Staphylococcus aureus is a common bacterium found widely in the environment and on human skin and mucous membranes. While often harmless, understanding its intricate structure is important for comprehending how it interacts with its surroundings and, at times, causes infections. This article will explore the specific architectural features of Staphylococcus aureus, from its basic shape to its sophisticated external components.
Basic Form and Gram-Positive Identity
Staphylococcus aureus typically appears as spherical cells (cocci), measuring about 0.5 to 1.5 micrometers in diameter. These cocci often arrange themselves in irregular, grape-like clusters, a characteristic arrangement that helps distinguish them. The bacterium is non-motile, meaning it lacks flagella or other structures for self-propulsion.
Its Gram-positive cell wall is considerably thick, ranging from approximately 20 to 80 nanometers. This robust structure primarily consists of multiple layers of peptidoglycan, a complex polymer of alternating N-acetylmuramic acid and N-acetylglucosamine units cross-linked by short peptide bridges. The extensive cross-linking provides significant structural rigidity and protection to the cell.
Interspersed within this peptidoglycan mesh are teichoic acids, polymers of ribitol phosphate or glycerol phosphate. Wall teichoic acids are covalently linked to the peptidoglycan, while lipoteichoic acids are anchored to the cytoplasmic membrane. These teichoic acids contribute to the overall negative charge of the cell surface and play roles in cell division and adhesion. The thick peptidoglycan layer retains the crystal violet stain during the Gram staining procedure, causing S. aureus to appear purple under a microscope.
The Inner Barrier: Cytoplasmic Membrane
Beneath the cell wall lies the cytoplasmic membrane, a dynamic barrier surrounding the internal components of the Staphylococcus aureus cell. This membrane is a phospholipid bilayer, with hydrophilic heads facing outward and hydrophobic tails forming the interior. Proteins are embedded within or associated with this lipid bilayer, contributing to its diverse functions.
The cytoplasmic membrane acts as a selective barrier, controlling the passage of ions, nutrients, and waste products into and out of the bacterium. This selective permeability maintains the cell’s internal environment and ensures its survival. Beyond its barrier role, the membrane also houses various enzyme systems involved in metabolic processes, including electron transport and ATP synthesis, which are fundamental for the cell’s energy production.
Internal Cellular Machinery
The interior of Staphylococcus aureus contains cytoplasm, a jelly-like substance composed primarily of water, dissolved salts, proteins, and organic molecules. This aqueous environment serves as the site for metabolic reactions, where enzymes facilitate the synthesis and breakdown of cellular components. Suspended within this cytoplasm are the specialized structures that carry out the bacterium’s life processes.
The genetic material of Staphylococcus aureus is in the nucleoid, a region lacking a surrounding membrane, unlike the nucleus in eukaryotic cells. The nucleoid houses a single, circular chromosome, a large DNA molecule that carries the bacterium’s primary genetic information. This compact organization allows for efficient replication and segregation during cell division.
In addition to the main chromosome, Staphylococcus aureus often possesses plasmids, smaller, extra-chromosomal DNA molecules. These plasmids are also circular and replicate independently of the main chromosome. They can carry genes that provide advantageous traits, such as those encoding specific enzymes. Ribosomes, granular structures composed of ribosomal RNA and proteins, are also present in the cytoplasm. These organelles translate genetic information from messenger RNA into proteins, a process fundamental for cellular function and structure.
External Features and Host Interaction
Beyond the cell wall, Staphylococcus aureus has features that mediate its interaction with the environment and host. Some S. aureus strains produce a polysaccharide capsule, an organized layer that surrounds the cell. This capsule, when present, provides a protective barrier, helping the bacterium evade immune recognition and destruction by host defenses.
A more common external feature is the slime layer (glycocalyx), a looser, less organized extracellular polymeric substance. This sticky layer, composed of polysaccharides and proteins, facilitates bacterial adhesion to surfaces and other bacteria. The slime layer plays a role in the formation of biofilms, structured communities of bacteria encased in a self-produced matrix, often found on medical devices or host tissues.
Surface proteins are anchored to or embedded within the Staphylococcus aureus cell wall, acting as adhesins to bind to host tissues. Protein A, for instance, is a cell wall-anchored protein that binds to the Fc region of antibodies, interfering with the host’s immune response. Other adhesins include clumping factors (ClfA and ClfB), which bind to fibrinogen, a protein found in blood plasma, leading to bacterial aggregation. Fibronectin-binding proteins (FnBPA and FnBPB) are also present, allowing the bacterium to attach to fibronectin, a component of the extracellular matrix.