Evolutionary Dynamics Leading to Tigecycline Resistance in Enterococcus faecalis
Shamoo, Yousif; Bartel, Bonnie
Doctor of Philosophy
The rising frequency of multidrug resistant pathogens poses an increasing threat to public health. There is a dire need for novel approaches to combat these deadly pathogens and maintain the efficacy of currently available antibiotics. In particular horizontal gene transfer threatens the therapeutic success of antibiotics by facilitating the rapid dissemination of resistance alleles among bacterial species. The conjugative mobile element Tn916 provides an excellent context for examining the role of adaptive parasexuality as it carries the tetracycline-resistance allele tetM and has been identified in a wide range of pathogens. For this thesis I used quantitative experimental evolution, a pipeline developed by our lab to identify clinically relevant resistance mechanisms, to study tigecycline resistance in a hospital strain of Enterococcus faecalis. Quantitative experimental evolution uses a combination of experimental evolution and allelic frequency measurements to gain insights into the adaptive trajectories leading to resistance and to predict what mechanisms of resistance are most likely to appear in the clinical setting. Here we show that antibiotic selection led to the near fixation of adaptive alleles that simultaneously altered TetM expression and produced remarkably increased levels of Tn916 horizontal gene transfer. In the absence of drug, approximately 1 in 120,000 of the non-adapted Enterococcus faecalis S613 cells had an excised copy of Tn916, whereas nearly 1 in 50 cells had an excised copy of Tn916 upon selection for resistance resulting in a more than 1,000-fold increase in conjugation rates. We also show that tigecycline, a translation inhibitor, selected for a mutation in the ribosomal S10 protein in the E. faecalis adapting populations. Furthermore, we show that mutation of S10 is an important allele for a broad range of Gram-positive and Gram-negative pathogens to adapt to tigecycline. These results show the first example of mutations that concurrently confer resistance to an antibiotic and lead to constitutive conjugal-transfer of the resistance allele. Selection created a highly parasexual phenotype and high frequency of Tn916 jumping demonstrating how the use of antibiotics can lead directly to the proliferation of resistance in, and potentially among, pathogens.