Step 1 · Microbiology

Neisseria & Moraxella

Name: Neisseria & Moraxella
Creator: Veli Bakalov, MD

A killer of young adults, a master of disguise and an ear-infection cousin

Microbiology·Bacteriology·4 min read·9 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Neisseria are gram-negative, oxidase-positive diplococci grown on Thayer-Martin; meningococcus ferments glucose and maltose, gonococcus glucose only.
Meningococcus has a polysaccharide capsule and LOS endotoxin, spreads by droplets in dorms and barracks, and causes meningitis and meningococcemia.
Meningococcemia progresses from petechiae to purpura fulminans, DIC and Waterhouse-Friderichsen bilateral adrenal hemorrhage within hours.
Terminal complement (C5–C9) deficiency or eculizumab predisposes to recurrent or severe Neisseria infection.
Treat meningococcal disease with ceftriaxone or penicillin G; close contacts get rifampin, ciprofloxacin or ceftriaxone (preferred in pregnancy).
MenACWY conjugate vaccine at 11–12 with a booster at 16; serogroup B needs protein-based MenB vaccines because its capsule mimics NCAM.
Gonococcal pili undergo antigenic and phase variation, so reinfection is common and there is no vaccine; gonococci lack a capsule.
Gonorrhea spans urethritis, cervicitis, PID, Fitz-Hugh-Curtis, pharyngitis, proctitis, DGI (tenosynovitis, pustules, polyarthralgia or knee monoarthritis) and ophthalmia neonatorum at days 2–5 (prevented by erythromycin ointment); treat with ceftriaxone plus doxycycline if chlamydia is not excluded.
Moraxella catarrhalis produces β-lactamase and causes otitis media, sinusitis and COPD exacerbations.

Identification

Shared features. Neisseria and Moraxella are gram-negative, oxidase-positive diplococci that pair like kidney beans. Neisseria grow on chocolate agar; Thayer-Martin agar (chocolate base with vancomycin, colistin, nystatin and trimethoprim) suppresses normal flora so they can be recovered from throat, genital or rectal specimens.

Three gram-negative diplococci

Feature	N. meningitidis	N. gonorrhoeae	M. catarrhalis
Sugars fermented	Glucose and maltose	Glucose only	Neither
Capsule and vaccine	Polysaccharide capsule; MenACWY and MenB vaccines	No capsule; no vaccine	No vaccine
Spread	Respiratory droplets	Sexual contact; vaginal delivery	Upper-airway colonizer
Main diseases	Meningitis, meningococcemia	Urethritis, cervicitis, PID, DGI, ophthalmia neonatorum	Otitis media, sinusitis, COPD exacerbations
Treatment	Ceftriaxone or penicillin G	Ceftriaxone ± doxycycline	Amoxicillin-clavulanate, cephalosporin or macrolide

Meningococcal disease

Carriage and spread. N. meningitidis colonizes the nasopharynx of healthy carriers and spreads by respiratory droplets, so crowding in college dormitories and military barracks favors outbreaks. Serogroups B, C and Y cause most US disease; serogroup A historically drove epidemics across the sub-Saharan meningitis belt.

Virulence. The polysaccharide capsule resists phagocytosis. Lipooligosaccharide (LOS), shed in large amounts in outer-membrane blebs, is the endotoxin behind the cytokine storm, and IgA protease and pili aid colonization.

Clinical syndromes. Meningitis brings fever, headache and neck stiffness. Meningococcemia starts with fever and petechiae that can coalesce into purpura fulminans with DIC, skin necrosis and limb loss. Bilateral adrenal hemorrhage (Waterhouse-Friderichsen syndrome) causes acute adrenal insufficiency and refractory shock. Patients deteriorate within hours, so antibiotics must not wait for imaging or lumbar puncture.

Terminal complement deficiency (C5–C9) and C5 inhibitors such as eculizumab (used for PNH and atypical HUS) remove the membrane attack complex that lyses Neisseria, causing recurrent or overwhelming meningococcal disease and disseminated gonococcal infection. Screen with CH50, and vaccinate before starting eculizumab.

Treating and preventing meningococcal disease

Treat the patientceftriaxone immediately; penicillin G once susceptibility is known; droplet precautions for the first 24 hours of therapy
Protect close contactshousehold and daycare contacts and anyone exposed to oral secretions receive rifampin, ciprofloxacin or single-dose IM ceftriaxone, the choice in pregnancy
VaccinateMenACWY conjugate at 11–12 years with a booster at 16; protein-based MenB vaccines for high-risk groups and outbreaks

High-risk groups. Asplenia, complement deficiency or complement-inhibitor therapy, HIV, military recruits, travelers to the meningitis belt, microbiologists and first-year students in residence halls. The serogroup B capsule is polysialic acid resembling human neural cell adhesion molecule, so it is poorly immunogenic; MenB vaccines target surface proteins such as factor H-binding protein.

Gonococcal disease

Immune evasion. Pili attach to urethral and cervical epithelium and undergo antigenic variation (pilin gene recombination) and phase variation (switching on and off), as do Opa proteins. With no capsule and an ever-changing surface, reinfection is common and no vaccine exists.

Men: purulent urethritis with dysuria.
Women: often silent cervicitis that ascends to PID (salpingitis, tubo-ovarian abscess, infertility, ectopic pregnancy) or Fitz-Hugh-Curtis perihepatitis with RUQ pain and violin-string adhesions.
Other mucosa: pharyngitis and proctitis.
Disseminated infection: tenosynovitis, a few painless extremity pustules and migratory polyarthralgia, or a purulent monoarthritis, usually of the knee; a classic cause of septic arthritis in sexually active young adults.
Neonates: hyperpurulent ophthalmia neonatorum 2–5 days after vaginal delivery, which can perforate the cornea.

Diagnosis and treatment. NAAT of urine or site-specific swabs is the test of choice; in symptomatic men, gram-negative diplococci inside neutrophils on a urethral Gram stain are diagnostic, and culture allows susceptibility testing. Uncomplicated infection is treated with one IM dose of ceftriaxone, adding 7 days of doxycycline when chlamydia has not been excluded; partners are treated and patients retested at about 3 months. Resistance has made fluoroquinolones obsolete.

Erythromycin 0.5% ophthalmic ointment at birth prevents gonococcal ophthalmia; established disease needs systemic ceftriaxone. Chemical irritation from prophylactic agents appears on the first day, whereas chlamydial conjunctivitis appears later (days 5–14) and is not reliably prevented by the ointment.

Moraxella catarrhalis

Airway pathogen. This gram-negative, oxidase-positive diplococcus colonizes the upper airway. It is the third common cause of otitis media and sinusitis in children, after S. pneumoniae and nontypeable H. influenzae, and triggers COPD exacerbations in adults. Nearly all strains produce β-lactamase, so amoxicillin alone fails; amoxicillin-clavulanate, a cephalosporin or a macrolide is used. On agar, its colonies slide intact across the plate like a hockey puck.