Background: Colistin is being administered as last-line therapy for patients that have failed to respond to other available antibiotics that are active against Escherichia coli. The underlying mechanisms of colistin resistance and heteroresistance remain largely uncharacterized. The present study investigated the mechanisms of resistance and heteroresistance to colistin in Escherichia coli  isolates from Wenzhou, China.
Materials and Methods: Colistin resistance was detected by the broth microdilution method (BMD). Colistin heteroresistance was determined by population analysis profiles (PAPs). The polymerase chain reaction (PCR) was conducted to detect mcr-1, mcr-2, mcr-3, pmrA, pmrB, phoP, phoQ  and mgrB, and quantitative real-time PCR (qRT-PCR) was used to determine the expression levels of mcr-1, pmrC, pmrA  and pmrB . Lipid A characterization was conducted by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS).
Results: 0.69% (2/291) of Escherichia coli  strains were resistant to colistin, whereas the heteroresistance rate reached 1.37% (4/291). mcr-1 , the mobile colistin-resistance gene, was present in the two resistant isolates. The substitutions in PmrB were detected in the two heteroresistant isolates. The transcripts levels of the pmrCAB  operon were upregulated in two of the heteroresistant isolates. carbonylcyanide m-chlorophenylhydrazone (CCCP) was able to reverse colistin resistance of all isolates tested and exhibited a significantly higher effect on colistin-heteroresistant isolates. MALDI-TOF MS indicated that the additional phosphoethanolamine (PEtn) moieties in lipid A profiles were present both in colistin-resistant and heteroresistant isolates.
Conclusion: The present study was the first to investigate the differential mechanisms between colistin resistance and heteroresistance. The development of colistin heteroresistance should be addressed in future clinical surveillance.
Keywords: Escherichia coli , colistin, mcr-1 , heteroresistance, lipid A