Neisseria gonorrhoeae, also known as gonococcus (singular) or gonococci (plural), is a species of Gram-negative diplococci bacteria first isolated by Albert Neisser in 1879. An obligate human pathogen, it primarily colonizes the mucosal lining of the urogenital tract; however, it is also capable of adhering to the mucosa of the nose, pharynx, rectum, and conjunctiva. It causes the sexually transmitted genitourinary infection gonorrhea as well as other forms of gonococcal disease including disseminated gonococcemia, septic arthritis, and gonococcal ophthalmia neonatorum.
N. gonorrhoeae is oxidase positive and a microaerophile that is capable of surviving phagocytosis and growing inside neutrophils. Culturing it requires carbon dioxide supplementation and enriched agar (chocolate agar) with various antibiotics (Thayer–Martin). It exhibits antigenic variation through genetic recombination of its pili and surface proteins that interact with the immune system.
Sexual transmission is through vaginal, anal, or oral sex. Sexual transmission may be prevented through the use of barrier protection. Perinatal transmission may occur during childbirth, though it is preventable through antibiotic treatment of the mother before birth and application of antibiotic eye gel on the eyes of the newborn. Gonococcal infections do not result in protective immunity; therefore, individuals may be infected multiple times. Reinfection is possible due to N. gonorrhoeae's ability to evade the immune system by varying its surface proteins.
Asymptomatic infection is common in both males and females. Untreated infection may spread to the rest of the body (disseminated gonorrhea infection), especially the joints (septic arthritis). Untreated infection in women may cause pelvic inflammatory disease and possible infertility due to the resulting scarring. Gonorrhoea is diagnosed through cultures, Gram staining, or nucleic acid tests (i.e. polymerase chain reaction) of urine samples, urethral swabs, or cervical swabs. Chlamydia co-testing and testing for other STIs is recommended due to high rates of co-infection.
Antibiotic resistance in N. gonorrhoeae is a growing public health concern, especially given its propensity to develop resistance easily. This ability of N. gonorrhoeae to rapidly adapt to novel antimicrobial treatments has been seen several times since the 1930s, making numerous treatment plans obsolete. Some strains have exhibited resistance to the current ceftriaxone treatments.
Microbiology
Neisseria species are fastidious, Gram-negative cocci (though some species are rod-shaped and occur in pairs or short chains) that require nutrient supplementation to grow in laboratory cultures. They are facultative intracellular pathogens, meaning they can persist and colonize within host cells but can also multiply outside the host cellular environment. They typically appear in pairs (diplococci), resembling the shape of coffee beans. Members of this genus do not form endospores and are nonmotile, except for pathogenic species, which are capable of moving using twitching motility; most are also obligate aerobes. Of the 17 species that colonize humans, only two are pathogenic: N. gonorrhoeae, which causes gonorrhea, and N. meningitidis, a leading cause of bacterial meningitis.
Culture and identification
N. gonorrhoeae can be isolated on Thayer–Martin agar (or VPN) agar in an atmosphere enriched with 3-7% carbon dioxide. Thayer–Martin agar is a chocolate agar plate (heated blood agar) containing nutrients and antimicrobials (vancomycin, colistin, nystatin, and trimethoprim). This agar preparation facilitates the growth of Neisseria species while inhibiting the growth of contaminating bacteria and fungi. Martin Lewis and New York City agar are other types of selective chocolate agar commonly used for Neisseria growth. N. gonorrhoeae is oxidase positive (possessing cytochrome c oxidase) and catalase positive (able to convert hydrogen peroxide to oxygen). When incubated with the carbohydrates lactose, maltose, sucrose, and glucose, N. gonorrhoeae will oxidize only the glucose.
Metabolism
Carbon Unlike other Neisseria species that can also metabolize maltose, N. gonorrhoeae is capable of using only glucose, pyruvate, and lactate as central carbon sources, and glucose is catabolized via both the Entner-Doudoroff (ED) and pentose phosphate (PP) pathways, and the ED pathway is the primary oxidative method. Use of these pathways is necessary as N. gonorrhoeae is incapable of glucose catabolism via the Embden-Meyerhof-Parnas (EMP) pathway due its lack of the phosphofructokinase (PFK) gene; however, the fructose 1,6-bisphosphatase enzyme is present to allow for gluconeogenesis to occur.
Glucose is first metabolized through the ED pathway to produce pyruvate and glyceraldehyde 3-phosphate, the latter of which can then further metabolized by enzymes of the EMP pathway to yield another molecule of pyruvate. The resultant pyruvate molecules are then converted into acetyl-CoA, which can then be incorporated as a substrate for the citric acid cycle (CAC) to yield high-energy electron carriers that will be used by the electron transport chain (ETC) for ATP production; however, the CAC is largely used for generating biosynthetic precursors rather than for catabolic purposes. This is due in part to inhibited expression of several CAC enzymes in the presence of glucose, pyruvate, or lactate. These enzymes, namely citrate synthase, aconitase, and isocitrate dehydrogenase, are needed for the incorporation of acetate. Instead, a partial CAC has been observed, where α-ketoglutarate is formed by glutamate dehydrogenase or transamination of oxaloacetate and glutamate by aspartate aminotransferase (yielding aspartate and α-ketoglutarate). The CAC then continues from there to yield oxaloacetate, which is an important precursor molecule for several biosynthetic pathways. Another differentiating aspect of the gonococcal CAC is the lack of malate dehydrogenase, which is instead replaced by a membrane-bound malate:quinone-oxidoreductase that operates independently of NAD+ by directly transferring electrons to ubiquinone.
Conversely, acetyl-CoA that does not enter the CAC but enters the phosphotransacetylase-acetate kinase (PTA-AckA) pathway, where it can be converted into acetate by phosphorylation (to form acetyl phosphate and release coenzyme A) and dephosphorylation to form ATP. While this acetate can enter the CAC for further oxidation, this does not occur so long as other carbon sources such as glucose or lactate are present, in which case it is excreted from the cell or incorporated for lipid synthesis. N. gonorrhoeae lack the glyoxylate shunt, preventing them from using acetate to form CAC intermediates to replenish the cycle.
A significant portion of the glyceraldehyde 3-phosphate formed in gonococci is recycled via the gluconeogenic pathway to reform glucose 6-phosphate, as well as the intermediate fructose 6-phosphate. Both of these can then be used for pentose synthesis in the PP pathway via the oxidative and non-oxidative pathways, respectively, for subsequent nucleotide formation as well as energy production.
N. gonorrhoeae, like other pathogenic members of the genus Neisseria, are capnophiles, meaning they require higher-than-normal concentrations of carbon dioxide (CO2) to grow, either in the form of CO2 or bicarbonate (HCO3−) depending on the bacterial strain. This requirement must be met exogenously during the lag and stationary growth phases, though it appears to be met through high metabolic CO2 productions in the exponential phase. Assimilation of this CO2 in Neisseria species is done by carbonic anhydrase and phosphoenolpyruvate enzymes in the periplasmic space and the cytoplasm, respectively.
Lactate catabolism is also of particular importance for gonococci, both for pathogenicity and for growth. External lactate is transported in to the cell via lactate permease (LctP). The N. gonorrhoeae genome encodes for three lactate dehydrogenase (LDH) enzymes for that allow for metabolism of both L-lactate and D-lactate: a cytoplasmic NAD+-dependent D-lactate dehydrogenase (LdhA), which is responsible for and two membrane-bound LDHs, one specific to L-lactate (LldD) and the other specific to D-lactate (LdhD). The membrane-bound LDHs have been determined to be flavoprotein-containing respiratory enzymes that directly oxidize lactate to reduce ubiquinone. While these enzymes do not directly pump protons (H+ ions) into the periplasmic space, it is proposed that the reduction of ubiquinone by these enzymes is capable of feeding into the larger ETC.
Electron transport chain and oxidative phosphorylation As an obligate human pathogen and a facultative anaerobic capnophile, Neisseria gonorrhoeae typically colonizes mucosal surfaces in microaerobic environments, such as those in the genitourinary tract. Growth in areas where oxygen concentrations are limited requires a terminal oxidase with a high affinity for oxygen; in gonococci, oxygen reduction is performed by a ccb3 -type cytochrome oxidase. In addition to aerobic respiration, gonococci can also perform anaerobic respiration via the reduction of nitrite (NO2) to nitric oxide (NO) as well as reduction of NO to nitrous oxide (N2O).
Several enzymes contribute electrons to the intramembranous ubiquinone pool, the first step in the ETC. These include the membrane-bound LDHs (LldD and LdhD), NADH:ubiquinone oxidoreductase (aka NADH dehydrogenase; Nuo complex I), Na+-translocating NADH dehydrogenase (Nqr), succinate dehydrogenase (SDH), and the membrane-bound NAD+-independent malate:quinone-oxidoreductase (MqR).
Following the initial transfer of electrons to ubiquinone, proposed schematics for the organization of the gonococcal ETC suggest the electrons can be further passed down the chain by reduction of the cytochrome bc1 complex or can be directly transferred to NO as a terminal electron acceptor by NO reductase (NorB). In the case of the former, electrons can then be passed from the bc1 complex along two alternative pathways via the reduction of either cytochrome c4 or c5. Both of these cytochromes transfer electrons to the terminal cytochrome ccb3 oxidase for the reduction of O2 to form H2O under aerobic conditions.
Gonococci also reduce NO2 via an inducible outer membrane-attached copper-containing nitrite reductase (AniA, a member of the NirK protein family) under anaerobic conditions, though this process has also been noted in microaerobic conditions as a means of supplementing growth. This leads to the formation of NO that is subsequently reduced to N2O in a partial denitrification pathway. The ccb3 oxidase of N. gonorrhoeae, dissimilarly to other members of the Neisseria genus, is a tri-heme protein that can transfer electrons not only to O2 (conserved across Neisseria species) but also to AniA for NO2 reduction. This is in addition to the typical process of receiving electrons transferred from cytochrome c5.
The general purpose of the ETC is the formation of the electrochemical gradient of hydrogen ions (H+ or protons), resulting from concentration differences across the plasma membrane, needed to power ATP production in a process known as oxidative phosphorylation. In gonococci, movement of protons into the periplasmic space is accomplished by the Nuo complex I, the cytochrome bc1 complex, and cytochrome ccb3. Subsequently, ATP synthesis is performed by the F1F0 ATP synthase, a two-part protein complex present in gonococci as well as numerous other species across phylogenetic domains. This complex couples proton translocation back into the cytoplasm along its gradient with mechanical rotation to generate ATP.
Iron