Experimental explorations of general patterns of epistatic interactions in clinical Escherichia coli isolates

Abstract

Resistance towards antimicrobial agents is an increasing medical problem nowadays caused by the elimination of susceptible bacteria leaving only the resistant ones to survive and evolve. Evolution through mutations that lead to antibiotic resistance in bacteria increases the diversity of bacterial genomes. Under antibiotic treatment, these mutations might be advantageous for the bacteria, but disadvantage in the absence of antibiotic due to fitness cost. By understanding, how mutations can have an impact on the fitness cost for individual resistance determinants and the interactions that occur within one genetic background can foster our understanding of epistasis. The importance of understanding epistatic interactions between genetic determinants that are responsible for a resistant phenotype can be essential in the treatment of bacterial infections with antimicrobial drugs. In this study, mutants of clinical Escherichia coli strains with reduced susceptibility towards trimethoprim and/or ciprofloxacin were generated to test whether epistatic interactions in certain combinations of mutants existed. Growth curve measurements were used to calculate the relative generation time as a measure of fitness. The result revealed that the generation time of double mutant TP+CIP22 was reduced with 7 minutes compared to the single mutant TP22, resulting in a potentially positive epistatic interaction. However, a negative epistatic interaction may have occurred due to a decrease in minimal inhibitory concentration (MIC) determined for trimethoprim in all double mutants (TP+CIP) compared to the respective single mutants (TP). The results provided here promote our understanding of epistatic interactions in bacteria. Hopefully, together with findings of further experiments, they will be implemented in innovative guidelines for antibiotic treatment.