Hospitals worldwide are becoming increasingly plagued by antibiotic-resistant pathogens; concomitantly, the number of patients who die from antibiotic-resistant pathogens is increasing. The rise of multi-drug resistant (MDR) pathogens has rendered many antibiotics obsolete. The streptogramin and aminoglycoside antibiotics are drugs of last resort against life-threatening, MDR Gram-positive (e.g., methicillin resistant Staphylococcus aureus (MRSA)) and -negative (e.g., Pseudomonas aeruginosa) pathogens, respectively; however, as microbial drug resistance continues to emerge, the efficacy of these two important drug classes is decreasing. One of the most common mechanisms by which pathogens become resistant to streptogramin and aminoglycoside antibiotics is enzymatic inactivation: the Vat and AAC(3) acetyltransferases are employed by pathogens to inactivate streptogramin and aminoglycoside antibiotics, respectively. There currently is a dire need to not only develop new antibiotics, but to find new, creative strategies to outwit microbial resistance mechanisms. One of these strategies is to rescue the activity of antibiotics through the discovery of antibiotic adjuvants. In the current study, adjuvants which rescue the activity of streptogramin and aminoglycoside antibiotics through inhibition of the resistance acetyltransferases, VatD and AAC(3)-Ia, have been discovered—through the development of a cell-based screening method, we have found the first inhibitors of VatD, as well as of AAC(3)-Ia and its homologues, AAC(3)-Ib, AAC(3)-Ic, and AAC(3)-Id. We have demonstrated that streptogramin and aminoglycoside resistance can be reversed both in vitro and in vivo by the protein kinase inhibitors, GW5074 and rottlerin. Steady state kinetics revealed these compounds to inhibit VatD and AAC(3) enzymes mainly through noncompetitive or mixed mechanisms. This study has also demonstrated that eukaryotic kinase inhibitor libraries may be repurposed for the discovery of not only new antibiotic adjuvants, but also new antimicrobial targets. The inhibitors described herein may someday serve as effective adjuvants of streptogramin and aminoglycoside antibiotics. / Thesis / Doctor of Philosophy (PhD)