Interactions between innate immune effectors and multidrug resistant bacteria

Abstract

Antibiotic resistance is an increasingly difficult problem in the clinic, where conventional antibiotics are failing, and new alternative solutions are in high demand. Infections caused by Gram-negative bacteria with multi drug resistance (MDR) mechanisms are increasing globally, and treatment options are limited. Plasmids encoding β-lactamases spread easily between bacteria, and the overuse of antibiotics select for MDR strains. β-lactamases are either serine-β-lactamases that are inhibited by certain β-lactamase inhibitors, and metallo-β-lactamases (MBLs), which are more difficult to inhibit with drugs. The current approach to fight MDR pathogens has mainly focused on finding β-lactamase inhibitors to use in combination with conventional antibiotics in the clinic.

The overall aim of this thesis was to study the role of the cellular micro-environment and the importance of the innate immune system for antibiotic resistant bacterial infections. In Paper I we hypothesized that human cells secreted factors that could impair β-lactamase function and thus restore antibiotic susceptibility in resistant bacterial isolates. We found that thiols produced by the cells acted as zinc chelators that inhibited the degradation of cephalosporin antibiotics in VIM-1 producing K. pneumoniae. Notably, free thiols in urine samples had the same effect, suggesting that the environment at the site of infection can be highly important for antibiotic susceptibility and possibly also for the effect of antibiotic treatment in a clinical situation. In Paper II, we hypothesized that induction of innate effector molecules would reduce intracellular growth of MDR K. pneumoniae and exert synergistic effects with conventional antibiotics. We tested this by infecting human macrophages with MDR K. pneumoniae. Notably, induction of innate immunity in these cells resulted in improved intracellular killing of MDR K. pneumoniae. The inducers were combined with traditional antibiotics, which resulted in an additive killing effect. The data suggests that inducing innate immune effectors can be an effective alternative or addition to conventional treatments in infections caused by MDR K. pneumoniae. Finally, in Paper III, we tested the hypothesis that ESBL E. coli would be more susceptible to innate effectors compared to non-ESBL isolates. The ESBL producing isolates had lower survival in serum and whole blood than non-ESBL isolates, suggesting a biological cost for resistant isolates. In vivo studies with zebrafish embryos showed that the non-ESBL isolates killed the embryos more efficiently than ESBL isolates. The biological cost was not related to the ESBL plasmid per se as shown by experiments where the ESBL plasmid was transferred from a clinical isolate to a neutral background in non-resistant E. coli.

Together, this thesis has highlighted the importance of considering the micro-environment at the site of infection, which may determine the effect of antibiotics. Next, I have shown that induction of innate immune effectors could be an alternative or additive treatment option for infections caused by MDR K. pneumoniae. Finally, I present data showing that ESBL E. coli are more susceptible to innate effectors than non-ESBL E. coli.