Correlation between the development of phage resistance and the original antibiotic resistance of host bacteria under the co-exposure of antibiotic and bacteriophage

Bacteriophages (phages) are viruses capable of regulating the proliferation of antibiotic resistant bacteria (ARB). However, phages that directly cause host lethality may quickly select for phage resistant bacteria, and the co-evolutionary trade-offs under varying environmental conditions, including the presence of antibiotics, remains unclear as to their impact on phage and antibiotic resistance. Here, we report the emergence of phage resistance in three distinct E. coli strains with varying resistance to β-lactam antibiotics, treated with different ampicillin (AMP) concentrations. Hosts exhibiting stronger antibiotic resistance demonstrated a higher propensity to develop and maintain stable phage resistance. When exposed to polyvalent phage KNT-1, the growth of AMP-sensitive E. coli K12 was nearly suppressed within 18 h, while the exponential growth of AMP-resistant E. coli TEM and super-resistant E. coli NDM-1 was delayed by 12 h and 8 h, respectively. The mutation frequency and mutated colony count of E. coli NDM-1 were almost unaffected by co-existing AMP, whereas for E. coli TEM and K12, these metrics significantly decreased with increasing AMP concentration from 8 to 50 μg/mL, becoming unquantifiable at 100 μg/mL. Furthermore, the fitness costs of phage resistance mutation and its impact on initial antibiotic resistance in bacteria were further examined, through analyzing AMP susceptibility, biofilm formation and EPS secretion of the isolated phage resistant mutants. The results indicated that acquiring phage resistance could decrease antibiotic resistance, particularly for hosts lacking strong antibiotic resistance. The ability of mutants to form biofilm contributes to antibiotic resistance, but the correlation is not entirely positive, while the secretion of extracellular polymeric substance (EPS), especially the protein content, plays a crucial role in protecting the bacteria from both antibiotic and phage exposure. This study explores phage resistance development in hosts with different antibiotic resistance and helps to understand the limitations and possible solutions of phage-based technologies.