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Sensitivity test : Results

     This first picture shows the sensitivity of bacteria containing the pCID909 plasmid to culture supernatant from two different microcin B17 expressing clones, microcin C7 expressing bacteria or MC1061 bacteria.

     Microcin B17 bacteria’s lawn shows a clear inhibition halo around the supernatant drop containing microcin C7. On the other hand, drops of microcin B17 (both clones) and MC1061 did not cause any inhibition halo.

     Microcin B17 bacteria seem to be resistant to their own microcin production but sensitive to microcin C7. Supernatant extracts of sensitive bacteria MC1061 culture do not seem to be toxic for microcin B17 bacteria.

     This second picture shows the sensitivity of bacteria containing the pp70 plasmid encoding microcin C7 to culture supernatant from two different microcin B17 expressing clones, microcin C7 expressing bacteria or MC1061 bacteria.

     We observe an inhibition halo on microcin C7 bacteria’s lawn around the supernatant drop containing microcin B17 (both clones) while microcin C7 and MC1061 drops did not cause any inhibition of bacterial growth.

     Microcin C7 bacteria seem then to be resistant to their own microcin production but sensitive to microcin B17. Sensitive strain’s supernatant had no impact on bacterial growth.

      This third picture shows the sensitivity of MC1061 E. coli strain to culture supernatant from two different microcin B17 expressing clones, microcin C7 expressing bacteria or MC1061 bacteria.

     We observe an inhibition halo on sensitive MC1061 bacterial’s lawn around the supernatant drop containing microcin C7 and a weak one around both supernatant drops containing microcin B17. MC1061 supernatant did not cause any inhibition halo.

     MC1061 is then sensitive to both microcin but microcin C7 seems to be more toxic than B17. As expected, MC1061 bacteria are not sensitive to their own supernatant.

Conclusion:

     Each microcin producing bacteria strain is immun against to its own microcin production: microcin B17 expressing bacteria against microcin B17 and microcin C7 expressing bacteria against microcin C7. Bacteria that do not express any microcin operon are sensitive to both microcin but microcin C7 seems more toxic than B17.

     This last observation follows theory: microcin B17 causes cessation of DNA replication and SOS response activation by inhibiting DNA gyrase while microcine C7 inhibits the translation by targeting the synthesis of aminoacylated tRNA^Asp.