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<p>The 20^th century introduction of antibiotics made once fatal infectious diseases readily treatable. This taken-for-granted therapy is now threatened by rising antibiotic resistance. The ability of pathogens to acquire numerous simultaneous resistance mechanisms has given rise to an alarming number of increasingly difficult to treat multi-drug resistant infections. When coupled with a sharp decline in development of novel antibiotic therapies, health practitioners today are left with limited therapeutic options. Several alternative methodologies have been employed to find novel therapeutics, including new techniques in natural product isolation and the production of semi-synthetic and synthetic antibiotics; however, there has been limited focus on targeting antibiotic resistance mechanisms directly to create synergistic therapies. We demonstrate the potential in using small molecules to target antibiotic kinases, thereby rescuing the antibiotic action of aminoglycosides and macrolides when used in combination. We conducted a thorough examination of these enzymes including: kinetic analysis; an assessment of phosphate donor specificity; and in-depth structural comparison, including a case study on the structure-function relationship of APH(4)-Ia. This analysis culminated in an intensive screening initiative of fourteen antibiotic kinases against a set of well defined protein kinase inhibitors. From this work, we have identified several inhibitors that have the potential for use in future combination therapeutics. This study illustrates the benefit of a structure-activity based approach to drug discovery, an important tool at a time when novel therapeutic strategies are required.</p> / Doctor of Philosophy (PhD)

Enterococcus faecalis are Gram-positive bacteria that colonize the gastrointestinal tracts of mammals, birds and invertebrates and are also found in sewage, soil, food and water. In addition to being commensal organisms, Enterococci can also cause nosocomial infections in humans including urinary tract infections, septicemia and endocarditis. Hospital-acquired infections often present a challenge in treatment due to the emergence of multi-drug resistant strains. Enterococcal plasmids may act as extremely stable reservoirs for resistance genes and other virulence factors. Pheromone responsive plasmids such as pCF10 mediate efficient transfer of genetic material within the species E. faecalis but may also be capable of transferring resistance genes across species and genus boundaries. Polymicrobial environments often found in nosocomial infections may expose plasmid-harboring enterococci to pathogenic species, poising cells for this type of promiscuous horizontal gene transfer of resistance determinants. Previous studies showed that prgW, which encodes the pCF10 replication initiation protein PrgW, is the minimal origin of replication for this plasmid. The replicon, which is usually limited to Enterococcal spp., can replicate in Lactococcus lactis if it is engineered to produce pre-cCF10. Three conserved cysteines (C78/C275/C307) are important for plasmid stability and allow for replication of the pCF10 replicon in L. lactis in the absence of pre-cCF10. PrgW has a predicted molecular weight of 38,635. Four polyclonal antibodies targeting PrgW at the N-terminus (aa 1-20), C-terminus (aa 314-333) and two internal regions (aa 64-80 and aa 250-271) were used in current experiments and retrospective studies. When PrgW was overexpressed in E. faecalis, four different apparent approximate molecular weights were detected by Western blotting (p40*, p36*, p24* and p18*), suggestive of processing. In Enterococci where the replicon is active, p36* was consistently detected by all four antisera; when PrgW was overexpressed in Streptococcus mutans where the replicon is non-functional, p49* and p40* were detected but p36* was not observed. PrgW p24* was detected by a mixture of the internally targeting antibodies as well as the C-terminal targeting antibody, but not the N-terminal targeting antibody, suggesting that the N-terminal domain of PrgW has been cleaved off in p24*. The p24* form may play a role in pCF10 stability. Mutations to three cysteines in PrgW (C78/C275/C307), which reduce the stability of pCF10, result in the loss of p24*. Enterococcal conjugative plasmids have been previously implicated in the transfer of antibiotic resistance genes. The pCF10 plasmid contains the conjugative transposon Tn925, which possesses the tetM tetracycline resistance gene. Proximity of donor and recipient cells is a key part of pheromone-responsive conjugation. Aggregation substance allows for formation of clumps of E. faecalis in liquid mating experiments. E. faecalis forms biofilms; in contrast to filter mating experiments, polymicrobial biofilms provide an in vitro model of a natural scenario during which horizontal gene transfer may occur. Rates of cross-genus genetic transfer of tetM between E. faecalis OG1RF(pCF10) donor cells and Staphylococcus aureus recipient cells growing on glass coverslips as mixed-species biofilm populations were determined to be 10-8 after pheromone induction of pCF10 conjugation. This biofilm transfer model also holds potential to test the efficacy of synthetic peptides in the reduction or even prevention of pCF10 transfer, and the consequential dissemination of antibiotic resistance determinants throughout the genus Enterococcus and beyond. / Microbiology and Immunology