Supplementary Material for: Biofilm Formation Restrained by Subinhibitory Concentrations of Tigecyclin in Acinetobacter baumannii Is Associated with Downregulation of Efflux Pumps

<i>Background:</i> Tigecycline, one of the few therapeutic options against multidrug-resistant <i>Acinetobacter baumannii,</i> reaches subinhibitory serum concentrations only with cautious clinical dosing and pharmacokinetics. Subinhibitory concentrations of tigecycline might induce an <i>A. baumannii</i> biofilm. <i>Methods:</i> Biofilm formation was assessed via the crystal violet staining method. We further analyzed the main biofilm components with NaIO4, proteinase K, and DNase. Real-time RT-PCR was applied for quantitative detection of biofilm potential-associated genes. <i>Results:</i> In this study, <i>A. baumannii </i>proved to be a strong biofilm producer, and we found that proteins and extracellular DNA are crucial components of the <i>A. baumannii </i>biofilm. Quantitative real-time RT-PCR revealed positive correlations between biofilm formation restrained by subinhibitory concentrations of tigecycline and the expression of biofilm potential-associated genes, especially the <i>AdeFGH</i> efflux pump gene. <i>Conclusion:</i> Our results suggest that downregulation of efflux pumps, especially the <i>AdeFGH</i> efflux pump, is probably responsible for the decline in biofilm formation in <i>A. baumannii</i> treated with subinhibitory concentrations of tigecyclin.