Static biofilm formations. (A) Static biofilm formation of SPI2 and rpoS deletion mutants. Following incubation at 25°C for 48 h in CFA medium, the level of biofilm formation is expressed as a percentage of WT SL1344 which had an A590 nm of 0.15 ± 0.02 in this experiment. The ΔssrA (JH3180) and ΔrpoS (JH3142) mutants showed significantly less attachment to polystrene than WT SL1344 after 24 h (data not shown) and 48 h of growth (* P = 0.001-0.02). Complementation of ΔssrA with a low copy plasmid encoding ssrAB (JH3181) restored the ability of this mutant to form WT biofilm after 48 h (* P = 0.002). The mean absorbance values from four wells are shown as a percentage of WT SL1344 and the error bars represent the SD between 4 technical replicates (n = 2). (B) Static biofilm formation of SL1344 over-expressing SsrAB (p1437-1) in CFA medium (+ 0.1% L-arabinose where indicated) for 24 h at 25°C. The level of biofilm formation is expressed as a percentage of WT SL1344, which had an A590 nm of 0.14 ± 0.01 in this experiment. The induction of SsrAB expression by an arabinose-inducible promoter significantly inhibited attachment (* P = 0.000007) when compared to WT SL1344. No significant difference in attachment was observed in the control strain over-expressing SsrAB in the reverse orientation (p1437-6). The mean absorbance values from six wells are shown and the error bars represent the SD between 4 technical replicates.