Staphylococcus aureus is a bacterium commonly found on human skin and in the nasal passages, often existing without causing harm. It is also a major human pathogen capable of causing a wide range of infections, from minor skin issues to severe, life-threatening conditions like pneumonia and sepsis. Understanding the physical makeup of S. aureus is helpful for understanding how it survives in diverse environments and resists immune defenses and antibiotic treatments. The bacterium’s complex, multilayered structure dictates its interaction with the human body and its ability to cause disease.
Visualizing the Staph Cell
Staphylococcus aureus is classified as a coccus, meaning its individual cells are spherical or nearly round in shape. These small cells typically measure about 0.5 to 1.0 micrometers in diameter. When observed under a microscope, they display a characteristic arrangement that resembles a cluster of grapes, which is the origin of the genus name “Staphylococcus.”
This distinctive clustering occurs because the cells divide in multiple planes but remain attached after division. The bacterium is non-motile, lacking flagella or other structures for independent movement. S. aureus is characterized as Gram-positive, a classification based on a staining technique that reveals its cell wall makeup. The thick cell wall retains the initial crystal violet stain, causing the cells to appear purple under the microscope.
The Defining Gram Positive Cell Wall
The most significant structural feature of S. aureus is its exceptionally thick cell wall, which surrounds the inner cytoplasmic membrane. This rigid envelope provides mechanical strength and protection against osmotic pressure. The cell wall is primarily composed of a robust, multilayered meshwork called peptidoglycan.
The peptidoglycan layer is substantial, typically ranging from 20 to 40 nanometers in thickness. This mesh is built from repeating glycan chains: alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc). Short peptide side chains are attached to the MurNAc residues and are cross-linked to neighboring chains by pentaglycine bridges. This extensive cross-linking creates a dense, three-dimensional structure that is highly rigid.
Embedded within this peptidoglycan matrix are teichoic acids, which are phosphate-rich polymers unique to Gram-positive bacteria. Wall teichoic acids (WTAs) are covalently linked to the peptidoglycan layer. Lipoteichoic acids (LTAs) are anchored to the underlying cytoplasmic membrane. These molecules help regulate cell division, contribute to the overall negative charge of the cell surface, and are involved in binding to host tissues.
Outer Layers and Biofilm Formation
External to the thick peptidoglycan wall, S. aureus possesses a loose, polysaccharide-based layer known as the glycocalyx, or capsule. This structure acts as the outermost protective shield for the bacterium. Composed of repeating sugar units, the capsule helps the cell evade the host’s immune system.
This outer layer makes the bacterial surface slippery, interfering with phagocytosis, the immune process where cells attempt to engulf and destroy the bacterium. The polysaccharide layer also plays a direct role in the initial attachment of the bacterium to surfaces, including host tissues or medical devices. This initial adherence is a precursor to a more complex, clinically significant mode of growth.
The ability to adhere to surfaces is followed by the formation of a biofilm, a structured community of cells encased in a self-produced extracellular matrix. This matrix is a complex mixture primarily made of polysaccharide intercellular antigen (PIA), proteins, and extracellular DNA (eDNA). Biofilm formation is a structural adaptation that allows S. aureus to persist in the body, providing a collective defense for the entire community.
Structural Features and Disease
The distinct structural components of S. aureus are directly linked to its capacity to cause disease, acting as functional virulence factors. One such structure is Protein A, a protein covalently anchored to the peptidoglycan layer. Protein A has a unique ability to bind to the Fc region of immunoglobulin G (IgG) antibodies.
By binding to the Fc region, Protein A effectively prevents the antibody from functioning normally, thereby inhibiting opsonization. This structural interference shields the bacterium from being recognized and cleared by phagocytic immune cells. The formation of a biofilm is another structural mechanism that contributes significantly to pathogenicity. The dense, protective matrix of the biofilm creates a physical barrier that shields the embedded cells from both the host immune response and many common antibiotics.
This structural defense mechanism is a major reason why biofilm-associated infections, particularly those involving implanted medical devices, are difficult to treat.