Abstract:

In these days that we are drifting into the post-antibiotic era, antibiotics called "last-resort" are begun to be used more frequently. Colistin is one of the last-resort antibiotics that act on Gram-negative bacteria. The aim of the study was to investigate antibiotic re-sensitization effect of lytic bacteriophages on colistin resistant E. coli O157:H7 in-vitro. In the study, four E. coli O157:H7 isolates (encoded 25KA, 44RA, 120RA and 168KA) were included. These isolates had different features such as harboring some of the mcr genes but not showing resistance to colistin, or demonstrating resistance to colistin without carrying any mcr genes. A lytic bacteriophage cocktail was prepared with three Myoviridae family member phages. In order to determine the effect of lytic bacteriophage application on the colistin resistance of E. coli O157:H7 strains before, during and after bacteriophage treatment, minimum inhibitory concentrations (MIC) of the isolates were determined by broth microdilution method. The results were interpreted according to EUCAST. According to the results, up to 3.6 log cfu/ml reductions in colistin resistant E. coli O157:H7 were detected within 6h incubation at 23°C. Colistin and phage combination showed synergistic effect. While strains 25KA and 168KA became susceptible to colistin, 44GA and 120RA were totally eliminated. The survivors of the phage treatment were also become sensitive to colistin. Phage-resistant mutants of 25KA and 168KA showed susceptibility to colistin (1 µg/ml and 0.5 µg/ml, respectively). In addition, 44GA and 120GA remained susceptible. The findings of this study highlight that in addition to taking advantage of the lytic activity of phages in biocontrol area, phages also play a major role in re-sensitization to a last-resort antibiotic like colistin. The results show the synergy between phage–antibiotic combination treatment and give the promising idea that this approach has the potential to extend the effective lifetime of antibiotics.

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