Microbiology of Neisseria Meningitidis

Notes on Classification, Morphology, Colony Characteristics, Biochemical Properties, Serogroups, Virulence factors, Pathogenesis, Clinical Syndrome, Epidemiology, Prophylaxis and Laboratory Diagnosis of Neisseria Meningitidis

Classification

Family: Neisseriaceae
Genus: Neisseria
Species: Neisseria Meningitidis

Morphology

• Gram negative diplococcus (paired)

  

• Size: 0.6-0.8 μm
• Bean shaped
• Non-motile
• Aerobe
• Encapsulated

Colony characterisitics

• Color: Bluish grey
• Shape: Round
• Size: About 1mm
• Surface: Smooth
• Elevation: Convex
• Opacity: Transluscent
• Consistency: Butyrous

Biochemical Properties:

• Oxidase positive
• Catalase positive
• Ferments glucose and maltose with acid production
• Gamma-glutamyl aminopeptidase positive
• Production of Deoxyribonuclease (DNAse)
• Nitrate negative
• Doesn’t ferment lactose, sucrose and fructose
• Colistin resistant

Serogroups

N. Meningitidis has been divided into atleast 13 serogroups on the basis of specificity of capsular polysaccharide antigens. These are A,B,C,D,X,Y,Z,W 135,29E,H,I,K and L. Serogroups A,B,C,X,Y,W 135 are most commonly associated with meningococcal disease.

Virulence Factors

• Adhesion factors : Pili for adhesion to mucosa of oropharynx and meningeal tissue
• Capsule : Antiphagocytic
• Lipopolysaccharide : Endotoxin
• IgA proteases : cleaves IgA protecting organism from effects of secretory IgA

Epidemiology

• Natural habitat and reservoir : Mucosal surfaces of the human nasopharynx, urogenital tract and anal canal
• Nasopharyngeal carriers : approximately 5-10% adults are asymptomatic carriers (may reach 90% in close communities)
• Modes of infection: Direct contact or respiratory droplets from the nose and throat of infected people

Pathogenesis (Steps)

1. Inhalation of contaminated droplets
2. Adherence of organism to nasopharyngeal mucosa
3. Local invasion and spread from nasopharynx to meninges through blood stream
4. In meninges, organsims are internalised into phagocytic cells
5. They replicate and migrate to subepithelial spaces
6. Incubation period : 3-4 days

Clinical Features

• Febrile illness : Mild and self limiting
• Pyogenic meningitis : High fever, stiff neck, Kernig’s sign, severe headache, vomiting, photophobia, chills
• Meningiococcal speticemia : acute fever with chills, malaise, prostation, Waterhouse-frederichsen syndrome, DIC
• Other Syndrome : Pneumonia, arthritis, urethritis, respiratory tract infection

Prophylaxis

a. Chemoprophylaxis :

• Rifampicin
• Minocycline
• Ciprofloxacin

b. Vaccination:

• A vaccine containing capsular polysaccharide of serotypes A and C : for infants below 2 years
• A quadrivalent vaacine constituted by polysaccharides of serotypes A,C,Y and W-135 : for children and adults

Laboratory Diagnosis

a. Specimen:

• CSF
• Blood

b. Examination of CSF:

• Increased Pressure
• Turbid

The collected CSF is divided into 3 portions (for microscopy, for biochemical tests and for culture)

Microscopy:

• Gram stained smear of CSF deposit commonly shows Gram negative intracellular diplococci.
• White cell count increases to several thousand per cubic mm with 90-99% PMNs.

Biochemical tests:

• Glucose is markedly diminished
• CSF protein is markedly raised

c. Culture:

CSF:

• Inoculated into chocloate agar
• Incubated at 37c in 5-10% Carbondioxide and high humidity
• After 24 hours bacterial colonies appear
• The organism is tested for biochemical and agglutination reaction

Blood:

• Blood culture is positive in over 40% cases of meningiococcal meningitis

Other Cultures:

• Nasopharyngeal swab
• Skin lesions
• Joint fluid
• Tracheal aspirate
• Urethral discharge

d. Detection of antigen:

For Detection of Meningiococcal DNA

• Polymerase Chain Reaction (PCR)

For detection of soluble polysaccharide antigen

• Counter current immunoelectrophoresis (CIEP)
• Latex agglutination test

e. Serogrouping