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Evidence for capsule gene sequences among pharyngeal isolates of nontypeable Haemophilus influenzae
J W St Geme1, A Takala, E Esko
1Edward Mallinckrodt Department of Pediatrics (Division of Infectious Diseases), Washington University School of Medicine, St. Louis, MO 63110.
Abstract:
Haemophilus influenzae is a common commensal organism of the human respiratory tract and is an important cause of localized and systemic disease. While isolates recovered from the respiratory tract are generally nonencapsulated (serologically nontypeable), isolates from systemic sites typically express a polysaccharide capsule. To explore the possibility that nontypeable strains evolved from encapsulated organisms, a series of serologically nontypeable isolates were examined for the presence of capsule gene sequences. Pharyngeal isolates (123) were collected from healthy 3-year-old Finnish children and examined by Southern hybridization with pUO38, a plasmid that contains one complete set of cap genes from an H. influenzae type b strain. Twenty-four isolates (20%) demonstrated homology with capsule-specific sequences. Of these 24, 18 in addition to 14 others had evidence of one or more copies of IS1016, an insertion element that has been associated with encapsulation in H. influenzae. These results support the hypothesis that nontypeable strains of H. influenzae arose from an encapsulated ancestor. Possibly the selective pressure driving the loss of encapsulation relates to the disadvantage associated with encapsulation during respiratory tract colonization.
Insights
Nontypeable Haemophilus influenzae strains may have evolved from encapsulated ancestors. Genetic analysis of respiratory isolates suggests capsule gene loss, possibly due to disadvantages during colonization.
Area of Science:
- Microbiology
- Genetics
- Epidemiology
Background:
- Haemophilus influenzae is a common bacterium in the human respiratory tract.
- Encapsulated strains typically cause systemic disease, while nonencapsulated strains colonize the respiratory tract.
Purpose of the Study:
- To investigate the evolutionary origins of nontypeable H. influenzae.
- To determine if nontypeable strains possess genetic remnants of capsule genes.
Main Methods:
- Collected 123 pharyngeal isolates from healthy Finnish children.
- Used Southern hybridization with a capsule gene probe (pUO38).
- Screened for the insertion element IS1016.
Main Results:
- 20% of isolates showed homology to capsule gene sequences.
- 18 of these isolates also had IS1016.
- An additional 14 isolates possessed IS1016, suggesting a link to encapsulation.
Conclusions:
- Nontypeable H. influenzae strains likely evolved from encapsulated ancestors.
- Loss of encapsulation may be advantageous for respiratory tract colonization.