title: Use of bexB to detect the capsule locus in Haemophilus influenzae.
abstract: Haemophilus influenzae strains are classified as typeable or nontypeable H. influenzae (NTHI) based upon the presence or absence of capsule. In addition to serotyping, which is subject to false-positive results, typeable strains can be identified through the detection of the capsular export gene bexA and one of six capsule-specific genes, but this method is resource intensive, especially in characterizing large numbers of strains. To address these challenges, we developed a bexB-based method to differentiate true NTHI strains from typeable strains. We validated a PCR-based method to detect bexB in 10 strains whose capsule status was well defined. Among 40 strains that were previously serotype positive in clinical microbiology laboratories, 5 lacked bexA, bexB, and capsule type-specific genes by PCR analysis and thus likely represent false-positive serotyping results. Among 94 additional otitis media, commensal, and serotype b-negative invasive strains, 85 were bexA and bexB negative and 9 contained either a complete or partial capsule locus, i.e., 8 were bexA and bexB positive and 1 was bexA negative but bexB positive. Finally, we adapted the method for use in a high-throughput DNA hybridization-based microarray method, which showed 98.75 and 97.5% concordance to the PCR methods for bexA and bexB, respectively. In addition, bexB showed 84% or greater nucleotide identity among strains containing the capsule locus. In this study, we demonstrate that bexB is a reliable proxy for the capsule locus and that its detection provides a simple and reliable method for differentiating strains that lack the entire capsule locus from those containing a partial or complete capsule locus.
authors.affiliation: Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor, Michigan, USA.
authors.forename: Gregg S
authors.initials: GS
authors.lastname: Davis
authors.forename: Sara A
authors.initials: SA
authors.lastname: Sandstedt
authors.forename: Mayuri
authors.initials: M
authors.lastname: Patel
authors.forename: Carl F
authors.initials: CF
authors.lastname: Marrs
authors.forename: Janet R
authors.initials: JR
authors.lastname: Gilsdorf
mesh_terms: D016667:Bacterial Capsules / Q000235:genetics*; D001426:Bacterial Proteins / Q000235:genetics*; D015373:Bacterial Typing Techniques / Q000379:methods*; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D006193:Haemophilus influenzae / Q000145:classification* / Q000235:genetics*; D006801:Humans; D026901:Membrane Transport Proteins / Q000235:genetics*; D046228:Microarray Analysis; D008969:Molecular Sequence Data; D009693:Nucleic Acid Hybridization; D016133:Polymerase Chain Reaction / Q000379:methods*; D017422:Sequence Analysis, DNA
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D026901:Membrane Transport Proteins; C068102:bexB protein, Haemophilus influenzae