Neisseria gonorrhoeae: Disease, Pathogenesis, Lab Diagnosis

Neisseria is the only pathogenic gram-negative cocci (precisely diplococci). Two species of Neisseria cause disease in humans; Neisseria meningitidis and Neisseria gonorrhoeae.

Human beings are only known hosts of N.gonorrhoeae. Neisseria gonorrhoeae (often called gonococcus) causes gonorrhoea, the second most common sexually transmitted disease (STDs) of worldwide importance (Chlamydial infections are more common). It causes disease only in humans.

Neisser described the causative agent of gonorrhea, N. gonorrhoeae, in 1879.

Morphology

1. Gram-negative cocci, usually seen in pairs (diplococci) with the adjacent sides flattened (typically kidney-shaped); (Looks like two coffee beans kissing).
2. In urethral discharge, it is predominately found within the polymorphs.

• Sensitive to dehydration and cold conditions.
• Gonococci do not contain capsules (meningococci do).

Mode of transmission

1. Neonates acquired Neisseria gonorrhoeae from the mother during passage through the birth canal. In newborn infants, Neisseria gonorrhoeae causes ophthalmia neonatorum (purulent conjunctivitis).
2. Sexual transmission: Acquired during unprotected sex with infected partner.

Virulence factors

1. Pili: Pili mediate attachment to mucosal cell surfaces and also are antiphagocytic. Piliated gonococci are usually virulent, whereas nonpiliated strains are avirulent.
2. Lipooligosaccharides (LOS)
3. Outer membrane proteins and Opa proteins eg. Porin A: mediates invasion of the epithelial cells.
4. IgA protease hydrolyzes secretory IgA, which could otherwise block the attachment of Neisseria gonorrheae to the mucosa.

Host defense against Neisseria gonorrhoeae

1. Antibodies (Mainly IgA and IgG)
2. Complement
3. Neutrophils

If you want to get detailed information about the virulence factors of Neisseria gonorrhoeae and how they play a role in infection, transmission, and disease pathogenesis, you can read my other blog post virulence factors of Neisseria gonorrhoeae here

Disease caused by Neisseria gonorrhoeae

Gonococci cause localized infections, usually in the genital tract, and disseminated infections.

1. Gonorrhoea in men is characterized primarily by urethritis accompanied by dysuria and a purulent discharge. Epididymitis can occur.
2. In women, infection is located primarily in the endocervix, causing a purulent vaginal discharge and intermenustrual bleeding (cervicitis). The most frequent complication in women is an ascending infection of the uterine tubes (salphingitis, pelvic inflammatory disease (PID)), which can result in sterility or ectopic pregnancy as a result of scarring of tissues.
3. In newly born children: Ophthalmia neonatorum (An eye infection which may develop within 2/3 days of vaginal delivery, affects cornea and can cause blindness)

Disseminated gonococcal infections occurs via the blood stream. Gonococcal strains causing disseminated infections are usually resistant to serum and complement. Disseminated infections commonly manifest as septic arthritis, tenosynovitis, pustules in the skin, endocarditis, and meningitis.

Laboratory diagnosis of Gonorrhoea

Sample

urethral/cervical/vaginal discharge

• To obtain a urethral specimen swab ( cotton or rayon swab) is inserted approximately 2cm in urethra and rotated gently before withdrawing.
• If there is profuse urethral discharge in male, it can be collected without inserting the swab.
• A few drops of first voided urine can be used in males, but the sensitivity is low compared to discharge.

Transport

Swabs collected for isolation of gonococci may be transported to the laboratory in modified Staurt’s or Amie’s charcoal transport media and held at room temperature until inoculated to culture media. Good recovery of gonococci is possible if swabs are cultured within 12 hours of collection.

Gram Staining 

For men, a gram-stained smear of urethral discharge (exudate) showing intracellular Gram-negative diplococci is diagnostic. But as women may carry normal vaginal flora such as Veillonella or occasional gram-negative coccobacilli , may resemble gonococci, same results in case of women may not be diagnostic so culture and identification process is needed, which is a confirmatory test.

Culture 

Modified Thayer Martin Medium {Chocolate agar containing antibiotics (vancomycin, colistin, trimethoprim, and nystatin)} is most often used.  Modified Newyork City Medium (MNC)  is also used for the culture of Neisseria gonorrhoeae. MNC also supports the growth of Mycoplasma.

Biochemical tests for Neisseria gonorrhoeae identification

1. Oxidase Test: Positive
2. Ferments glucose but not maltose, sucrose or lactose
3. DNase Test: Negative
4. Beta-galactosidase (ONPG) Test: Negative
5. Glutamyl-aminopeptidase (GAP) Test: Negative

Serology

Serological tests to determine the presence of antibody to gonococci are not useful for diagnosis.

Molecular Diagnosis

Presence of gonococcal nucleic acids in patient specimens, using amplification or non-amplification method. These tests are highly sensitive and specific.

Drug Resistance

Antimicrobial resistance in Neisseria gonorrhoeae has been categorized by the CDC as an “urgent threat” .

Currently, the CDC recommends an intramuscular shot of an extended-spectrum cephalosporin (ESC), ceftriaxone, plus one oral dose of azithromycin as an empirical treatment for uncomplicated or pharyngeal gonococcal infections.

TIMELINE OF RESISTANCE:

1. 1935s: Antimicrobial treatment of gonorrhea started in 1935 after discover of sulfanilamide. Neisseria gonorrhoeae developed widespread resistance to sulfanilamide within the first ten years resulting in treatment failures.
2. 1940s: Penicillin is used as a treatment of choice for gonorrhea. Penicillin remained effective for next few decades despite a continuous and insidious increase in MICs.
3. 1976: End of era of penicillin with discovery and worldwide spread of Neisseria gonorrhoeae harboring plasmid-mediated beta-lactamases.
4. 2010s: Alarming decrease in cefixime susceptibility reported from around the world.
5. 2018s: Combination of azithromycin and ceftriaxone – has failed to treat the disease (report from BBC)

After the start of the standard double regimen for the treatment of N.gonorrhoeae infection, Azithromycin resistance among N. gonorrhoeae is on the rise. Reduced susceptibility to ceftriaxone and cefixime has also been reported. Till date, all patients are successfully treated as azithromycin is rarely used as a monotherapy for gonorrhea, and most azithromycin-resistant isolates are still susceptible to ceftriaxone. However, as the organism continues to develop resistance to ceftriaxone, we might end up with isolates highly resistant to both agents in the near future.

Further Resources:

1. Ng, L. K., & Martin, I. E. (2005). The laboratory diagnosis of Neisseria gonorrhoeae. The Canadian journal of infectious diseases & medical microbiology = Journal canadien des maladies infectieuses et de la microbiologie medicale, 16(1), 15–25. https://doi.org/10.1155/2005/323082
2. Gonococcal Infections: CDC
3. When the clap hits back: Antimicrobial Resistance Threats in Neisseria gonorrhoeae: ASM