Haemophilus influenzae biogroup aegyptius is a common cause of purulent conjunctivitis. It is also responsible for severe invasive disease with a similar presentation to meningococcal septicaemia, characterised by a rapid disease onset. First identified in the state of São Paulo, Brazil, the disease was referred to as Brazilian purpuric fever (BPF). We aim to investigate how these strains of H. aegyptius became so virulent, and what might separate BPF strains of H. aegyptius from non BPF strains.
H. aegyptius encodes a number of virulence factors with homology to known adhesins in non-typeable Haemophilus influenzae (NTHi). H. aegyptius encodes two high molecular weight (HMW) adhesins, Hae HMW1 and Hae HMW2, which are related to, yet distinct from, the phase variable HMW1/2 adhesins which have previously been shown to play an important role in colonisation by NTHi. We have previously demonstrated that the HMW1 and HMW2 adhesins of NTHi bind to human-specific glycans found throughout the respiratory tract. H. aegyptius also encodes multiple variable trimeric autotransporter (TAA) proteins, named the Tab/Tah proteins. Many TAAs function as adhesins, and a related TAA in NTHi, Hia, is required for host colonisation, and like the HMW proteins, also binds host-specific glycans. We hypothesise that these adhesins of H. aegyptius, Hae HMW1/2 and the TAAs Hae Tab and Hae Tah, bind host specific glycans that are distinct from those bound by homologues in NTHi. We cloned and over-expressed Hae HMW1/2 and Hae Tab/Tah proteins in E. coli BL21, and utilized a glycobioanalytical approach to determine the binding affinities of these proteins. Each adhesin was screened using the Institute for Glycomics glycan array, and specific protein-glycan interactions characterised using surface-plasmon resonance. By characterising the glycan binding profile of these adhesins, we are better able to understand the factors behind the unusual virulence of a subset of H. aegyptius strains that has so far not been elucidated since the first emergence of BPF over thirty years ago.