After three weeks, one of the two bottom pots termed inducer side was inoculated with 20 ml of a bacterial suspension in tap water (OD560560= 2,corresponding to 86109 cfu ml21 for strain Sb4-23, 56109 cfu ml21 for Mc2-Re2, 46109 cfu ml21 for Mc5-Re2, 16109 cfu ml21 for E. coli, or 1.261010 cfu ml21 for G12).