Chromosome-Mediated Colistin Resistance in Clinical Isolates of Klebsiella pneumoniae and Escherichia coli: Mutation Analysis in the Light of Genetic Background

Abstract

Purpose

Colistin resistance mechanisms involving mutations in chromosomal genes associated with LPS modification are not completely understood. Mutations in genes coding for the MgrB regulator frequently account for colistin resistance in Klebsiella pneumoniae, whereas mutations in genes coding for PhoPQ and PmrAB are frequent in E. coli. Our aim was to perform a genetic analysis of chromosomal mutations in colistin-resistant (MIC ≥4 µg/mL) clinical isolates of K. pneumoniae (n = 8) and E. coli (n = 7) of different STs.

Methods

Isolates were obtained in a 3-year period in a university hospital in Santiago, Chile. Susceptibility to colistin, aminoglycosides, cephalosporins, carbapenems and ciprofloxacin was determined through broth microdilution. Whole genome sequencing was performed for all isolates and chromosomal gene sequences were compared with sequences of colistin-susceptible isolates of the same sequence types.

Results

None of the isolates carried mcr genes. Most of the isolates were susceptible to all the antibiotics analyzed. E. coli isolates were ST69, ST127, ST59, ST131 and ST14, and K. pneumoniae isolates were ST454, ST45, ST6293, ST380 and ST25. All the isolates had mutations in chromosomal genes analyzed. K. pneumoniae had mutations mainly in mgrB gene, whereas E. coli had mutations in pmrA, pmrB and pmrE genes. Most of the amino acid changes in LPS-modifying enzymes of colistin-resistant isolates were found in colistin-susceptible isolates of the same and/or different ST. Eleven of them were found only in colistin-resistant isolates.

Conclusion

Colistin resistance mechanisms depend on genetic background, and are due to chromosomal mutations, which implies a lower risk of transmission than plasmid-mediated genes. Colistin resistance is not associated with multidrug-resistance, nor to high-risk sequence types.

Keywords:

• sequence type
• LPS-modifying enzymes
• polymorphism vs potential mutation
• MgrB regulator
• PmrA-PmrB and PhoP-PhoQ three-component systems

Acknowledgments

The authors acknowledge the expert technical assistance of the staff of Laboratory of Microbiology from Pontificia Universidad Católica de Chile for their help in technical aspects of this work.

Disclosure

The authors declare no conflicts of interest in this work.

Additional information

Funding

This work was supported by research funds from SENTRY (Antimicrobial Resistance Surveillance Program), the Red de Salud UC-Christus and the Department of Clinical Laboratories at the School of Medicine of Pontificia Universidad Católica de Chile.