Neisseria meningitidis: An Overview
The History of Discovery
The meningococcus (Neisseria meningitidis) was primary isolated from the cerebrospinal fluid of patients with meningitis and studied in details in 1887 by A. Weichselbaum.
Classification of Meningococci
Meningococci pertain to the family Neisseriaceae, genus Neisseria, and species Neisseria meningitidis. They are further subdivided according to their antigenic features into serogroups and serotypes.
Structure and Properties of Meningococci
- Meningococci are gram-negative, bean-shaped pathogenic diplococci that similarly to other gram-negative bacteria are surrounded by an outer membrane. It is composed of lipids, outer membrane proteins (OMPs), and lipopolysaccharides.
- Pathogenic meningococci are enveloped by a polysaccharide capsule attached to this outer membrane. Menigococci are non-sporeforming non-motile organisms. They possess multiple pili and fimbriae.
- Meningococci are fastidious bacteria and can’t grow on basic nutrient media.
- They should be cultured on media with blood, serum or ascitic fluid (ascitic agar), better in atmosphere, supplemented with 5-10% CO2 (capnophilic bacteria).
- Optimum temperature for growth is 36-37°С. Bacteria can’t grow at 22°C. After 48 h of cultivation on solid media they produce transparent, convex, glistening, and elevated small colonies without hemolysis. In serum broth turbidity and a precipitate at the bottom of the test tube appears.
- Meningococci are aerobic or facultatively anaerobic bacteria. They produce oxidase and catalase.
- Generally meningococci show poor biochemical activity – the bacteria utilize only glucose and maltose with acid formation and don’t possess proteolytic activity.
- Meningococci carry multiple antigenic polysaccharides and proteins in their cell wall and capsule.
- They demonstrate more genetic diversity than most of other pathogenic human bacteria. This is explained by horizontal intraspecies recombination and gene incorporation from closely related Neisseria species.
- Because of this striking variability 13 serogroups by capsule antigens (A, B, C, D, Y, W-135, etc.) and 20 serotypes identifying outer membrane proteins (OMP) were defined.
- On the ground of antigenic properties of lipopolysaccharide, termed lipooligosaccharide (LOS) because of its relatively short sugar chain, another 13 immunotypes were described. Further additional typing is possible according to the antigenic properties of immunoglobulin Al (IgAl) proteases and pili.
- Pili and outer membrane proteins are the major adhesins that contribute to meningococcal attachment to mucosal cells.
- The most essential bacterial virulence factor for survival in the bloodstream is its polysaccharide capsule, which protects bacteria against complement-mediated bacteriolysis and phagocytosis by neutrophils.
- IgA proteases of meningococci break down human IgAs, thus impairing mucosal immunity. Hyaluronidase and neuraminidase promote microbial invasion.
- Disintegration of meningococci leads to the release of a highly toxic LOS-based endotoxin. Its liberation produces large amounts of proinflammatory cytokines such as tumor necrosis factor-alpha (α-TNF), IL-1, IL-6, IL-8, γ-interferon, and various colony-stimulating factors.
- Unlike other endotoxins, meningococcal LOS can be actively secreted by bacteria within membrane microvesicles, and microbial cells retain their viability after LOS shedding.
Meningococcus is the microbial agent of low stability – it is destroyed by drying in a few hours. By heating to a temperature of 60°С the bacterium is killed in 10 minutes and to 80°C in 2 minutes. When treated with 1% phenol, the culture becomes inactivated in 1 minute. Meningococci are very sensitive to low temperatures.
Pathogenesis and Clinical Findings in Meningococcal Infections
- Meningococcal infections affect only humans (anthroponotic disease).
- Meningococcal disease occurs worldwide. The bacteria from serogroups В and С cause the majority of infections in industrialized countries. Strains of serogroups A and, to a lesser extent, С dominate in third-world countries.
- Meningococcal carriers are the predominant source of infection. The causative agent is localized primarily in their nasopharynx. About 10% of adult population may become the carriers of meningococci through the lifetime.
- Infants and children remain to be the most susceptible group for the disease.
- The infection is transmitted by the air droplet route.
- Several forms of meningococcal infection exist: meningococcal carriage, meningococcal nasopharyngitis, meningitis, and meningococcemia (including fulminant meningococcal sepsis).
- Meningococcal carriage and meningococcal nasopharyngitis are the predominant forms of infection, being most spread in population.
- Nevertheless, some patients develop severe acute meningococcal disease: meningitis and meningococcemia. Meningococcal meningitis isregarded now as a form of systemic meningococcal disease, which is always followed by microbial dissemination.
- Systemic meningococcal infection is the invasive disease. It is occurred after exposure to a pathogenic strain and colonization of the naso-oropharyngeal mucosa, followed by microbial passage through mucosal tissues, and survival of meningococci in the bloodstream.
- Damage of the nasopharyngeal ciliated epithelium may be the first step that provokes colonization. After primary adherence to CD46, further microbial attachment is promoted by interaction of outer membrane proteins (OMP) to CD66 receptors.
- Microbial binding to CD66 on phagocytic and endothelial cells activates phagocytosis and cytokine production and stimulates the engulfment of meningococci by epithelial cells resulting in their transcellular passage.
- Microbial entry into the bloodstream leads to bacterial dissemination and endotoxin release. It triggers massive proinflammatory cytokine liberation that may cause toxic shock. High cytokine concentrations reflect the depth of shock. Extensive disseminated intravascular coagulation (DIC) and tissue damage are the most severe complications of meningococcal endotoxemia.
- Incubation period lasts from several hours to several days, i.e., acute meningococcal disease is one of the most swift-progressing infections.
- In some patients with low degrees of bacteremia, meningococci can be eliminated spontaneously.
- Other patients demonstrate sudden attack of the disease with high fever 39-40°С, vomiting, rigidity of the occipital muscles, severe headache, and hemorrhagic skin rashes. Involvement of the cranial nerves results from the increase of the intracranial pressure. A large number of neutrophils are found in the cerebrospinal fluid.
- In case of meningitis the inflammatory response is localized predominantly in an extravascular compartment.
- If meningococcal sepsis (meningococcemia) has abnormally high fatality rate (20-50% and even more), meningococcal meningitis develops lower rate of lethality (about 1-5%) and post-infectious neurological sequelae (in 10-20% of patients).
- Immunity to meningococcal infection is associated with the presence of specific bactericidal complement-dependent antibodies in patient’s serum. These antibodies arise in the course of infection. They can be type-specific and/or group-specific. Antimicrobial antibodies prevent the development of invasive disease. Recurring infections are not common.
- Infants are generally protected from the infection for 3-5 months by passive immunity via IgG antibodies transferred from the mother.
Laboratory Diagnosis of Meningococcal Infections
- Nasopharyngeal swabs and blood samples are taken for culture. Specimens of cerebrospinal fluid (CSF) and skin petechiac biopsy are taken for microscopy, culture, and microbial antigen detection.
- Meningococcal antigens can be rapidly determined in CSF by precipitation or ELISA test.
- Microbial DNA in CSF is detected by molecular genetic tests (PCR).
- Microscopy of gram-stained slides with the samples of centrifuged cerebrospinal fluid detects typical gram-negative bean-shaped diplococci within polymorphonuclear leukocytes (incomplete phagocytosis) or extracellularly.
- Cultivation of clinical specimens is performed in serum, ascitic or blood agar, supplemented with antibiotics, suppressing gram-positive microflora (vancomycin, amphotericin or ristomycin). After incubation for 48 h in aerobic atmosphere with 5-10% CO2 pure cultures of meningococci can be recovered from CSF or blood.
- The bacteria are further identified by carbohydrate fermentation and agglutination with group and type-specific sera.
- Antibodies to meningococcal polysaccharides (serological diagnosis) can be measured by latex agglutination or ELISA. Test for antibodies is elaborated mainly in cases of unclear meningococcal infection.
Treatment and Prophylaxis of Meningococcal Infections
- Taking into account the fulminant character of disseminated meningococcal infection, it is generally accepted that the therapy should never be delayed by diagnostic procedures, and antibiotics are the cornerstone of treatment.
- Beta-lactam antibiotics (penicillin G or third-generation cephalosporins) are the drugs of choice for treatment of meningococcal disease. Azalides or chloramphenicol can be used in allergic persons.
- Treatment of shock includes fluid resuscitation, administration of glucocorticoids, transfusion of fresh-frozen plasma, mechanical lung ventilation if required.
- For specific prophylaxis various polysaccharide chemical vaccines based on group A and С capsular antigens were developed.
- Currently, a quadrivalent vaccine containing the antigens of serogroups A, C, W, and Y is available. Vaccination is highly effecient in the control of outbreaks and epidemics of meningococcal infection conferring the protective immunity at least for 2-3 years. However, vaccination doesn’t affect carriers.
- The major drawback of these vaccines is the absence of activity against group В meningococci. It has been found, that group В polysaccharide mimics the human neuronal cell adhesion molecules; therefore, the use of group В capsular antigen for immunization elevates the risk of autoimmune response.
- Now experimental group В vaccines based on meningococcal outer membrane proteins are under the clinical trials.