Plasmid-Associated Lactocin RN78 Production in A Lactobacillus RN78 Strain Isolated from A Dairy Sample in Iran

A bacteriocin, lactocin RN78 produced by a Lactobacillus RN78 strain isolated previously from a cheese sample, was found inhibitory against many pathogenic Gram-negative and Gram-positive bacteria including Kl. pneumoniae, L. ivanovii, L. monocytogenes, Ps. aeruginosa, and S. aureus. Characterization of the inhibitory agent produced by RN78 strain revealed it to be heat stable (100 and 121°C for 90 and 15 minutes respectively), and active at wide pH range of 3 to 9. Moreover, the inhibitory activity was completely destroyed after treatment with proteolytic enzymes (Pronase and Trypsin); while no change in its antimicrobial activity was recorded after treatment with the enzyme catalase. A rapid cell death of the indicator strain (106 to 107 cells) within 2 hours of incubation in presence of 10240 AU mL-¹ of lactocin RN78 indicated the bactericidal nature of this bacteriocin against L. monocytogenes. Treatment of lactocin RN78 with 0.5 mM EDTA resulted in expansion of the inhibitory spectrum and it inhibited the growth of E. coli and S. typhi to which it was previously non-influential. Curing of the producer strain RN78 with acridine orange resulted in Bac-mutant defective in bacteriocin production. Plasmid analysis of the wild type and mutant's defective in bacteriocin production indicated the absence of a 42Kb plasmid that was originally present in the wild producer strain. All the cured strains retained their immunity to lactocin RN78 which indicated the possibility of the immunity genes to be on the chromosomes instead of being on the plasmid RN.