FIG. 1.

Expression of bacteriocin MC4-1 and sibling killing. (A) Inhibition of bacteriocin MC4-1 activity by gelatinase. Central inocula of isogenic strains TX5128 (gelatinase negative [Gel⁻]) (left) and OG1RF (gelatinase positive [Gel⁺]) (right) on Todd-Hewitt broth (Becton, Dickinson and Co., Sparks, MD) agar were incubated for 24 h before the inoculation of the agar with susceptible indicator lawns (dilute broth cultures swabbed onto the agar surface) and the bacteriocin producer E. faecalis SFJ1. Indicator lawns are E. faecalis JH2-2 and E. faecium 409. Clear strips alongside TX5128 and OG1RF are areas not inoculated. Zones of inhibition created by SFJ1 extend up to TX5128 growth. However, zones of inhibition produced by SFJ1 were absent within the approximate area of gelatinase diffusion extending from OG1RF (as measured under similar conditions on gelatin-containing agar) (data not shown). (B) Gelatinase-negative SFTX3 (TX5128/pAMS1) as a producer kills siblings used as indicators on solid medium. SFTX3 is not killed by plasmid-free TX5128. SFTX3, but not the plasmid-free TX5128, exhibits bacteriocin activity against the gelatinase-positive OG1RF. (C) Nonisogenic clinical isolates without and with pAMS1 (upper and lower rows, respectively). Only when carrying pAMS1 do producers kill siblings used as indicators on solid medium. (D) Hosts from the JH2 family (SFJ1 [JH2-2/pAMS1] and SFJS1 [JH2SS/pAMS1]) exhibit no sibling killing. However, following the transfer of the plasmid from SFJS1 to the plasmid-free TX5128, sibling killing by the transconjugant SFTX2 occurs. Sibling killing by TX5128 before the acquisition of the plasmid is not observed.