Screening E. coli for Bacteriocin Sensitivity

Studied E. coli strains were screened for bacteriocin sensitivity using a collection of indicator strains producing colicins and microcins (BZB collection, University of Ljubljana, Slovenia). The effects of group A and B colicins, as well as class I and II microcins were evaluated [23 (link)]. Colonies of the bacteriocinogenic indicator strains were grown on Luria-Bertani agar (LB-agar; Amresco, Solon, OH, USA) in a Petri dish (9 cm) for 24 h. The grown colonies were treated with chloroform vapors for 15 min. A suspension of the analyzed E. coli strain (standardized to 2.0 according to MacFarland) was added to the melted (46 °C) 0.6% LB soft agar, mixed, and poured over the prepared vaporized colonies. The plates were then incubated for 24 h at 37 °C. The next day, the plates were examined and growth inhibition zones were measured. Inhibition zones of 1 mm or more were considered to reveal sensitive E. coli strains. Using the same method, except that the strains were grown as colonies and the DH5α strain, known to be sensitive to bacteriocins, was used as the overlaid indicator strain, all studied strains were also tested for their ability to produce bacteriocins themselves.