Neisseria meningitidis

Neisseria meningitidis

The bacterium  Neisseria meningitidis, the meningococcus, is a Gram-negative, oxidase-positive diplococcus, identical in its staining and morphological characteristics to Neisseria gonorrhoeae. However, at an ultrastructural level, N. meningitidis has a prominent polysaccharide capsule not seen in the gonococcus. The capsule is antiphagocytic and is an important virulence factor in meningococcal disease. N. meningitidis strains are grouped on the basis of their capsular polysaccharides, into 12 serogroups, some of which are subdivided according to the presence of outer membrane protein and lipopolysaccharide antigens.

Neisseria meningitidis is usually cultivated in a peptone-blood base medium in a moist chamber containing 5-10% CO₂. All media must be warmed to 37 degrees prior to inoculation as the organism is extremely susceptible to temperatures above or below 37 degrees. This trait is rather unique among bacteria. Also, the organism tends to undergo rapid autolysis after death, both in vitro and in vivo. This accounts for the dissemination of lipopolysaccharide (endotoxin) during septicemia and meningitis.

The organism tends to colonize the posterior nasopharynx of humans, and humans are the only known host. Individuals who are colonized are carriers of the pathogen who can transmit disease to nonimmune individuals. The bacterium also colonizes the posterior nasopharynx in the early stages of infection prior to invasion of the meninges. Most individuals in close contact with a case of meningococcal meningitis become carriers of the organism. This carrier rate can reach 20 percent of the contact group before the first case is recognized, and may reach as high as 80 percent at the height of an epidemic.

Structure and Classification

Meningococcal capsular polysaccharides provide the basis for grouping the organism. Twelve serogroups have been identified (A, B, C, H, I, K, L, X, Y, Z, 29E, and W135). The most important serogroups associated with disease in humans are A, B, C, Y, and W135. The chemical composition of these capsular polysaccharides is known. The prominent outer membrane proteins of N. meningitidis have been designated class 1 through class 5. The class 2 and 3 proteins function as porins and are analogous to gonococcal Por. The class 4 and 5 proteins are analogous to gonococcal Rmp and Opa, respectively. Serogroup B and C meningococci have been further subdivided on the basis of serotype determinants located on the class 2 and 3 proteins. A handful of serotypes are associated with most cases of meningococcal disease, whereas other serotypes within the same serogroup rarely cause disease. All known group A strains have the same protein serotype antigens in the outer membrane. Another serotyping system exists based on the antigenic diversity of meningococcal LOS (lipooligopolysaccharide).

Meningitis

The term meningitis refers to inflammation the meninges of the brain or spinaL cord. Meninges are any of the three membranes that envelope the brain and spinal cord. The disease meningitis is caused by a number of different bacteria and viruses. Bacterial causes include Haemophilus influenzae, Escherichia coli, Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus aureus, and Neisseria meningitidis. Although a variety of cocci cause meningitis, the term meningococcus is reserved for the Gram-negative, bean-shaped diplococcus, Neisseria meningitidis. Like its relative N. gonorrhoeae, the organism tends to occur intracellularly in the cytoplasm of neutrophils which are attracted to the site of inflammation in the meninges, so this type of infection is called pyogenic or suppurative, to mean pus-forming.

Marchiafava and Celli were the first to report observing Gram-negative diplococci in cerebrospinal fluid of a fatal case of meningitis in 1884. In 1887, Weichselbaum isolated the bacterium from six cases of meningitis and established the isolates as a distinct species and proven to be the cause of meningitis.

Pathogenesis

Infection with N. meningitidis has two presentations, meningococcemia, characterized by skin lesions, and acute bacterial meningitis. The fulminant form of disease (with or without meningitis) is characterized by multisystem involvement and high mortality.

Infection is by aspiration of infective bacteria, which attach to epithelial cells of the nasopharyngeal and oropharyngeal mucosa, cross the mucosal barrier, and enter the bloodstream. If not clear whether blood-borne bacteria may enter the central nervous system and cause meningitis.

The mildest form of disease is a transient bacteremic illness characterized by a fever and malaise; symptoms resolve spontaneously in 1 to 2 days. The most serious form is the fulminant form of disease complicated by meningitis. The manifestations of meningococcal meningitis are similar to acute bacterial meningitis caused by other bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and E. coli. Chills, fever, malaise, and headache are the usual manifestations of infection.  Signs of meningeal inflammation are also present.

Clinical manifestations of N. meningitidis infection

The onset of meningococcal meningitis may be abrupt or insidious. Infants with meningococcal meningitis rarely display signs of meningeal irritation. Irritability and refusal to take food are typical; vomiting occurs early in the disease and may lead to dehydration. Fever is typically absent in children younger than 2 months of age. Hypothermia is more common in neonates. As the disease progresses, apnea, seizures, disturbances in motor tone, and coma may develop.

In older children and adults, specific symptoms and signs are usually present, with fever and altered mental status the most consistent findings. Headache is an early, prominent complaint and is usually very severe. Nausea, vomiting, and photophobia are also common symptoms.

Neurologic signs are common; approximately one-third of patients have convulsions or coma when first seen by a physician. Signs of meningeal irritation such as spinal rigidity, hamstring spasms and exaggerated reflexes are common.

Petechiae (minute hemorrhagic spots in the skin) or purpura (hemorrhages into the skin) occurs from the first to the third day of illness in 30 to 60% of patients with meningococcal disease, with or without meningitis. The lesions may be more prominent in areas of the skin subjected to pressure, such as the axillary folds, the belt line, or the back.

Fulminant meningococcemia occurs in 5 to 15% of patients with meningococcal disease and has a high mortality rate. It begins abruptly with sudden high fever, chills, myalgias, weakness, nausea, vomiting, and headache. Apprehension, restlessness, and delirium occur within the next few hours. Widespread purpuric and ecchymotic skin lesions appear suddenly. Typically, no signs of meningitis are present. Pulmonary insufficiency develops within a few hours, and many patients die within 24 hours of being hospitalized despite appropriate antibiotic therapy and intensive care.

Virulence Factors

For a time, the virulence of Neisseria meningitidis was attributed to the production of an "exotoxin" that could be recovered from culture filtrates of the organism. But when studies revealed that antitoxin reacted equally well with washed cells as culture filtrate, it was realized that the bacteria underwent autolysis during growth and released parts of their cell walls in a soluble form. Hence, the major toxin of N. meningitidis is its lipooligosaccharide, LOS, and its mechanism is endotoxic. The other important determinant of virulence of N. meningitidis is its antiphagocytic polysaccharide capsule.

The human nasopharynx is the only known reservoir of N. meningitidis. Meningococci are spread via respiratory droplets, and transmission requires aspiration of infective particles. Meningococci attach to the nonciliated columnar epithelial cells of the nasopharynx. Attachment is mediated by fimbriae and possibly by other outer membrane components. Invasion of the mucosal cells occurs by a mechanism similar to that observed with gonococci. Events involved after bloodstream invasion are unclear and how the meningococcus enters the central nervous system is not known.

Purified meningococcal LOS is highly toxic and is as lethal for mice as the LOS from E. coli or Salmonella typhimurium; however, meningococcal LOS is 5 to 10 times more effective than enteric LPS in eliciting a dermal Shwartzman phenomenon (a characteristic type of inflammatory reaction) in rabbits. Meningococcal LOS has been shown to suppress leukotriene B4 synthesis in human polymorphonuclear leukocytes. The loss of leukotriene B4 deprives the leukocytes of a strong chemokinetic and chemotactic factor.

Host Defenses

N. mengingitidis establishes systemic infections only in individuals who lack serum bacterial antibodies directed against the capsular or noncapsular (cell wall) antigens of the invading strain, or in patients deficient in the late-acting complement components.

The integrity of the pharyngeal and respiratory epithelium appears to be important in protection from invasive disease. Chronic irritation of the mucosa due to dust or low humidity, or damage to the mucosa resulting from a concurrent  upper respiratory infection, may be predisposing factors for invasive disease.

The presence of serum bactericidal IgG and IgM is probably the most important host factor in preventing invasive disease. These antibodies are directed against both capsular and noncapsular surface antigens. The antibodies are produced in response to colonization with carrier strains of N. meningitidis, as well as N. lactamica, and other nonpathogenic Neisseria species that are normal inhabitants of the upper respiratory tract. Protective antibodies are also stimulated by cross-reacting antigens on other bacterial species such as Escherichia coli. The role of bactericidal antibodies in prevention of invasive disease explains why high attack rates are seen in infants from 6 to 9 months old, the time at which maternal antibodies are being lost. Individuals with complement deficiencies (C5, C6, C7, or C8) may develop meningococcemia despite protective antibody. This emphasizes the importance of the complement system in defense against meningococcal disease.

Epidemiology

The meningococcus usually inhabits the human nasopharynx without causing detectable disease. This carrier state may last for a few days to months and is important because it not only provides a reservoir for meningococcal infection but also stimulates host immunity. Between 5 and 30% of normal individuals are carriers at any given time, yet few develop meningococcal disease. Carriage rates are highest in older children and young adults. Attack rates highest in infants 3 months to 1 year old. Meningococcal meningitis occurs both sporadically (mainly groups B and C meningococci) and in epidemics (mainly group A meningococci), with the highest incidence during late winter and early spring. Whenever group A strains become prevalent in the population, the incidence of meningitis increases markedly.

Treatment

Penicillin is the drug of choice to treat meningococcemia and meningococcal meningitis. Although penicillin does not penetrate the normal blood-brain barrier, it readily penetrates the blood-brain barrier when the meninges are acutely inflamed. Either chloramphenicol or a third-generation cephalosporin such as cefotaxime or ceftriaxone is used in persons allergic to penicillins.

Meningococcal disease is contracted through association with infected individuals, as evidenced by the 500- to 800-fold greater attack rate among household contacts than among the general population. Because such household members are at high risk, they require chemoprophylaxis. Sulfonamides were the chemoprophylactic agent of choice until the emergence of sulfonamide-resistant meningococci. At present, approximately 25 percent of clinical isolates of N. meningitidis in the United States are resistant to sulfonamides; nowadays, rifampin is the chemoprophylactic agent of choice.

Control

Groups A, C, AC, and ACYW135 capsular polysaccharide vaccines are available. However, the polysaccharide vaccines are ineffective in young children (in children under 1 year old, antibody levels decline rapidly after immunization) and the duration of protection is limited in children vaccinated at 1 to 4 years of age.  Routine vaccination is not currently recommended because the risk of infection is low. The group B capsular polysaccharide is a homopolymer of sialic acid and is not immunogenic in humans. A group B meningococcal vaccine consisting of outer membrane protein antigens has recently been developed, but is not licensed in the United States.

Search for a universal vaccine for meningococcal meningitis

There is an obvious need for a universal vaccine for meningococcal meningitis,  but the development of an effective vaccine against all forms of N. meningitidis has been hampered by the high degree of variation in the proteins on the surface of the bacterium which leads to the occurrence of many different antigenic types.

More than 10% of the population may be carrying the bacterium at any one time on the mucosal surfaces of the nose and throat. The majority of these carriers will not have any symptoms of the disease, but this continual exposure to the immune system puts pressure on the bacterium to mutate its surface components in order to survive. Thus, natural selection is the driving force for the emergence of new antigenic variants.

Among the class 2 and 3 outer membrane proteins of N. meningitidis, Por A has been considered a primary target for a vaccine-induced antibody.  PorA is a major component of the outer membrane of N. meningitidis, and anti-PorA antibodies are thought to be a critical component in immunity.  Interactions between antibodies and  PorA have been studied. Different strains of the bacterium have different PorA amino acid sequences within the region of the protein that specifically binds to antibody molecules. PorA  has several large amino acid "loop" regions that protrude from the surface, and it is these loops that are targets for antibody binding.

In the laboratory, the antigen-binding fragment (Fab) of anti-PorA antibodies can be crystallized and reacted with the antigenic loop regions of PorA  in order to determine the specificity of binding between antigen and antibody. Slight changes in PorA amino acid sequence have been shown to cause loss in the ability to bind to antibody molecules.  In nature, the bacterium mutates to insert new amino acid residues into the tip of the loop, which alters or eliminates many of the interactions with  antibody and allows the bacterium to bypass previous immune responses.