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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.

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