Neisseria gonorrhoeae
Also known as: gonococcus
Neisseria gonorrhoeae is a gram-negative diplococcus that causes gonorrhea, a sexually transmitted infection. It produces urethritis, cervicitis, pelvic inflammatory disease, septic arthritis, and neonatal conjunctivitis.
Neisseria gonorrhoeae, the gonococcus, is a gram-negative diplococcus — paired, kidney-bean-shaped cocci often seen inside neutrophils on Gram stain. It is transmitted sexually or perinatally and infects mucosal surfaces of the urethra, cervix, rectum, pharynx, and conjunctiva.
Clinically, gonococcal infection causes urethritis with purulent discharge in men and cervicitis in women, though many infections — especially in women — are asymptomatic. Untreated infection in women can ascend to cause pelvic inflammatory disease (PID), risking infertility, ectopic pregnancy, and perihepatitis (Fitz-Hugh-Curtis syndrome). Disseminated gonococcal infection produces the triad of polyarthralgia, tenosynovitis, and dermatitis, and N. gonorrhoeae is the most common cause of septic arthritis in sexually active young adults. Newborns exposed during delivery can develop ophthalmia neonatorum, which is why erythromycin eye ointment is given prophylactically at birth.
Several microbiologic features are exam favorites. The organism ferments glucose but not maltose — distinguishing it from Neisseria meningitidis, which ferments both. It has no polysaccharide capsule and no vaccine, and its pilus proteins undergo antigenic variation, which allows reinfection because prior infection confers no lasting immunity. Antibiotic resistance is a growing concern, and co-treatment for chlamydia is standard because coinfection is common.
The USMLE Step 1 exam tests N. gonorrhoeae in its gram-negative cocci microbiology content and again in musculoskeletal pathology as a cause of infectious (septic) arthritis, emphasizing the contrast with N. meningitidis and the antigenic variation of its pili.
Key takeaways
- N. gonorrhoeae is a gram-negative diplococcus causing gonorrhea, often seen within neutrophils on Gram stain.
- It causes urethritis, cervicitis, PID, disseminated infection, and neonatal conjunctivitis.
- It is the most common cause of septic arthritis in sexually active young adults.
- It ferments glucose but not maltose, unlike N. meningitidis, and has no capsule and no vaccine.
- Antigenic variation of pili prevents lasting immunity — a favorite USMLE Step 1 testing point.