P31 The effects of topical 5-fluorouracil treatment of <i>in situ</i> squamous cell carcinomas on antimicrobial resistance and small colony variant formation in <i>Staphylococcus aureus</i>

Abstract Background Squamous cell carcinoma (SCC) is a prevalent skin malignancy. Its precursor lesion, in-situ squamous cell carcinoma (iSCC), is often treated with topical 5-fluorouracil (5-FU). 5-FU may be linked with antibiotic resistance in Staphylococcus aureus, a common skin commensal with pathogenic potential. However, the effects of 5-FU on antibiotic resistance and pathogenicity in S. aureus remain unclear. Objectives To investigate the effects of Topical 5-FU exposure in iSCC on S. aureus, with a focus on the formation and characteristics of Small Colony Variant (SCVs) and implications for antibiotic resistance, pathogenicity, biofilm formation and infection risk. Methods Antibiotic susceptibility and biofilm formation were tested in S. aureus NCTC 6571 SCVs and normal colony variants (NCVs). A 21 day evolution experiment, followed by biofilm formation and antibiotic sensitivity testing, as well as a protease assay were performed on S. aureus NCTC 6571, F77 and ATCC 25923 isolates. Clinical sampling to collect S. aureus from iSCC lesions was then conducted. Results Minor differences in antibiotic susceptibility were observed among S. aureus NCTC 6571 SCVs and normal colony variants (NCVs). The MIC of linezolid was reduced against SCVs compared to NCVs. Biofilm formation was increased in some SCVs and decreased in the ancestor and NCVs. S. aureus NCTC 6571, ATCC 25923 and F77 biofilms were then exposed to 5-FU for 21 days. Long-term 5-FU exposure led to increased SCVs in S. aureus F77. Biofilm formation was varied, with 5-FU associated reductions in NCTC 6571 and ATCC 25923, while F77 displayed minimal changes. Protease activity in NCTC 6571 was reduced following long-term exposure. 43% of clinical AK isolates displayed S. aureus colonization, but no SCVs. Conclusions Our study contributes to a comprehensive understanding of the effects of 5-FU on S. aureus skin infections, particularly regarding the wider implications for iSCC and SCC treatment and antimicrobial resistance. Isolates from this study require further characterization, including identification of phenotypic characteristics, and antibiotic susceptibility testing. The observed strain-specific responses to 5-FU emphasize the need for continued research involving various strains. The emergence of SCVs may have clinical consequences, particularly contributing to infection persistence, resistance to treatment and increased risk of infection and recurrence in patients with SCC. Further studies are also needed to clarify the long-term implications.