Identification and characterization of a nontypeable Haemophilus influenzae putative toxin-antitoxin locus

Dayle A Daines,Arnold L Smith,Justin Jarisch

doi:10.1186/1471-2180-4-30

Abstract

BackgroundCertain strains of an obligate parasite of the human upper respiratory tract, nontypeable Haemophilus influenzae (NTHi), can cause invasive diseases such as septicemia and meningitis, as well as chronic mucosal infections such as otitis media. To do this, the organism must invade and survive within both epithelial and endothelial cells. We have identified a facilitator of NTHi survival inside human cells, virulence-associated protein D (vapDHi, encoded by gene HI0450). Both vapDHi and a flanking gene, HI0451, exhibit the genetic and physical characteristics of a toxin/antitoxin (TA) locus, with VapDHi serving as the toxin moiety and HI0451 as the antitoxin. We propose the name VapXHi for the HI0451 antitoxin protein. Originally identified on plasmids, TA loci have been found on the chromosomes of a number of bacterial pathogens, and have been implicated in the control of translation during stressful conditions. Translation arrest would enhance survival within human cells and facilitate persistent or chronic mucosal infections.ResultsIsogenic mutants in vapDHi were attenuated for survival inside human respiratory epithelial cells (NCI-H292) and human brain microvascular endothelial cells (HBMEC), the in vitro models of mucosal infection and the blood-brain barrier, respectively. Transcomplementation with a vapDHi allele restored wild-type NTHi survival within both cell lines. A PCR survey of 59 H. influenzae strains isolated from various anatomical sites determined the presence of a vapDHiallele in 100% of strains. Two isoforms of the gene were identified in this population; one that was 91 residues in length, and another that was truncated to 45 amino acids due to an in-frame deletion. The truncated allele failed to transcomplement the NTHi vapDHi survival defect in HBMEC. Subunits of full-length VapDHi homodimerized, but subunits of the truncated protein did not. However, truncated protein subunits did interact with full-length subunits, and this interaction resulted in a dominant-negative phenotype. Although Escherichia coli does not contain a homologue of either vapDHi or vapXHi, overexpression of the VapDHi toxin in trans resulted in E. coli cell growth arrest. This arrest could be rescued by providing the VapXHi antitoxin on a compatible plasmid.ConclusionWe conclude that vapDHi and vapXHi may constitute a H. influenzae TA locus that functions to enhance NTHi survival within human epithelial and endothelial cells.

Highlights

Certain strains of an obligate parasite of the human upper respiratory tract, nontypeable Haemophilus influenzae (NTHi), can cause invasive diseases such as septicemia and meningitis, as well as chronic mucosal infections such as otitis media
Mutation of vapDHi results in attenuated survival in human endothelial cells When the Rd vapDHi mutant strain was used to invade the in vitro model of the blood-brain barrier, human brain microvascular endothelial cells (HBMEC) in 12-well plates, the amount of gentamicin-resistant bacteria recovered from the monolayer after three hours declined to approximately 60% of wild type levels, an average of 2.2 × 103 CFU/ml for the wild-type strain Rd versus 1.3 × 103 CFU/ml for the Rd vapDHi mutant (n = 3; P < 0.05, Student's t test)
No significant difference was observed between the wild type and vapDHi mutant in adherence to the human cell monolayers: the average number of cellassociated bacteria recovered for strain Rd was 1.5 × 105 CFU/ml and 1.8 × 105 CFU/ml for the Rd vapDHi mutant (n = 3; P > 0.05)

Summary

Introduction

Certain strains of an obligate parasite of the human upper respiratory tract, nontypeable Haemophilus influenzae (NTHi), can cause invasive diseases such as septicemia and meningitis, as well as chronic mucosal infections such as otitis media. We have identified a facilitator of NTHi survival inside human cells, virulence-associated protein D (vapDHi, encoded by gene HI0450). Both vapDHi and a flanking gene, HI0451, exhibit the genetic and physical characteristics of a toxin/antitoxin (TA) locus, with VapDHi serving as the toxin moiety and HI0451 as the antitoxin. H. influenzae adheres to and penetrates into and between cultured human respiratory epithelial cells, a mechanism that may contribute to its persistence in chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF) patients [1,2]. We have found in a recent in vivo study that H. influenzae can persist in a human bronchiolar xenograft for at least three weeks [5] This suggests that the organism can survive and persist in protected biological compartment(s)

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