Neisseria

• TRANSMISSION ROUTE: Direct contact, inhalation

Definition

Neisseria is a gram-negative, nonmotile, non-spore-forming, aerobic coccus often found in pairs. The bacterium was named for Albert Neisser, who discovered N. gonorrhoeae. Many species of Neisseria are normal flora in the nasopharynx of humans and other animals, but some are human pathogens.

Natural Habitat and Features

Most Neisseria spp. are commensal organisms of the upper respiratory mucosa in humans and other animals. Some species show pathogenicity in humans, but pathogenicity has not been reported in other animals. N. meningitidis is the most pathogenic of the respiratory species. Of the other respiratory Neisseria spp., some are opportunistic pathogens that cause infections in people who are immune compromised or otherwise debilitated, while other species are nonpathogenic.

N. gonorrhoeae is an obligate human parasite of the urogenital mucosa. In the laboratory, Neisseria spp. grow best on media enriched with the coenzyme nicotinamide adenine dinucleotide (NAD) and with blood and yeast extract. Incubation is best at 98.6° Fahrenheit (37° Celsius) in a moist chamber enriched with carbon dioxide. N. meningitidis strains are especially sensitive to temperature fluctuations, and media must be warmed to 98.6° F before inoculation. Neisseria spp. are also subject to drying and ultraviolet light and rarely survive long when away from mucous membranes. Most species appear under the microscope as paired cocci that are somewhat flattened at the junction. N. elongata appears as a short rod.

N. gonorrhoeae, also called gonococcus, is the only Neisseria sp. that is always parasitic and never commensal, and humans are its only known host. It is the causative agent of both gonorrhea and neonatal conjunctivitis, and it has been associated with meningitis and other systemic infections. Although this species is nonencapsulated, it can still evade the human immune system. Immunity is rare, and reinfection is common, in part because of the extreme variability and changeability of the surface antigens of N. gonorrhoeae.

The bacteria can be transmitted by sexual contact or during the birthing process. Gonorrhea is a common sexually transmitted disease, and it is estimated that between 650,000 and 700,000 persons are infected in the United States each year. About one-half of the women infected are asymptomatic, while other women show varying degrees of symptoms, including vulvovaginitis, urethritis, and pelvic inflammatory disease. Infected men are usually symptomatic and most commonly show urethritis. Systemic infection is quite rare but can occur in all genders.

The usual antibiotic treatment for gonorrhea includes third generation cephalosporins such as cefixime or ceftriaxone, which have replaced penicillins and quinolines because N. gonorrhoeae strains that are resistant to these antibiotics have become more common. Azithromycin, an antichlamydial, is often also given in conjunction with cephalosporins because many persons with gonorrhea are also infected with Chlamydia spp. An infected female can pass N. gonorrhoeae to her fetus during the birthing process. The most common consequence is neonatal conjunctivitis, but systemic infections can occur. In the United States, neonatal conjunctivitis is usually treated with antibiotic ointment containing erythromycin, neomycin, or tetracycline. Silver nitrate was the preferred treatment, but its use has been discontinued as better treatments emerged. Outside the United States, povidone-iodine is the preferred treatment because it is less irritating and is a broader-spectrum bactericide. No effective vaccine against N. gonorrhoeae has been developed, which is not surprising because natural immunity rarely, if ever, develops.

N. meningitidis, also called meningococcus, the most pathogenic of the respiratory Neisseria, is host specific to humans, as is gonococcus. It is biochemically and structurally identical to N. gonorrhoeae except for its polysaccharide capsule, which it uses to evade the immune system. N. meningitidis is subdivided into twelve groups, each defined by capsular antigens. In the United States, group B meningococci are most commonly seen. Other major human pathogenic strains fall into groups A, C, Y, and W. Humans exposed to meningococcus can develop natural immunity to the capsular antigens. The immunity is group-specific, but there is some overlap. About 30 percent of people infected with N. meningitidis become asymptomatic carriers. The rest develop pharyngitis or other upper respiratory infections. In a small percentage, systemic infection ensues; the most serious of these is meningitis. Although considered a respiratory organism, N. meningitidis strains have been found in the urogenital mucosa associated with gonorrhea. Meningococcal infections are most often spread in schools, barracks, or other places where groups of children or young adults, who have less chance of having developed immunity, congregate.

Although strains resistant to penicillin have emerged, penicillin G is still the preferred treatment. Cephalosporins are also commonly used, especially if penicillin resistance is suspected. When an N. meningitidis infection is discovered, all the infected person’s contacts are given prophylactic antibiotic treatment. Vaccines against group B strains have been developed and are commonly administered in the United States. In sub-Saharan Africa, a region known as the meningitis belt, group A strains are the most common pathogenic strains. Although group B vaccines have some effect on group A and other strains, work is underway to develop better broad-spectrum vaccines that will be effective against all groups of N. meningitidis.

Among the other Neisseria spp., N. lactamica and N. cinerea are common opportunistic human pathogens. Some human commensals, such as N. elongata and N. subflava, have occasionally been seen in pathogenic infections. Other species are species-specific nonpathogenic commensals in humans and other animals: N. canis in dogs, N. cuniculus in rabbits, and N. macacae in Rhesus monkeys. In the twenty-first century, there were ongoing challenges to combatting Neisseria pathogens, as well as advances made in better understanding them. For example, scientists discovered a new protein in Neisseria gonorrhoeae called TfpC, which provided insight into the structure of the bacteria. New vaccines were also developed against N. meningitidis serogroup B. However, antibiotic-resistant N. gonorrhoeae continued to be a serious and growing public health concern.

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