GenBank Accession IDs for Haemophilus influenzae Type b Capsids
The GenBank accession IDs for Haemophilus influenzae type b (Hib) capsular polysaccharide loci include M19995, X78559, and AF549211, which represent the complete cap locus sequences for H. influenzae serotype b strains. 1
Understanding H. influenzae Type b Capsule
H. influenzae exists in two main forms:
- Encapsulated (typeable) strains - categorized into six serotypes (a-f)
- Unencapsulated (nontypeable) strains
The type b capsule is a critical virulence factor composed of polyribosylribitol phosphate (PRP) and is encoded by a specific variation of the capsule biosynthesis locus (cap-b). 2, 3
Capsular Locus Organization
The cap locus of H. influenzae type b has a distinctive structure:
- Central DNA segment specific to serotype b
- Flanking DNA segments common across all serotypes
- Contains genes for both capsule synthesis and export 4
A unique feature of the type b capsule locus is:
- Near-total duplication of the cap region (17 kb region organized as direct repeats)
- This duplication is not found in other serotypes
- The duplicated structure appears to confer survival advantages to type b strains 4, 3
Molecular Identification of H. influenzae Type b
The Centers for Disease Control and Prevention (CDC) recommends laboratory confirmation for H. influenzae type b identification, as taxonomy ID alone is insufficient. 2
Key Genes in the Capsular Locus
The complete cap locus sequence reveals:
- Region I: Contains genes for capsule transport (bexA-D)
- Region II: Contains serotype-specific genes for capsule biosynthesis
- Region III: Contains hcsA and hcsB genes involved in post-polymerization modification 1
Modern Identification Methods
Several molecular methods are available for identifying H. influenzae type b:
- PCR targeting capsule-specific genes - considered the gold standard for serotyping 5
- Software tools like "hicap" for in silico serotype prediction from genome sequences 6
- These methods overcome problems of cross-reaction and autoagglutination associated with traditional serotyping 5
Clinical Significance
The accurate identification of H. influenzae type b is critical because:
- Before effective vaccines, 1 in 200 children developed invasive Hib disease by age 5
- 60% of these cases involved meningitis with 3-6% mortality
- 20-30% of meningitis survivors experienced permanent sequelae 7, 2
Vaccination with Hib conjugate vaccines has dramatically reduced type b disease incidence, but accurate identification remains important for:
- Surveillance of vaccine effectiveness
- Identifying vaccine failures
- Monitoring potential emergence of other serotypes 2
Important Considerations
When working with H. influenzae type b capsid sequences:
- Standard taxonomy IDs typically only identify to species level, not serotype level
- Biochemical testing can identify H. influenzae to species level but requires both hemin (X factor) and NAD (V factor) for growth
- Molecular methods are necessary for definitive serotype identification 2
For researchers studying Hib capsids, accessing these GenBank sequences provides valuable reference data for understanding capsule structure, function, and evolution, which remain important despite the success of vaccination programs.