Gram-negative bacteria are distinguished by the unique architecture of their cell wall. This structure plays a role in the bacterium’s survival and its interactions with the environment. The cell wall acts as a protective barrier, allowing these microorganisms to thrive in diverse conditions. Understanding this design is important for comprehending their biology and impact on various ecosystems, including the human body.
The Layers of the Gram-Negative Cell Wall
The gram-negative cell wall is a multi-layered envelope. The innermost layer is the cytoplasmic membrane, also known as the inner membrane, a lipid bilayer that regulates the passage of substances into and out of the cytoplasm. Surrounding this inner membrane is the periplasmic space, a gelatinous region containing various proteins and enzymes.
Within the periplasmic space lies a thin layer of peptidoglycan, a mesh-like polymer of sugars and amino acids. This peptidoglycan layer provides structural support and helps the bacterium maintain its shape, preventing it from bursting due to osmotic pressure. Unlike some other bacteria, the peptidoglycan in gram-negative species is notably thin, often consisting of a single layer. The outermost component is the outer membrane.
The Outer Membrane’s Unique Molecules
The outer membrane contains unique molecules. A prominent component is lipopolysaccharide (LPS), embedded in the outer leaflet of this membrane. LPS is composed of three main parts: Lipid A, the core polysaccharide, and the O-antigen.
Porins are another type of protein found in the outer membrane, forming channels that allow for the passive diffusion of small, hydrophilic molecules, such as nutrients, into the periplasmic space. These channels are selective, controlling what substances can enter the cell. Lipoproteins anchor the outer membrane to the underlying peptidoglycan layer, maintaining structural integrity.
Impact on Human Health
The gram-negative cell wall has implications for human health. The Lipid A component of LPS is an endotoxin that can trigger inflammatory responses. When gram-negative bacteria are present in large numbers during severe systemic infections, high levels of LPS can be released, leading to excessive cytokine production by immune cells like macrophages. This can result in harmful systemic effects, potentially contributing to conditions such as septic shock.
The outer membrane also acts as a permeability barrier, contributing to the resistance of gram-negative bacteria to many antibiotics. This barrier prevents antibiotic molecules from reaching their targets within the bacterial cell. Some gram-negative bacteria further enhance their resistance through efflux pumps, specialized protein systems that actively expel antibiotics that manage to enter the cell.
Distinguishing Gram-Negative from Gram-Positive Bacteria
The fundamental differences in cell wall structure distinguish gram-negative from gram-positive bacteria through Gram staining. Gram-negative bacteria possess a thin peptidoglycan layer sandwiched between an inner cytoplasmic membrane and an outer membrane. In contrast, gram-positive bacteria lack an outer membrane and instead have a much thicker peptidoglycan layer, which can consist of up to 30 layers.
During the Gram stain procedure, crystal violet dye is applied, followed by a decolorizing solution. Gram-positive bacteria retain the purple crystal violet due to their thick peptidoglycan layer, appearing purple under a microscope. The decolorizing solution washes the crystal violet from gram-negative bacteria because their thin peptidoglycan layer and outer membrane do not retain the dye effectively. A counterstain, typically safranin, is then applied, which stains the decolorized gram-negative cells pink or red.