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Open Access Research article

There is a specific response to pH by isolates of Haemophilus influenzae and this has a direct influence on biofilm formation

Nadiah Ishak12, Alexandra Tikhomirova12, Stephen J Bent2, Garth D Ehrlich3456, Fen Z Hu3456 and Stephen P Kidd12*

Author Affiliations

1 Research Centre for Infectious Diseases, The University of Adelaide, North Terrace Campus, Adelaide, South Australia 5005, Australia

2 School of Molecular and Biomedical Sciences, The University of Adelaide, North Terrace Campus, Adelaide, South Australia 5005, Australia

3 Center for Genomic Sciences, Institute of Molecular Medicine and Infectious Disease, Philadelphia, PA 1902, USA

4 Center for Advanced Microbial Processing, Institute of Molecular Medicine and Infectious Disease, Philadelphia, PA 1902, USA

5 Department of Microbiology and Immunology, Drexel University College of Medicine, Philadelphia, PA 19102, USA

6 Department of Otolaryngology-Head and Neck Surgery, Drexel University College of Medicine, Philadelphia, PA 19102, USA

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BMC Microbiology 2014, 14:47  doi:10.1186/1471-2180-14-47

Published: 21 February 2014

Abstract

Background

Haemophilus influenzae colonizes the nasopharynx as a commensal. Strain-specific factors allow some strains to migrate to particular anatomical niches, such as the middle ear, bronchi or blood, and induce disease by surviving within the conditions present at these sites in the body. It is established that H. influenzae colonization and in some cases survival is highly dependent on their ability to form a biofilm. Biofilm formation is a key trait in the development of chronic infection by certain isolates. This is exemplified by the contrast between the biofilm-forming strains found in middle ear infections and those isolates that survive within the blood and are rarely associated with biofilm development.

Results

Screening a group of H. influenzae strains revealed only slight variations in their growth across a range of pH conditions. However, some isolates responded to a pH of 8.0 by the formation of a biofilm. While the type b capsular blood isolate Eagan did not form a biofilm and grew at the same rate regardless of pH 6.8-8.0, transcriptomic analyses demonstrated that at pH 8.0 it uniquely induced a gluconate-uptake and metabolism pathway, which concurrently imports H+. A non-typeable H. influenzae, isolated from the middle ear, induced biofilm formation at pH 8.0, and at this pH it induced a series of iron acquisition genes, consistent with previous studies linking iron homeostasis to biofilm lifestyle.

Conclusions

Different strains of H. influenzae cope with changes in environmental factors using strain-specific mechanisms. These pathways define the scope and mode of niche-survival for an isolate. The pH is a property that is different from the middle ear (at least pH 8.0) compared to other sites that H. influenzae can colonize and infect. The transcriptional response to increasing pH by H. influenzae varies between strains, and pH is linked to pathways that allow strains to either continue free-living growth or induction of a biofilm. We showed that a biofilm-forming isolate induced iron metabolism pathways, whereas a strain that does not form biofilm at increasing pH induced mechanisms for growth and pH homeostasis based on sugar acid transport.

Keywords:
Biofilm; H. influenzae; pH stress; Transcriptomics