The genus Neisseria encompasses a diverse group of bacteria defined by a specific set of characteristics. This group is notable because it includes species significant to human health and many that are harmless residents of the body. Some members are well-known for causing disease, establishing infections that have impacted human populations globally.
Conversely, many Neisseria species are commensal, meaning they live in the human body, particularly the upper respiratory tract, without causing harm. The presence of both pathogenic and non-pathogenic species within a single genus makes it a subject of scientific interest for microbiology and medicine.
Microscopic and Colonial Appearance
When viewed under a microscope after a Gram stain procedure, Neisseria species consistently appear as Gram-negative. This means they do not retain the primary crystal violet stain, instead taking up the red counterstain. Their shape is that of a coccus, but they are most frequently observed in pairs, a configuration known as diplococci. The individual cells within these pairs are not perfectly round; their adjacent sides are flattened, giving them a distinct appearance often compared to coffee beans or kidney beans.
These bacteria are non-motile, meaning they lack structures for self-propulsion, and they do not form spores. The cells are small, measuring between 0.6 and 1.0 micrometers in diameter. When grown on laboratory agar plates, Neisseria colonies have a recognizable appearance. The colonies are small, round, and may appear grayish or white, and often look smooth and moist. For example, on a medium like chocolate agar, Neisseria meningitidis forms relatively large, gray, and smooth colonies.
Physiological and Metabolic Properties
Neisseria species have specific requirements for growth, thriving in environments that mimic the human body. They are aerobic, meaning they need oxygen to fuel their metabolic processes, and many species are also described as capnophilic. This signifies a requirement for an increased concentration of carbon dioxide for optimal cultivation. The optimal temperature for the growth of most Neisseria species is around 35–37°C, which corresponds to human body temperature. These specific growth needs mean that specialized media and incubators are often necessary to successfully culture them in a laboratory.
A few biochemical tests are fundamental for identifying bacteria of this genus. All Neisseria are oxidase-positive; this test detects the presence of the enzyme cytochrome c oxidase. Most species are also catalase-positive, meaning they can break down hydrogen peroxide into water and oxygen. Differences in how various species metabolize carbohydrates are also used for identification. For instance, N. gonorrhoeae produces acid from glucose only, while N. meningitidis produces acid from both glucose and maltose.
Key Neisseria Species and Their Roles
Within the Neisseria genus, two species are of particular medical importance: Neisseria gonorrhoeae and Neisseria meningitidis. N. gonorrhoeae, often called gonococcus, is the causative agent of gonorrhea, a sexually transmitted infection. Humans are the only natural host for this bacterium, and it can cause infections in various sites, including the urethra, cervix, rectum, and pharynx.
Neisseria meningitidis, or meningococcus, is a major cause of bacterial meningitis and meningococcemia, a severe bloodstream infection. Unlike N. gonorrhoeae, N. meningitidis possesses a polysaccharide capsule, which is a factor in its ability to cause invasive disease. There are 12 different serogroups of meningococcus based on the composition of this capsule.
Factors Contributing to Pathogenicity
Pathogenic Neisseria species possess several attributes, often called virulence factors, that enable them to cause disease. A prominent feature is the presence of pili, which are hair-like filaments extending from the bacterial surface. These pili are used by the bacteria to attach to the surfaces of host cells, which prevents them from being washed away by bodily fluids. The outer membrane contains various proteins and a molecule called lipooligosaccharide (LOS), which acts as an endotoxin. When released, LOS can trigger a strong inflammatory response responsible for much of the tissue damage seen in these diseases.
Pathogenic Neisseria have also developed mechanisms to evade the host’s immune system. N. meningitidis is surrounded by a thick polysaccharide capsule that protects it from being engulfed by immune cells. Both N. gonorrhoeae and N. meningitidis produce an enzyme called IgA protease, which breaks down IgA antibodies found on mucosal surfaces. Furthermore, these bacteria can change the proteins on their surface, a process known as antigenic variation, which helps them avoid recognition by the immune system.