Neisseria is a genus of bacteria known for its distinct microscopic appearance. These bacteria are gram-negative, meaning they do not retain the crystal violet stain used in Gram staining. They appear in pairs, giving them the name diplococci, and their adjacent sides are flattened, resembling coffee or kidney beans. The Neisseria genus includes many harmless species, but it is primarily recognized for two human pathogens responsible for well-known diseases.
Pathogenic Neisseria Species
The two pathogenic species, Neisseria gonorrhoeae and Neisseria meningitidis, cause distinct clinical conditions with serious health consequences. They share some structural similarities, but differences, such as the presence of a protective outer capsule on N. meningitidis, contribute to their different modes of infection and the diseases they cause.
Neisseria gonorrhoeae is the causative agent of gonorrhea, a common sexually transmitted infection. In men, the infection often presents with symptoms including a discharge from the penis, a burning sensation during urination, and pain or swelling in the testicles. Many women with gonorrhea, however, do not experience any symptoms, making it possible to unknowingly transmit the infection. When symptoms do appear in women, they may include vaginal discharge, pain in the lower abdomen, and bleeding between periods.
Untreated gonorrhea can lead to long-term health problems. In women, the infection can spread to the reproductive organs, causing pelvic inflammatory disease (PID). PID can result in scarring of the fallopian tubes, which increases the risk of ectopic pregnancy and can lead to infertility. In men, untreated gonorrhea can cause epididymitis, an inflammation of the tube that carries sperm, which may also result in infertility.
The other major pathogen, Neisseria meningitidis, is responsible for bacterial meningitis and a severe blood infection called meningococcemia. Meningitis is an infection of the meninges, the protective membranes surrounding the brain and spinal cord. Its hallmark symptoms are a sudden onset of fever, severe headache, and a stiff neck. Meningococcemia, or sepsis caused by these bacteria, can produce fever, fatigue, and a characteristic dark purple rash. Both forms of meningococcal disease are serious and can progress rapidly, with a case-fatality rate of 10-15% even with antibiotic treatment.
Commensal Neisseria Species
While some Neisseria species are pathogenic, the majority are considered commensal, meaning they live in and on the human body without causing harm. These harmless bacteria are a normal part of the human microbiota. They are most commonly found on the mucosal surfaces of the upper respiratory tract, such as in the nose and throat.
There are numerous species of commensal Neisseria, with Neisseria lactamica being one well-known example. These bacteria coexist peacefully with their human host and can sometimes play a protective role. Research suggests that some commensal species may help inhibit the growth of their pathogenic relatives, N. gonorrhoeae and N. meningitidis, by competing for resources and space.
The presence of these commensal organisms is an aspect of a healthy microbiome. They contribute to the overall balance of microbial life in the respiratory tract.
Transmission and Prevention
The methods of transmission and prevention differ between the two pathogenic Neisseria species. Neisseria gonorrhoeae is transmitted almost exclusively through sexual contact, including vaginal, anal, and oral sex. Consequently, prevention strategies are centered on safe sex practices. The consistent and correct use of condoms is highly effective at reducing the transmission of gonorrhea. Open communication with sexual partners about sexual health history is another component of prevention.
In contrast, Neisseria meningitidis spreads through respiratory and throat secretions, such as saliva and spit. Transmission requires close or prolonged contact, like kissing, or living in crowded settings such as college dormitories or military barracks. It is not as contagious as common viruses like the cold or flu.
The most effective method for preventing meningococcal disease is vaccination. There are two main types of vaccines available that protect against the most common serogroups, or strains, of the bacteria. The MenACWY vaccine protects against serogroups A, C, W, and Y, while the MenB vaccine protects against serogroup B. Widespread vaccination has led to a significant decline in the incidence of meningococcal disease.
Diagnosis and Treatment
For Neisseria gonorrhoeae, the most reliable diagnostic methods are nucleic acid amplification tests (NAATs). These highly sensitive tests can detect the bacterium’s genetic material from a urine sample or a swab taken from the cervix, urethra, rectum, or throat.
For a suspected Neisseria meningitidis infection, which is a medical emergency, diagnosis involves analyzing samples of blood or cerebrospinal fluid. A blood culture can identify the bacteria in cases of meningococcemia. If meningitis is suspected, a lumbar puncture, or spinal tap, is performed to collect cerebrospinal fluid, which is then tested for the presence of the bacteria.
Antibiotics are the primary treatment for both infections. However, antibiotic resistance is a growing challenge, particularly with N. gonorrhoeae. The bacterium has developed resistance to many of the antibiotics once used to treat it, making infections more difficult to cure. Current treatment guidelines recommend dual therapy with two different antibiotics to combat resistance. Meningococcal disease requires immediate hospitalization and the administration of intravenous antibiotics to be effective.