Nov 4: Mechanisms and evolution of colistin resistance in gram-negative bacteria
PhD research by Axel Janssen at UMC Utrecht has revealed new insights into the mechanisms and evolution of antimicrobial resistance, in particular of resistance of Gram-negative bacteria against colistin. There was a great variety in mutations that can lead to colistin resistance, but such mutations often occur in the same genes. These observations may be helpful to identify new proteins that can serve as targets for new antimicrobials.
Infections by multidrug‑resistant Gram‑negative (Gr-) bacteria are increasingly common, prompting a renewed interest in the use of the antimicrobial agent colistin. Colistin specifically targets Gr- bacteria by interacting with lipopolysaccharides, leading to membrane destabilization and cell death. Colistin plays a vital role as a last‑resort drug in the treatment of multidrug‑resistant Gr- bacteria. Considering the lack of development of novel antibiotics active against such bacteria, antimicrobial resistance against colistin is a reason for concern.
Colistin resistance
There is a gap in our understanding of the evolution and mechanisms of colistin resistance in Gr- bacteria. Therefore, Axel Janssen (Department of Medical Microbiology, UMC Utrecht) aimed in his PhD research to elucidate the mechanisms and evolution of colistin resistance such bacteria. Through the use of clinically relevant strains, he investigate the evolution of, and mechanisms through which colistin resistance may occur. In addition, he studied the ability of these bacteria to spread amongst patients, and the impact of colistin resistance on fitness and virulence characteristics.
Key findings
- Axel Janssen and colleagues investigated the prevalence and mechanisms of colistin resistance in Escherichia strains isolated from bloodstream infections. They showed the low prevalence of colistin resistance, and mcr carriage, in the E. coli strains isolated from bloodstream infections. He showed the modification of lipid A with phosphoethanolamine in the nosocomial strains as a mechanism to diminish electrostatic charges on lipid A. Through a combination of whole genome sequencing (WGS), comparative genomics, and construction of transgene insertion mutants they showed that mutations in the genes encoding the BasRS two‑component regulatory system contributed to colistin resistance in E. coli strains.
- They also investigated the mechanisms of colistin resistance in Gr- bacteria isolated from ICU patients who received colistin‑containing SDD treatment. By collecting colistin‑resistant strains through rectal swabs, the investigators showed transient gut colonization. They showed the mutations through which these bacteria can develop colistin resistance in vivo and observed that there was no clonal relationship between the colistin‑resistant strains, outside of those isolated from the same patient. In addition, they showed in‑patient microevolution of colistin resistance by analyzing a group of colistin‑resistant E. coli strains isolated from the same patient.
- The investigators used a collection of isogenic colistin‑susceptible, and colistin‑resistant ESBL‑producing K. pneumoniae strains isolated from ICU patients receiving SDD treatment. They showed the development of colistin resistance in these strains, through chromosomal mutations, and confirmed the role of these mutations in colistin resistance through in trans complementation experiments and susceptibility determinations.
- The investigators performed an in vitro evolution experiment by passaging cultures of K. pneumoniae complex strains in increasing concentrations of colistin. They demonstrated the evolutionary pathways through which K. pneumoniae strains can develop resistance to colistin. Through population sequencing they observed the swift and selective fixation of mutations located in genes associated with LPS modification and assembly, and capsule synthesis within the evolving populations. They showed the absence of a general impact on fitness in these K. pneumoniae strains upon development of colistin resistance.
PhD defense
Axel Janssen (1991, Nijmegen) defended his PhD thesis on November 4, 2020 at Utrecht University. The title of his thesis was “Mechanisms and evolution of colistin resistance in clinical Enterobacteriaceae”. Supervisors were prof. dr. Rob Willems and prof. dr. ir. Willem van Schaik (Department of Medical Microbiology, UMC Utrecht).