Genomic profiling reveals distinct routes to complement resistance in Klebsiella pneumoniae.
Short FL., Di Sario G., Reichmann NT., Kleanthous C., Parkhill J., Taylor PW.
The serum complement (C') system is a first line of defense against bacterial invaders. Resistance to killing by serum enhances the capacity of Klebsiella pneumoniae to cause infection, but is an incompletely understood virulence trait. Identifying and characterising the factors responsible for preventing activation of, and killing by, serum C' could inform new approaches to treatment of K. pneumoniae infections. We have used functional genomic profiling to define the genetic basis of C' resistance in four diverse serum-resistant K. pneumoniae strains (NTUH-K2044, B5055, ATCC43816 and RH201207), and explored their recognition by key complement components. Over 90 genes contributed to resistance in one or more strains, but only three, rfaH, lpp and arnD, were common to all four. Deletion of the anti-terminator rfaH, controlling expression of capsule and O-side chains, resulted in dramatic C' resistance reductions in all strains. The murein lipoprotein gene lpp promoted capsule retention through a mechanism dependent on its C-terminal lysine residue; its deletion led to modest reductions in C' resistance. Binding experiments with the C' components C3b and C5b-9 showed that the underlying mechanism of evasion varied in the four strains: B5055 and NTUH-K2044 appeared to bypass recognition by C' entirely, while ATCC43816 and RH201207 were able to resist killing despite being associated with substantial levels of C5b-9. All rfaH and lpp mutants bound C3b and C5b-9 in large quantities. Our findings show that, even amongst this small selection of isolates, K. pneumoniae adopts differing mechanisms and utilises distinct gene sets to avoid C' attack.