Neisseria meningitidis, or meningococcus, is a bacterium capable of causing severe infections in humans, including meningitis, an inflammation of the brain and spinal cord membranes, and sepsis, a life-threatening systemic infection. Understanding its structure is important for developing effective strategies to combat these diseases. The unique features of its outer layers and internal components contribute to its ability to colonize, invade, and cause illness.
Basic Bacterial Blueprint
Neisseria meningitidis is classified as a Gram-negative bacterium, a distinction based on its cell wall composition and how it reacts to a specific laboratory staining procedure. It typically appears as a diplococcus, meaning its round-shaped cells commonly form pairs, often described as kidney-bean or coffee-bean shaped. This bacterium possesses a layered structure, starting with an outer membrane, followed by a thin peptidoglycan layer, then a cytoplasmic membrane, and finally the internal cytoplasm.
Outer Layer Components
The outermost structures of Neisseria meningitidis are important for its survival and interaction with the human immune system. Surrounding the bacterium is a polysaccharide capsule, which acts as a protective layer. This capsule helps the bacterium evade host immune responses by preventing phagocytosis, a process where immune cells engulf and destroy foreign invaders. There are at least 13 different types of capsules, known as serogroups (e.g., A, B, C, X, Y, W), and these distinctions are used for classification and are fundamental for vaccine development.
Beneath the capsule lies the outer membrane, a complex structure composed of lipopolysaccharide (LPS), also referred to as lipooligosaccharide (LOS), and various proteins. LPS functions as a potent endotoxin, triggering strong immune responses in the host that contribute significantly to the symptoms of meningococcal disease, such as septic shock and hemorrhaging. The outer membrane also contains porin proteins, such as PorA and PorB, which regulate the passage of nutrients and other molecules into the cell. Other outer membrane proteins, including Opa (opacity-associated) and Opc (outer membrane protein C), play a role in the bacterium’s ability to adhere to and invade host cells.
Extending from the bacterial surface are hair-like appendages called pili, specifically Type IV pili. These structures are important for the initial attachment of Neisseria meningitidis to host cells, especially in the nasopharynx, where the bacterium often first colonizes. Pili also facilitate the formation of bacterial aggregates, known as microcolonies, and can mediate interactions with host cell receptors, initiating signaling cascades within the host cells.
Internal Cellular Machinery
The cell wall, composed of a thin peptidoglycan layer, is situated in the periplasmic space, which is the region between the outer membrane and the cytoplasmic membrane. This peptidoglycan layer provides structural integrity to the bacterial cell and helps maintain its characteristic kidney-bean shape. The peptidoglycan structure can be O-acetylated, which contributes to its resistance against host enzymes like lysozyme.
The cytoplasmic membrane, also known as the inner membrane, is located directly beneath the peptidoglycan layer. This membrane acts as a selective barrier, carefully controlling what substances enter and exit the cytoplasm, and it also plays a role in the bacterium’s energy production. It is primarily composed of phospholipids.
Filling the interior of the cell is the cytoplasm, a jelly-like substance that houses the bacterium’s genetic material. This includes a single, circular bacterial chromosome, and sometimes smaller, circular DNA molecules called plasmids, which can carry genes for antibiotic resistance or other traits. The cytoplasm also contains ribosomes, which are responsible for protein synthesis, and various enzymes necessary for metabolic processes that sustain the bacterium. Neisseria meningitidis is non-motile, as it lacks flagella, the whip-like appendages used for movement by other bacteria.