen bond and reducing vancomycin binding affinity by 1,000-fold.

Why can old or stationary-phase cultures of Gram-positive bacteria sometimes stain Gram-negative (Gram-variable)?

Gram positivity depends on the physical integrity of the multilayered peptidoglycan sacculus to trap the crystal violet–iodine complex during ethanol dehydration. When Gram-positive cultures (such as Bacillus or Clostridium) enter late stationary phase, nutrient depletion, or exposure to β-lactam antibiotics and autolysins, endogenous peptidoglycan hydrolases degrade the glycan cross-links, allowing the ethanol decolorizer to wash out the dye complex so the cells take up the pink safranin counterstain.

How does the lipopeptide antibiotic daptomycin kill Gram-positive bacteria without affecting Gram-negative bacteria?

Daptomycin is a large (1,620 Da) cyclic anionic lipopeptide that cannot penetrate the outer membrane of Gram-negative bacteria. In the presence of physiological Ca²⁺ ions, daptomycin adopts an amphipathic conformation that diffuses through the porous Gram-positive peptidoglycan wall and inserts its decanoyl fatty-acid tail into phosphatidylglycerol-rich domains of the cytoplasmic membrane, forming oligomers that delocalize cell-wall synthesis proteins (MurG) and depolarize the membrane potential.

Why are Mycoplasma species classified phylogenetically with Gram-positive bacteria despite staining Gram-negative and lacking a cell wall?

Mycoplasma and Ureaplasma species (phylum Mycoplasmatota / Tenericutes) evolved from low-G+C Gram-positive Bacillota ancestors through reductive genome evolution, losing the genes for peptidoglycan synthesis while adopting an obligate parasitic lifestyle in osmotically stable host tissues and incorporating host cholesterol into their plasma membrane for mechanical stability. Because they lack a cell wall, they do not retain crystal violet and are intrinsically resistant to all β-lactam and glycopeptide antibiotics.

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