Staphylococci as a screening resource and toolkit for antibacterial drug discovery

Antibiotic resistance is a growing threat to modern-day medicine and the problem is compounded by an innovation gap in antibiotic discovery; no new antibiotic classes have been introduced in recent decades. Investigating the secondary metabolites of microorganisms other than the soil-dwelling bacteria traditionally screened for antibiotics represents one potential route to identifying novel antibacterial drug candidates. Staphylococci have been key players in the antimicrobial resistance problem in recent decades. The underpinning concept of this thesis is that these organisms also have the potential to make a useful contribution towards a solution to antimicrobial resistance, both in terms of proving a source of novel antimicrobials and acting as a tool to facilitate the discovery process. In this study, a diverse collection of 2035 staphylococcal isolates were tested for the production of antimicrobial substances (AMS). Of this collection, 74 isolates (~4%) were found to produce an AMS capable of inhibiting the growth of the indicator organism, Staphylococcus aureus SH1000. All AMS producers were tested for antibacterial activity against all of the others, allowing deprioritization of strains likely producing the same AMS. Also, methods of excluding undesirable AMS like bacteriocins applied. Together, these experiments achieved a reduction in the number of AMS suitable for further characterization from 74 to 5. One of the AMS was subsequently purified using bioactivity-guided chromatography and identified through use of nuclear magnetic resonance (NMR) experiments as cyclo(L-Tyr-L-Phe). This is the first report of cyclo(L-Tyr-L-Phe) being produced by a staphylococcus strain and showing anti-staphylococcal activity. However, further investigation of the compound is needed.
The second part of this work focused on addressing a perennial issue in natural product antibiotic discovery: the issue of cross-resistance of novel antibiotics with previously characterized resistance mechanisms. To address this, a cross-resistance panel of staphylococci genetically engineered to display resistance to common antibacterial classes was established and validated. The panel will allow high-throughput cross-resistance detection to guide future antibiotic discovery efforts.