What sexually transmitted disease (STD) is caused by a gram-negative diplococcus?

Neisseria gonorrhoeae is the STD Caused by a Gram-Negative Diplococcus

Neisseria gonorrhoeae, the causative agent of gonorrhea, is the sexually transmitted disease characterized by gram-negative diplococci. 1, 2

Microbiological Characteristics

• Morphology: N. gonorrhoeae appears as gram-negative, coffee-bean shaped diplococci 3
• Biochemical properties: Oxidase-positive and catalase-positive 3
• Microscopic identification: In symptomatic males, the presence of intracellular gram-negative diplococci in urethral specimens has >95% sensitivity and >99% specificity 1, 4
• Growth requirements: Requires specialized media (Thayer-Martin) for culture 5

Diagnostic Methods

Gram Stain

• Highly reliable in symptomatic males: Visualization of polymorphonuclear leukocytes with intracellular gram-negative diplococci in urethral specimens is considered diagnostic 1, 4
• Limited utility in other scenarios: Not recommended for endocervical, pharyngeal, or rectal specimens due to lower sensitivity 1
• Not reliable in asymptomatic patients: A negative gram stain does not rule out infection 4

Nucleic Acid Amplification Tests (NAATs)

• Gold standard: Highest sensitivity (>95%) and specificity (>99%) 4
• Specimen versatility: FDA-cleared for endocervical swabs, vaginal swabs, male urethral swabs, and urine specimens 1, 4

Culture

• Specific indications: Recommended for:
  • Nongenital sites (rectum, pharynx)
  • Cases with legal implications
  • Antimicrobial susceptibility testing
  • Pediatric cases 4

Clinical Presentation

• Males: Typically symptomatic with urethritis 2
• Females: Up to 50% of infections can be asymptomatic 2, 6
• Infection sites: Can affect urethra, cervix, pharynx, rectum, and conjunctiva 1, 6
• Complications: Can lead to pelvic inflammatory disease, tubal scarring, infertility, and ectopic pregnancy in women 1

Antimicrobial Resistance

• Widespread resistance: N. gonorrhoeae has developed resistance to penicillin, tetracycline, fluoroquinolones, and increasing resistance to macrolides 3, 5, 7
• Current treatment challenges: Extended-spectrum cephalosporins remain the last reliable treatment option, though resistance is emerging 2, 7
• Multi-drug resistance: Studies show high rates of multi-drug resistant strains (80% in some populations) 5

Common Pitfalls in Diagnosis

• Relying on gram stain for non-urethral specimens: Low sensitivity in females and extragenital sites 4
• Failing to test for co-infections: Chlamydia co-infection is common and requires simultaneous testing 4
• Inadequate specimen collection: Not collecting appropriate specimens based on sexual practices can miss extragenital infections 4
• Cross-reactivity in pharyngeal specimens: Other Neisseria species can lead to false positives 4

Clinical Implications

The accurate identification of N. gonorrhoeae is critical due to:

• High rates of asymptomatic infection, particularly in women
• Potential for serious sequelae including PID and infertility
• Increasing antimicrobial resistance patterns
• Need for appropriate partner notification and treatment

Understanding that gonorrhea is caused by a gram-negative diplococcus is fundamental for laboratory diagnosis, especially in settings where microscopy may be the primary diagnostic tool available.