Identification and characterization of a nontypeable Haemophilus influenzae putative toxin-antitoxin locus
1 Nonproliferation, Arms Control, and International Security Directorate, Lawrence Livermore National Laboratory, L-501, 7000 East Avenue, Livermore CA 94550-9698, USA
2 Bacterial Pathogenesis Program, Seattle Biomedical Research Institute, 307 Westlake Avenue North, Suite 500, Seattle, WA 98109-5219, USA
BMC Microbiology 2004, 4:30 doi:10.1186/1471-2180-4-30Published: 26 July 2004
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. 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,
Isogenic 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.
We conclude that vapDHi and vapXHi may constitute a H. influenzae TA locus that functions to enhance NTHi survival within human epithelial and endothelial cells.