The study of antibiotic resistance mechanisms is crucial in order to combat the serious issue of antibiotic-resistant bacterial infections. One way that bacteria are able to evade antibiotic stress is through the efflux of antibiotics by efflux pumps, which are proteinaceous transporters able to expel drugs and toxins from the bacterial cell. Serial passaging methods to isolate mutant bacteria resistant to trans-translation inhibitor KKL-73 yielded two strains of C. crescentus with decreased susceptibility to several structurally dissimilar compounds. Whole-genome sequencing analysis of the strains uncovered several mutations present in CCNA_03832, a putative TetR-family transcriptional regulator. This novel gene was found to be located in an operon coding for a MDR efflux system that also contains a putative MDR efflux pump and a major facilitator superfamily transporter. Results from inactivation of CCNA_03832 in wild-type C. crescentus reveal an increase in resistance to several classes of antibiotics, suggesting that the transcription factor is a negative regulator of the efflux system operon and that mutation of the transcription factor resulted in overexpression of the efflux system. Techniques to inactivate the genes coding for the MDR efflux pump and major facilitator superfamily transporter were carried out in order to create knockout strains, which will be used for further characterization of the efflux system. The study of this novel three-gene MDR efflux system may reveal clinically relevant information about the potential for drug resistance in Burkholderia and Pseudomonas aeruginosa and may allow for development of efflux pump inhibitors (EPIs).
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