Neisseria meningitidis is a bacterium known for causing serious human diseases, including meningitis and bloodstream infections. The bacterium’s ability to cause illness is directly related to its structure, which allows it to colonize a human host and bypass immune defenses. Understanding its components is foundational for creating effective treatments to combat the diseases it causes.
Overall Form and Classification
Neisseria meningitidis is a Gram-negative bacterium, a classification based on the Gram stain test which differentiates bacteria by cell wall composition. Gram-negative bacteria possess a thin peptidoglycan cell wall between an inner and outer membrane, a feature preventing them from retaining the test’s violet stain. Under a microscope, the bacterium appears as a diplococcus, meaning its spherical cells are arranged in pairs.
The individual cells have a “coffee-bean” shape, facing each other on their flattened sides. This bacterium is non-motile and does not form spores. For laboratory growth, it requires specific conditions, including an environment with elevated carbon dioxide and a temperature of 35-37°C.
The Protective Outer Capsule
The most external layer of Neisseria meningitidis is a polysaccharide capsule. This structure is a primary defense mechanism, shielding the bacterium from the host’s immune system. The capsule’s composition helps it prevent being engulfed by immune cells, a process known as phagocytosis, and offers resistance against complement-mediated killing.
The chemical makeup of these polysaccharides is not uniform across all meningococcal bacteria, and these variations define the different serogroups. At least 13 serogroups have been identified, with groups A, B, C, W, Y, and X responsible for most disease cases worldwide. This diversity is a consideration for public health, as vaccines are designed to target specific serogroups, meaning a vaccine for one may not protect against another.
The Dynamic Outer Membrane
Beneath the capsule is the outer membrane, a hallmark of Gram-negative bacteria. This membrane contains proteins and molecules instrumental to how the bacterium interacts with the human host. A significant component is lipooligosaccharide (LOS), an endotoxin embedded in the outer membrane. When the bacteria are destroyed, LOS is released and can trigger a powerful inflammatory response, contributing to symptoms like fever and septic shock.
The outer membrane also contains proteins called porins, which form channels for nutrient passage. Two main porins, PorA and PorB, are notable for nutrient uptake and interacting with host cells. Other surface proteins, known as opacity proteins (Opa and Opc), function as adhesins, helping the bacterium stick to human cells, and can undergo phase variation, allowing the bacterium to adapt and evade the immune system.
Pili: Tools for Attachment and Interaction
Extending from the bacterial surface are numerous hair-like appendages known as Type IV pili. These structures extend beyond the capsule, making them one of the first points of contact with host tissues. Their primary function is adhesion, enabling N. meningitidis to attach firmly to the epithelial cells lining the human nasopharynx, the initial site of colonization.
Beyond attachment, these pili are dynamic and facilitate a movement called twitching motility, where the pili extend, attach to a surface, and then retract, pulling the bacterium along. This allows the bacteria to move across cell surfaces and form microcolonies. Pili also play a part in natural competence, the bacterium’s ability to take up free DNA from its environment, which contributes to genetic adaptation and antibiotic resistance.
Core Bacterial Machinery
Internal to the outer membrane are the components required for the bacterium’s survival and replication. The cell wall contains a thin layer of peptidoglycan, which provides structural integrity and shape to the cell. The peptidoglycan in N. meningitidis can be modified to help it resist degradation by host enzymes like lysozyme.
Beneath the peptidoglycan layer lies the cytoplasmic, or inner, membrane. This barrier controls the transport of molecules into and out of the cell’s interior and is the site of energy production. The cytoplasm is filled with ribosomes for protein synthesis and contains the bacterium’s genetic material in a single, circular chromosome located in a region called the nucleoid.