FIG 4.

Antibiotics inhibit motility and induce the repulsion response. (A) Swarming assays in which satellite positions are spotted with 0.5 mg/ml gentamicin (Gent) and wild-type, ΔrhlAB, ΔpqsA, and ΔrhlAB ΔpqsA strains of P. aeruginosa. The dashed lines indicate the boundaries of the initial spots. (B) Repulsion radii of wild-type and mutant P. aeruginosa strains that were spotted with 0.5 mg/ml gentamicin (Gent), 40 mg/ml fosfomycin (Fos), or 25 mg/ml kanamycin (Kan). (C and D) Swarming assay (C) and corresponding repulsion radii (D) in which the initial culture of the ΔrhlAB strain was spotted with fosfomycin (concentrations in mg/ml). White dots indicate the centers of positions where antibiotics, cells, or the combination of antibiotics and cells was spotted. Bars in panels B and D indicate averages from at least 6 or 2 independent experiments (n = 6 or 2), respectively. Error bars indicate standard deviations. The swarming assays for the kanamycin and fosfomycin treatments depicted in panel B are shown in Fig. S3A and B in the supplemental material. Overnight cultures were mixed with antibiotics to the indicated concentrations, and 6-μl aliquots of the mixtures were spotted at satellite positions on antibiotic-free swarming media.