Neisseria meningitidis are bacteria that have a characteristic coffee bean shape when observed in stained smears under a regular light microscope used in microbiology laboratories (figure 2). N. meningitidis may be present in the nose and throat of humans without causing any symptoms or signs of disease. This is referred to as colonization or the carrier state. There are 13 serogroups of N. meningitidis but groups A, B, C, Y and W-135 are associated most frequently with serious disease in humans (figure 3). The type of sugar material forming the outer coat or capsule of an organism determines the serogroup. The distribution of meningococcal serogroups causing disease in the United States has shifted in recent years (figure 4). Serogroups B, C, and Y each account for approximately 30% of reported cases, but serogroup distribution may vary by geographic location and time. Group A has been associated frequently with epidemic disease elsewhere in the world, primarily in Saharan Africa and Asia. Outbreaks due to serogroups B, C, and Y have been reported in the United States since 1990. Figure 5 shows serogroup C meningococcal outbreaks by year from 1981 to 1993. A schematic diagram of the meningococcus is shown in figure 6. The inside or cytoplasm of the meningococcus is surrounded by a cell envelope consisting of three layers, an inner layer called the cell membrane, a middle layer composed of peptidoglycans, and an outer layer called the cell wall. The cell wall is made up of sugar-like molecules (lipopolysaccharides or LPS) and outer membrane proteins (OMPs), which are linked together to form a mesh or sheet, giving the bacterium its shape. Meningococci that cause disease also have a sugar (polysaccharide) capsule, which is attached to the outer cell wall. This meningococcal capsule forms an additional layer around the bacterium, protecting it from ingestion and killing by certain white blood cells, a process referred to as phagocytosis. The meningococcal cytoplasm contains water, salts, deoxyribose nucleic acid (DNA), ribose nucleic acid (RNA), various structures (e.g., ribosomes), proteins, enzymes, and other substances necessary for life.