Neisseria gonorrhoeae

Microbiology

Neisseria species are fastidious Gram-negative cocci that require nutrient supplementation to grow in laboratory cultures. To be specific, they grow on chocolate agar with carbon dioxide. These cocci are facultatively intracellular and typically appear in pairs (diplococci), in the shape of coffee beans. Of the 11 species of Neisseria that colonize humans, only two are pathogens. N. gonorrhoeae is the causative agent of gonorrhea (also called "the clap") and is transmitted via sexual contact.

Neisseria is usually isolated on Thayer-Martin agar (or VPN agar)—an agar plate containing antibiotics (vancomycin, colistin, nystatin, and trimethoprim) and nutrients that facilitate the growth of Neisseria species while inhibiting the growth of contaminating bacteria and fungi. Further testing to differentiate the species includes testing for oxidase (all clinically relevant Neisseria species show a positive reaction) and the carbohydrates maltose, sucrose, and glucose test in which N. gonorrhoeae will oxidize only the glucose.

N. gonorrhoeae possess many dynamic polymeric protein filaments called type IV pili, which allow the bacteria to adhere and move along surfaces. The bacterial movement is mediated by twitching motility powered by the extension and retraction of the type IV pilus. Pili extend and attach to a substrate that signals the pilus to retract, dragging the cell forward. N. gonorrhoeae is able to pull 100,000 times its own weight, and it has been claimed that the pili used to do so are the strongest biological motor known to date, exerting one nanonewton. The pulling of type IV pilus is powered by a set of ATPases proteins: Pil F (extension) and Pil T (retraction). The type IV pilus in N. gonorrhoeae adhesive properties also play a role in microcolony aggregation and biofilm formation.

N. gonorrhoeae is naturally competent for DNA exchange, which is also dependent on type IV pilus dynamics, specifically proteins Pil Q and Pil T. These processes allow for the DNA of N. gonorrhoeae to acquire or spread new genes. Especially dangerous from the aspect of healthcare is the ability to conjugate, since this can lead to antibiotic resistance.

N. gonorrhoeae has surface proteins called Opa proteins, which bind to receptors on immune cells. In so doing, N. gonorrhoeae is able to prevent an immune response. The host is also unable to develop an immunological memory against N. gonorrhoeae – which means that future reinfection is possible. N. gonorrhoeae can also evade the immune system through a process called antigenic variation, in which the N. gonorrhoeae bacterium is able to alter the antigenic determinants (sites where antibodies bind) such as the Opa proteins and Type IV pili that adorn its surface. The many permutations of surface proteins make it more difficult for immune cells to recognize N. gonorrhoeae and mount a defense.

Genome

The genomes of several strains of N. gonorrhoeae have been sequenced. Most of them are about 2.1 Mb in size and encode 2,100 to 2,600 proteins (although most seem to be in the lower range). For instance, strain NCCP11945 consists of one circular chromosome (2,232,025 bp) encoding 2,662 predicted ORFs and one plasmid (4,153 bp) encoding 12 predicted ORFs. The estimated coding density over the entire genome is 87%, and the average G+C content is 52.4%, values that are similar to those of strain FA1090. The NCCP11945 genome encodes 54 tRNAs and four copies of 16S-23S-5S rRNA operons.

Horizontal gene transfer

In 2011, researchers at Northwestern University found evidence of a human DNA fragment in a Neisseria gonorrhoeae genome, the first example of horizontal gene transfer from humans to a bacterial pathogen.

Transmission

N. gonorrhoeae is transmitted from person to person during sexual relations. Traditionally, the bacteria was thought to move attached to spermatozoon, but this hypothesis did not explain female to male transmission of the disease. A recent study suggests that rather than “surf” on wiggling sperm, N. gonorrhoeae bacteria uses hairlike structures called pili to anchor onto proteins in the sperm and move through coital liquid. It can also be transmitted to the fetus in utero and afterward become apparent as a neonatal infection.

Disease

Symptoms of infection with N. gonorrhoeae differ, depending on the site of infection. Note also that 10% of infected males and 80% of infected females are asymptomatic. Men who have had a gonorrhea infection have a significantly increased risk of having prostate cancer.

Infection of the genitals can result in a purulent (or pus-like) discharge from the genitals, which may be foul-smelling. Symptoms may include inflammation, redness, swelling, and dysuria.

N. gonorrhoeae can also cause conjunctivitis, pharyngitis, proctitis or urethritis, prostatitis, and orchitis.

Conjunctivitis is common in neonates (newborns), and silver nitrate or antibiotics are often applied to their eyes as a preventive measure against gonorrhoea. Neonatal gonorrheal conjunctivitis is contracted when the infant is exposed to N. gonorrhoeae in the birth canal and can lead to corneal scarring or perforation, resulting in blindness in the neonate.

Disseminated N. gonorrhoeae infections can occur, resulting in endocarditis, meningitis or gonococcal dermatitis-arthritis syndrome. Dermatitis-arthritis syndrome presents with arthralgia, tenosynovitis, and painless non-pruritic (non-itchy) dermatitis.

Infection of the genitals in females with N. gonorrhoeae can result in pelvic inflammatory disease if left untreated, which can result in infertility. Pelvic inflammatory disease results if N. gonorrhoeae travels into the pelvic peritoneum (via the cervix, endometrium and fallopian tubes). Infertility is caused by inflammation and scarring of the fallopian tube. Infertility is a risk to 10 to 20% of the females infected with N. gonorrhoeae.

Diagonistic methods

Culture is the most common test for Gonorrhea, followed by DNA probe (Hybridization probe), Polymerase chain reaction assay, and Ligase chain reaction (LCR). DNA probe is an antigen detection test that uses a probe to detect gonorrhea DNA in specimens.

Specific culture of a swab from the site of infection is a criterion standard for diagnosis at all potential sites of gonococcal infection. Cultures are particularly useful when the clinical diagnosis is unclear, when a failure of treatment has occurred, when contact tracing is problematic, and when legal questions arise.

In patients who may have disseminated gonoccocal infection (DGI), all possible mucosal sites should be cultured (eg, pharynx, cervix, urethra, rectum), as should blood and synovial fluid (in cases of septic arthritis). Three sets of blood cultures should also be obtained.

Treatment and prevention

If N. gonorrhoeae is resistant to the penicillin family of antibiotics, then ceftriaxone (a third-generation cephalosporin) is often used. Sexual partners should also be notified and treated.

Antibiotic resistance in gonorrhea has been noted by beginning in the 1940s, when gonorrhea was treated with penicillin, but doses had to be continually increased in order to remain effective. By the 1970s, penicillin- and tetracycline-resistant gonorrhea emerged in the Pacific Basin. These resistant strains then spread to Hawaii, California, the rest of the United States, and Europe. Fluoroquinolones were the next line of defense, but soon resistance to this antibiotic emerged as well. Since 2007, standard treatment has been third-generation cephalosporins, such as ceftriaxone, which are considered to be our “last line of defense.”

Recently, a high-level ceftriaxone-resistant strain of gonorrhea, called H041, was discovered in Japan. Lab tests found it to be resistant to high concentrations of ceftriaxone, as well as most of the other antibiotics tested. Within N. gonorrhoeae, there are genes that confer resistance to every single antibiotic used to cure gonorrhea, but thus far they do not coexist within a single gonococcus. Because of N. gonorrhoeae’s high affinity for horizontal gene transfer, however, antibiotic-resistant gonorrhea is seen as an emerging public health threat.

Patients should also be tested for other sexually transmitted infections (there is a fivefold increase of HIV transmission), especially Chlamydia infections, since co-infection is frequent (up to 50% of cases). Antibacterial coverage is often included for Chlamydia because of this.

Transmission can be reduced by the usage of latex barriers, such as condoms or dental dams, during intercourse, oral and anal sex, and by limiting sexual partners.

Vaccine

Due to the relative frequency of infection and the rapid evolution of antibiotic resistance in strains of N. gonorrhoeae, vaccines are thought to be an important goal in the prevention of infection. Development of a vaccine has been complicated by the ongoing evolution of resistant strains and strains with diverse cell membrane structure and composition.

Survival of gonococci

The exudates from infected individuals contain many polymorphonuclear leukocytes (PMN) with ingested gonococci. These gonococci stimulate the PMN to release an internal oxidative burst involving reactive oxygen species in order to kill the gonococci. However, a significant fraction of the gonococci can resist killing and are able to reproduce within the PMN phagosomes.

Stohl and Seifert showed that the bacterial RecA protein, that mediates recombinational repair of DNA damage, plays an important role in gonococcal survival. The protection afforded by RecA protein may be linked to transformation, the process by which recipient gonococci take up DNA from neighboring gonococci and integrate this DNA into the recipient genome through recombination. Michod et al. have suggested that an important benefit of transformation in N. gonorrhoeae may be recombinational repair of oxidative DNA damages caused by oxidative attack by the hosts phagocytic cells.