Fig. 2: Changes in biofilm growth behavior in silver-adapted A. baumannii.
A, B Comparative analysis of chromosomal mRNA transcript levels of cell-to-surface adherence associated genes (bap, bfmR) and EPS synthesis genes (pgaABCD) in WT, NAgR and AgT, upon exposure to low silver concentrations (LN = ‘low’ nanosilver; 0.5 µg Ag/mL [0.5 × MIC], LA = ‘low’ ionic silver; 1 µg Ag/mL [0.5 × MIC] and high silver concentrations (HN = ‘high’ nanosilver; 3 µg Ag/mL [3 × MIC], HA = ‘high’ ionic silver; 3 µg Ag/mL [3 × MIC]). The differentially expressed genes (log2-fold change cut-off threshold ≥0.58 (≥1.5-fold change), p-adjusted ≤0.01, grey cells indicate statistically insignificant p-adj >0.01 changes) were captured at 30 min of silver exposures in the exponential growth phase. Also shown are the physiological gene expression changes when in the absence of silver. C Fluorescent microscopy images of biofilms formed by WT and the silver-adapted strains when exposed to silver (0.5 × MIC of NAg and/or Ag+). Also shown are the respective cell-only (untreated) control samples. Cyan color showed surface-attached bacterial colonies, while red color showed EPS mass, scale bar = 10 µm. D Quantitative analysis revealed total colony (µm3/µm2) and EPS mass (µm3/µm2) of each strain under each respective silver exposures. Each data point represents calculated biomass surface coverage from individual images, with the horizontal bar showing the mean data from 15 to 20 images, per strain, per treatment. Statistical analysis (unpaired t-test with Welch’s correction) showed statistically significant differences in bacterial colony and EPS abundance between strains and treatments (***p < 0.001, ****p < 0.0001). The mRNA and phenotypic work were performed with a minimum of three biological replicates (single isolated colonies).
