Many chronic and acute diseases of the respiratory tract are either caused or exacerbated by infections with non-typeable Haemophilus influenzae (NTHi), a WHO priority pathogen requiring immediate further action due to increasing disease severity and antibiotic resistance. NTHi thrives at sites of infection, and here we have investigated the molecular mechanisms underlying the remarkable resistance of H. influenzae to host-produced bactericidal molecules such as hypochlorite.
To overcome cell envelope stress caused by hypochlorite exposure, all H. influenzae strains contain two to three highly conserved, periplasmic S-oxide reductases that are expressed in response to hypochlorite exposure and form a first line of defense against cellular damage. The loss any one of these sulfoxide reductases reduced H. influenzae fitness in in vivo and also in TC culture infection models despite very different roles in protection from S-oxide stress.
The non-traditional, Mo -containing S-oxide reductase MtsZ reduced free methionine-sulfoxide (MetSO) to Met, thus preventing uptake of MetSO and its incorporation into newly synthesized proteins. The reaction of MtsZ is linked to the Hi respiratory chain, allowing this reaction to be used for energy generation and redox balancing.
In contrast, the MsrAB S-oxide reductase was required for Hi hypochlorite resistance, and repairs MetSO residues in HOCl-damaged proteins which includes key H. influenzae OMPs that are highly susceptible to HOCl induced damage.
Unexpectedly, a loss of the MsrAB protein led to specific changes in the host response to infection with NTHi. Increased production of AMPs such as cathelicidin (9x) and BPI (4x) was observed, while expression of two key antiapoptotic proteins, XIAP and BIRC3 was reduced. Thus the conserved extracellular NTHi S-oxide reductases not only prevent and repair HOCl induce damage, but also have previously undocumented links to key virulence relevant processes such as manipulating host responses and successful energy generation.