Neisseria

Classification

General Characteristics

• Many Neisseria species are normal inhabitants of the mucosal surfaces of many animals.
• Of the 10+ species that infect humans, only 2 are strictly human pathogens: N. gonorrhoeae (gonococcus) and N. meningitidis (meningococcus).
• Gram-negative diplococci (paired cocci)
• Facultative intracellular organisms
• Non-motile
• Oxidase and catalase positive
• Sugar fermentation:
  • N. meningitidis uses maltose and glucose.
  • N. gonorrhoeae uses glucose only.
• Contain lipooligosaccharides (LOS) with strong endotoxin activity
• Require special culture media to grow:
  • N. meningitidis is less fastidious and grows on sheep agar.
  • N. gonorrhoeae requires modified Thayer-Martin agar (MTM) for optimal growth: enriched chocolate agar with antimicrobial agents that selectively favor growth of Neisseria by inhibiting growth of other bacteria and fungi.

Neisseria Gonorrhoeae

Epidemiology

• Worldwide distribution
• Incidence of 583,405 cases in the United States in 2018, with a 5% increase from the year before and a higher rate in males than females (212.8:145.8 per 100,000 people)
• A nationally notifiable disease

Transmission

• Sexual contact with the penis, vagina, mouth, or anus of an infected individual
• Perinatal transmission (during vaginal delivery)

Pathogenesis

• Exclusively pathogenic for humans

General virulence factors: 

• Pili (fimbriae): 
  • Enhance attachment to host cells
  • Furnish resistance to phagocytosis
  • Provide antigenic variation among different strains 
• Por proteins in cell membrane: 
  • Components of the proteins of the outer membrane vesicles (OMVs)
  • May prevent intracellular killing by neutrophils by interfering with phagosome-lysosome fusion
• Opacity-associated (Opa) proteins (outer membrane proteins): aid in attachment to host cell receptors
• Lipooligosaccharide (LOS, endotoxin) envelope protein:
  • LOS is an oligosaccharide with a lipid A component. 
  • Responsible for most of the toxicity in gonococcal infections (e.g., causes fever, ciliary loss, and mucosal cell death in the fallopian tube)
  • Not to be confused with the capsular polysaccharide of meningococci
• IgA1 protease: inactivates IgA1 immunoglobulin and reduces host’s defense

Mechanisms to evade immune recognition:

• LOS mimics human cell membrane glycosphingolipids and can be sialylated, which renders them resistant to serum killing.
• Can rapidly switch from one antigenic form of LOS, pilin, and Opa to another

Mechanisms of antimicrobial resistance:

• Plasmids code for:
  • Penicillinase (similar to Haemophilus spp. and other gram-negative bacteria)
  • Resistance to tetracycline (from the conjugative streptococcal plasmid)
• Most gonococci have plasmids, but meningococci rarely have them.

Clinical presentation

• The incubation period ranges from 1–14 days (usually < 7).
• The clinical presentation can vary greatly depending on the age and immunologic state of the host, as well as the mode of transmission. 
• Some patients can present asymptomatically.
  • More common in young women
  • Higher risk of spreading the infection and developing long-term complications

Diagnosis

• Specimens used: 
  • Pus/secretions from various sites 
  • First-catch urine, without pre-cleansing, for genitourinary tract lesions
  • Blood and joint fluid for culture (only 30% sensitivity) needs a special transport medium.
  • Smears: Sensitivity and specificity vary among sites.
    • > 90% in urethral exudates from men
    • 50% sensitivity for endocervical exudates 
    • < 10% for joint fluid
    • Gram stain is not helpful for throat or rectum samples because non-pathogenic Neisseria often colonize these sites. 
• Culture on modified Thayer-Martin medium:
  • Not routinely used because nucleic acid amplification test (NAAT) is much faster and reliable
  • Used to determine antibiotic resistance if patient failed initial therapy
  • Identification by:
    • Positive oxidase test, only glucose is oxidized
    • Immunologic (coagulation) testing
    • MALDI-TOF (matrix-assisted laser desorption ionization-time of flight) mass spectrometry for same-day identification

Neisseria Meningitidis

Epidemiology

• Meningococcal meningitis occurs in 2 settings:
  • Epidemic waves, associated with crowding
    • Military bases/encampments
    • School dormitories
    • Religious pilgrims
    • Sub-Saharan Africa (the “meningitis belt,” from Senegal to Ethiopia)
  • Sporadic “interepidemic” cases
    • Older infants, children, and adolescents have the highest incidence in developed countries.
• In the United States, the annual incidence is approximately 1/100,000.

Transmission

• Transmission occurs via respiratory droplets and oral secretions, and requires close contact.
• Factors that increase the risk of infection: 
  • Complement component deficiencies
  • Anatomic or functional asplenia
  • HIV positivity
  • Clinical microbiologists have a higher risk of becoming infected
  • Children exposed to secondhand smoke

Pathogenesis

Virulence factors:

• Capsular polysaccharides are antigenic and form 6 important serogroups, responsible for most invasive meningococcal disease (IMD): 
  • A, B, C, X, Y, W (formerly W-135)
  • Serogroup A causes most meningitis in sub-Saharan Africa.
  • Serogroup B causes most sporadic meningitis.
• Other virulence factors are similar to those of N. gonorrhoeae.
  • Lipooligosaccharide (LOS, endotoxin) envelope protein
    • LOS mimics human cell membrane glycosphingolipids.
    • Can be sialylated→ resistant to serum killing
    • Lipid A disaccharide (endotoxin) component causes much of the toxicity.
  • IgA protease: inactivates IgA1 immunoglobulin and reduces host’s defense
  • Can rapidly switch antigenic forms of LOS, pilin, Opa

Pathogenesis:

• The nasopharynx is the portal of entry, attaching to mucosal cells with the aid of pili.
• Organisms reach the bloodstream → IMD, which has a rapid course and manifests as sepsis (meningococcemia) and meningitis.
• Bacteria may also reach the meninges from nearby infected structures or through a congenital or acquired defect in the skull or spine. 
• Much of the neurologic injury is caused by the immune response to the endotoxin and other bacterial components rather than direct bacteria-induced tissue injury.
  • Inflammatory cytokines and chemokines → intense acute inflammatory response, vasogenic edema, and purulent exudate (visible on the brain surface)
  • Interstitial and cytotoxic edema, vasculitis, and thrombosis → ischemia, infarction, and cerebral herniation

Clinical presentations

Identification

• Specimens used:
  • Blood: culture (50%–60% sensitive) and real-time polymerase chain reaction (RT-PCR) if available (96% sensitivity and 100% specificity)
  • CSF: smear and culture and RT-PCR
  • Petechiae: smear or biopsy for culture and RT-PCR
  • Nasopharyngeal swab: for culture and RT-PCR: for carrier surveys but not for stand-alone diagnosis as many healthy carriers are in population
• Lumbar puncture: if no contraindications (elevated ICP, coagulation disorders, thrombocytopenia, uncorrected shock)
• Culture: N. meningitidis is less fastidious than N. gonorrhoeae but special transport media should still be used, as well as culture on modified Thayer-Martin medium.

Prevention

Prevention is done by quadrivalent meningococcal polysaccharide conjugate vaccine:

• Contains A, C, Y, and W 
• Recommended for individuals 11–18 years, with a booster dose 3 years later
• A single dose only if > 16 years of age
• Vaccination at 2 months and older for at-risk groups of people
• Efficacy: 82% first year after vaccination, waning to 59% at 3–6 years