Identification of the factor responsible for C-5691 (Δ pnp ) aggregative phenotype. A. Cell aggregation in C-1a (pnp+), C-5691 (Δpnp) and C-5691 derivatives carrying mutations in genes encoding for adhesion determinants (ΔpgaC, C-5937; ΔbcsA, C-5929; ΔcsgA, C-5931; ΔwcaD, C-5935). Cell aggregates were stained with crystal violet for better visualization. B. Surface adhesion of the same set of strains to polystyrene microtiter plates. The adhesion unit values, assessed as previously described , are the average of three independent experiments and standard deviation is shown. The overall p-value obtained by ANOVA was p = 5.11x10-12. Letters provide the representation for posthoc comparisons. According to posthoc analysis (Tukey’s HSD, p < 0.05), means sharing the same letter are not significantly different from each other. C. Phenotype on Congo red-supplemented agar plates. D. Phase contrast micrographs (1,000 x magnification) of pnp+ (C-1a), Δpnp (C-5691), ΔpgaC (C-5936), and Δpnp ΔpgaC (C-5937) strains grown overnight in M9Glu/sup medium at 37°C. The images were acquired with a digital CCD Leica DFC camera.