Auto aggregation assay was performed according to Del Re et al. Overnight cultures were centrifuged at 2000 × g for 10 min at 4 ◦C. The pellets thus obtained were washed twice in sterile phosphate-buffered saline (PBS, pH 7·2) and resuspended in PBS to a final OD600 nm of 0.5. The suspension (4 ml) was mixed by gentle vortexing for 10 s. Absorbance was measured immediately and after 1 h of incubation. The percentage of auto aggregation was expressed as: 1 − (At/A0) × 100, where At represents the absorbance at 1 h and A0 the absorbance at 0 h. All experiments were performed in triplicates.