Division-arrest induced filamentation protects uropathogenic Escherichia coli from killing by the cathelicidin antimicrobial peptide LL-37

Abstract

Antimicrobial peptides are a key component of the innate immune system, serving both as signaling and immunomodulatory molecules and also as direct antimicrobials toward numerous bacterial species. One of the key antimicrobial peptides in humans is LL-37 but the mechanism of bacterial killing remains poorly characterized. Using live-cell imaging, we observe that LL-37 preferentially targets actively dividing E. coli cells. Permeabilization occurs during late-stage division at the bacterial septum with both of the newly dividing cells being killed at the same time. Death occurs downstream of the recruitment and dissembly of FtsZ and FtsN and concomitantly with AmiB association at the divisome, consistent with killing occurring during late division. Model membrane tubes that topologically mimic the late-stage cytoplasmic membrane intermediates during division also undergo rapid size- and lipid composition-dependent fission, which likely reflect the ability of LL-37 to disrupt membranes. Intriguingly, we show that inducing bacterial filamentation results in resistance to LL-37 mediated killing. Resistance to LL-37 appears to be a general feature of bacterial filamentation as overexpression of queE, sulA or damX all resulted in reduced killing by LL-37. As filamentation is often observed during UPEC infection, we hypothesize that filamentation associated resistance to host innate immunity may be a key feature of UPEC pathogenesis that protects bacteria during extrusion from infection bladder epithelial cells, when exposure to soluble and cellular host defences is likely highest. This observation may provide an important avenue for therapeutic intervention. SignificanceAntimicrobial peptides are a key component of innate immunity. The human cathelicidin peptide, LL-37, is membrane active but the mechanism of sensitization in living systems is poorly understood. We show that dividing E. coli cells are highly susceptible to LL-37 mediated disruption. Killing occurs downstream of the assembly and activation of functional divisomes. In vitro membrane nanotubes also demonstrate sensitivity to LL-37 in a lipid composition and diameter-dependent manner. Cells with stalled division are protected from LL-37. Urinary tract infections caused by uropathogenic E. coli (UPEC) are a major cause of morbidity globally. UPEC form filamentous bacteria late in the human infection cycle. UPEC filaments are protected from LL-37 killing and suggest that interfering with bacterial filamentation might potentiate innate immune control of UTIs.