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Background Fluoroquinolones are utilized in Staphylococcal prosthetic joint infections due to their anti-biofilm activity. When antibiotic dosing is not optimized or antibiotics do not reach the site of infection, additional virulence factors may upregulate. We aimed to determine whether exposure to sub-MIC concentrations of levofloxacin and delafloxacin affect biofilm formation in Staphylococcus aureus. Methods This study utilized 50 diverse methicillin-susceptible S. aureus (MSSA) clinical isolates collected between 2004 and 2018. Sources included blood, skin/tissue, bone, and joint fluid. Minimum inhibitory concentrations and minimum bactericidal concentrations were identified according to CLSI. Biofilm assays were conducted as previously described by our program. Biofilm quantification was categorized as strong (OD570 ≥ 2), moderate (OD570 ≥ 1 and &lt; 2), or weak (OD570 &lt; 1). Prevention assays were conducted with the addition of increasing concentrations of delafloxacin or levofloxacin. We evaluated the amount of isolates that demonstrated increased biofilm formation in the presence of sub-MIC concentrations and extent of biofilm enhancement. Percent change was calculated between OD570 of the isolate growth control without antibiotic exposure and peak biofilm OD570 when exposed to the antibiotic. Results Of the 50 MSSA isolates, 14 (28%) exhibited moderate/strong formation and 36 (32%) exhibited weak biofilm formation. 52% and 58% of the isolates demonstrated a ≥50% increase in formation when exposed to sub-MIC concentrations of delafloxacin and levofloxacin, respectively. None of the strong biofilm formers demonstrated a ≥50% peak increase in formation when exposed to the antibiotics. Of the isolates that demonstrated a ≥50% peak increase, the average percent change was 267% (±29) with levofloxacin and 258% (±33) with delafloxacin. Conclusion Sub-MIC concentrations of delafloxacin and levofloxacin increased biofilm formation in S. aureus isolates that normally exhibit weak or moderate biofilm formation when not in the presence of antibiotics. Maintaining appropriate fluoroquinolone concentrations at the site of action is critical in preventing enhancement of biofilm formation. Further research is needed to identify the mechanism behind this increase. Disclosures All authors: No reported disclosures.

620. Sub-MIC Concentrations of Levofloxacin and Delafloxacin Enhance Staphylococcus aureus Biofilm Formation: Significance of Maximizing Exposure