Neisseria gonorrhoeae: An overview
The History of Discovery
Gonococci, the causative agents of gonorrhoea, were first described by A. Neisser in 1879. Later in 1885 E. Baum obtained the pure culture of these bacteria.
Gonococci belong to the family Neisseriaceae, genus Neisseria, and species Neisseria gonorrhoeae.
Structure and Properties of Neisseria gonorrhoeae
- Gonococci are similar with meningococci (about 70% of genetic similarity). Bacteria are visualized as gram-negative, bean-shaped diplococci.
- Gonococci are non-sporeforming non-motile microorganisms. Unlike meningococci, Neisseria gonorrhoeae is lack of capsule.
- The bacteria express multiple pili and fimbriae. They carry a large number of plasmids. Some of them confer resistance of gonococci to antimicrobial drugs resulting from beta-lactamase expression.
- N. gonorrhoeae are even more fastidious than meningococci and can’t multiply on basic nutrient media.
- They are cultivated on media, containing blood, serum or ascitic fluid (blood, serum or ascitic agar) better in atmosphere with 5-10% CO2 at pH 7.2-7.6. Optimal growth temperature is 37°C; the bacteria lose viability out of range 25-42°C.
- Gonococci produce very small convex colonies, opaque or transparent, depending on Opa protein expression.
- Gonococci are mostly aerobic or facultatively anaerobic bacteria. The bacteria yield minimal biochemical activity.
- They ferment solely glucose with acid end products and have no proteolytic activity.
- Similar to other members of the genus, gonococci produce oxidase and catalase.
- N. gonorrhoeae harbors various antigenic determinants of polysaccharide and protein nature.
- The pathogens are able to alter surface antigen expression to evade host immune response. Bacterial pili contain protein pilin, which significantly varies among gonococcal strains (about 100 serovars).
- Gonococci express a number of porins, namely PorA and PorB proteins. Multiple serovars are determined according to Por antigen variations.
- Adhesive Opa (opacity) proteins also render antigenic activity.
- Polysaccharide epitopes of gonococci are confined within bacterial cell wall lipooligosaccharide (LOS).
- N. gonorrhoeae can switch the synthesis of various antigenic molecules, e.g pilins, Opa proteins or LOS residues triggering alternate gene expression.
- Overall, gonococci are regarded as the bacteria with highest genetic variability and genetic exchange with other bacterial species
- Bacterial adhesins, including pili and Opa proteins promote microbial attachment to the host cells. Opa proteins principally bind to the cells bearing CD66 carcinoembryonic antigen.
- Opa and Por proteins stimulate intracellular invasion of gonococci and inhibit phagocytosis, preventing phagosome-lysosome fusion.
- Microbial lipooligosaccharide displays evident endotoxin activity. LOS antigenic mimicry with human glycosphingolipids support gonococci to escape host defensive reactions.
- Gonococci produce IgA1 protease that cleaves human mucosal IgA1. Many bacterial strains express plasmid-encoded beta-lactamases.
- Gonococci demonstrate a low viability, being very sensitive to external influences. They can’t resist cooling, drying, or UV irradiation. Gonococci best survive in the moist conditions in various human discharges.
- Bacteria are killed at temperature of 56°C within 5 minutes. They are readily inactivated by treatment with ordinary disinfectants.
Pathogenesis and Clinical Findings in Gonorrhoea
- Gonococcus is the strictly human pathogen.
- Gonorrhoea is a typical sexually transmitted disease that affects predominantly urogenital tract. Unprotected sexual intercourse results in 50% likelihood of disease contraction in women and 30-50% in men.
- Also gonococci produce gonorrhoeal conjunctivitis in adults and ophthalmia neonatorum (or blennorrhoea) in newborn infants transmitted by contact route.
- Infectious dose of bacteria is generally low – about 103 cells of virulent strains
- Only piliated opaque gonococcal cultures, containing multiple adhesins (e.g., Opa proteins), are able to adhere and invade host tissues inflammation with extensive tissue fibrosis and seminal duct obliteration that may cause male infertility.
- In females the primary penetration of bacteria occurs in the endocervical epithelium. The infection extends to the urethra and vagina, and affects uterine tubes thus provoking salpingitis.
- Fibrosis and obturation of uterine tubes result in female infertility. Female gonorrhoea may be asymptomatic.If not treated the disease easily becomes chronic.
- In some rare cases the infection breaks tissue barriers, and the bacteria enter the bloodstream. This leads to hematogenous microbial spread with hemorrhagic skin rashes. Gonococcal dissemination may produce specific arthritis or endocarditis.
- Blennorrhoea or ophthalmia neonatorum evolves as the result of neonate infection, when newborns pass through infected maternal canal. Specific gonococcal eye injury can cause infant blindness.
- The immunity doesn’t confer the resistance against gonococci albeit specific antibodies and immune cells can appear in human secretions.
Laboratory Diagnosis of Gonorrhoea
- Specimens are collected from the discharge of urethra, vagina, vulva, cervix, rectum or conjunctiva in case of ophthalmia neonatorum.
- Gram-stained smears of secretions show typical gram-negative bean-shaped cocci within polymorphonuclear leukocytes (incomplete phagocytosis) or extracellularly.
- More sensitive and specific is immunofluorescent test.
- Gonococcal antigens in clinical specimens are determined by ELISA.
- To obtain microbial culture the collected specimens are inoculated immediately into serum or ascitic agar. The media are supplemented with antibiotics, suppressing concomitant bacteria and fungi (vancomycin, amphotericin or ristomycin).
- For men the culture is not necessary in case of positive microscopic examination, but cultures for women are indispensable.
- After incubation for 48 h in chamber with 5% CO2 the specimens can yield pure cultures. They are further confirmed by microscopy with Gram stain, fermentation tests and microbial antigens determination.
- Serological reactions are of limited use in gonorrhoea.
- As confirmatory tests for detection of microbial nucleic acids, PCR and other nucleic acid amplification tests (NAATs) are used.
Treatment and Prophylaxis of Gonorrhoea
- Because of rapidly growing resistance of gonococci to antimicrobial agents, third generation cephalosporins (e.g., ceftriaxone) and macrolides/azalides (azithromycin) are currently recommended for treatment of gonorrhoea..
- However, in 2011 the first ceftriaxone-resistant isolates of gonococci were registered. Later in 2013 a new drug combination of azithromycin and gentamycin was introduced into clinical practice that is efficient against multiresistant gonococcal strains.
- For treatment of chronic gonorrhoea the injections of gonococcal killed vaccine can be administered to stimulate host immunity.
- For protection of newborns against ophthalmia neonatorum, urgent eye instillations of sulfacetamide (sulfacyl-sodium) solution as well as applications of tetracycline or azithromycin ophthalmic ointments are administered immediately after birth.