Gram-positive bacteria are defined by the composition of their cell envelope, which mediates all interaction between the cell and its environment. The definitive answer to whether Gram-positive bacteria possess an outer membrane is no; they are entirely surrounded by only one lipid membrane. This single-membrane architecture, known as a monoderm, means the environment is separated from the cell interior by a cytoplasmic membrane and a massive, external cell wall.
The Cell Envelope of Gram-Positive Bacteria
The cell envelope of a Gram-positive bacterium begins with the plasma, or cytoplasmic, membrane, a standard lipid bilayer that regulates the passage of substances into and out of the cell. Immediately external to this membrane lies the defining feature of the group: an extraordinarily thick layer of peptidoglycan. This peptidoglycan wall, which can measure between 20 and 80 nanometers thick, forms a dense, mesh-like protective shell that can account for up to 90% of the entire cell wall’s mass.
This multi-layered wall is composed of glycan chains cross-linked by short peptides, creating a rugged structure that provides mechanical strength and resistance to osmotic pressure. Threaded throughout this thick wall are specialized molecules called teichoic acids (TAs), which are long anionic polymers.
There are two main types: wall teichoic acids (WTAs), which are covalently bound to the peptidoglycan, and lipoteichoic acids (LTAs), which are anchored to the plasma membrane’s lipid head groups and extend outward through the wall. Teichoic acids contribute to the overall negative charge of the cell surface and play roles in cell division and the binding of cations.
How Structure Influences the Gram Stain Result
The thick, multi-layered peptidoglycan wall is the direct reason Gram-positive bacteria retain the characteristic purple color during the Gram staining procedure. The staining begins with the application of crystal violet, which is then fixed by an iodine solution, forming a large crystal violet-iodine complex inside the cell.
The application of a decolorizing agent, usually alcohol or acetone, is the next step. When this dehydrating agent is applied, the extensive peptidoglycan wall of the Gram-positive cell shrinks and becomes tightly constricted. This significant dehydration effectively closes the pores within the mesh-like structure, physically trapping the large crystal violet-iodine complex inside the cell.
Because the dye complex cannot escape the dehydrated wall, Gram-positive cells remain purple even after the decolorizer is washed away. When the counterstain, safranin, is applied, its pink color is completely masked by the darker, retained purple of the primary stain.
Structural Differences in Gram-Negative Bacteria
For comparison, Gram-negative bacteria possess a fundamentally different, two-membrane architecture, termed a diderm. These bacteria have an inner cytoplasmic membrane, which is functionally similar to the Gram-positive plasma membrane. External to this membrane is a thin layer of peptidoglycan, which is situated within a space called the periplasm.
The most significant structural difference is the presence of an outer membrane that surrounds the entire periplasm and thin peptidoglycan layer. This outer membrane is an asymmetric lipid bilayer, meaning its two leaflets are chemically distinct. The inner leaflet is composed of phospholipids, while the outer leaflet is primarily made up of lipopolysaccharide (LPS), a large molecule often referred to as endotoxin.
The outer membrane also contains specialized pore-forming proteins called porins, which allow for the passive diffusion of small, hydrophilic molecules across this second barrier. This distinct outer membrane, rich in LPS, is the structural feature entirely absent in Gram-positive bacteria.
Permeability and Antibiotic Sensitivity
The architectural difference between the cell envelopes affects the cell’s permeability and its susceptibility to various substances, including antibiotics. The thick peptidoglycan layer of Gram-positive bacteria, while physically strong, is relatively porous, allowing many small molecules to pass through it easily. The absence of a secondary, outer membrane means that drugs targeting the cell’s interior or the cell wall itself have relatively unimpeded access to their targets.
This lack of an outer membrane makes Gram-positive bacteria more susceptible to antibiotics that interfere with cell wall synthesis, such as penicillin and its derivatives. In contrast, the outer membrane of Gram-negative bacteria acts as a highly effective permeability barrier.
The LPS-containing outer leaflet restricts the entry of many large or hydrophobic compounds, including several classes of antibiotics. This selective barrier function contributes to the intrinsic resistance of Gram-negative bacteria, making them inherently less permeable and often more challenging to treat than their Gram-positive counterparts.