Effect of Different Tolerable Levels of Constitutive Expression on Escherichia coli.

To study the effect of different tolerable levels of constitutive expression on Escherichia coli, and to provide direct evidence for moderate resistance mediated by , construction of E. coli strains carrying on the chromosome with promoters of different strengths was conducted using λ-red recombination. Our results demonstrated that over-high expression of cannot be tolerated, and seven constructs with more than 200-fold transcriptional expression differences were obtained. The colistin MICs of the seven strains increased with the increase of MCR-1 levels, and the highest MIC was 8 μg/mL. Lower expression of didn't demonstrate many effects on bacteria, while higher tolerable expression of tended to show fitness costs in growth rate, competitive ability, and cell structures, but no obvious change of virulence was observed in mice. Bacteria demonstrated colistin MICs of 4-8 μg/mL at expression levels similar to clinical isolates, which were the expression levels with relatively lower fitness costs. The effects of relatively lower tolerable levels of were not evaluated thoroughly, and direct evidence for moderate resistance mediated by was lacking. In the present study, we made constructs carrying on the E. coli K12 chromosome under the control of serial constitutive promoters of different strengths and studied the effects of different tolerable levels of expression and . The results demonstrated that generally, except QH0007 (the construct with the highest expression that showed some extent of cell death), the fitness costs of tolerable expression on bacteria were not apparent or low. Bacteria demonstrated colistin MICs of 4-8 μg/mL at expression levels similar to clinical isolates, which corresponded to the lower levels of expression that can lead to colistin resistance, indicating the cleverness of bacteria to balance the benefit and cost of MCR-1-mediated colistin resistance.