FIG. 4.

agr activation and inhibition by culture supernatants. (A and B) One-tenth volume of a post-exponential-phase culture supernatant (sup) or CY broth was added to mid-exponential-phase cultures, growth was continued for an additional hour with shaking at 37°C, and samples were then assayed for β-lactamase by the nitrocefin method (11) as adapted for the microplate reader (4). The group I supernatant was from RN6734, the group II supernatant was from RN6607, the group III supernatant was from RN8465, and the group IV supernatant was from RN4850. Note that β-lactamase activity in samples treated with group III supernatant (cross-hatched bars) represents the endogenous β-lactamase activity of the group III strain and does not represent activation of group IV. (C) Northern hybridization analysis of the effects of the RN4850 (group IV) supernatant on agr activation in RN8465 (group III). One-tenth volume of a post-exponential-phase RN4850 supernatant (+) or broth (−) was added to a mid-exponential-phase culture of RN8465, and hourly samples were taken for Northern hybridization analysis using an RNAIII-specific probe.