Inhibiting Gram-negative bacteria growth in microdilution by Nisin and EDTA treatment

The outer membrane composition of Gram-negative bacteria provides more resistance against the action ofantimicrobial agents from the class of bacteriocins, in comparison to Gram-positive bacteria, naturally moresusceptible. Aiming the control of proliferation Escherichia coli, Pseudomonas aeruginosa and SalmonellaTyphimurium, antimicrobial assays were performed with nisin as biocidal agent (concentrations 123 to 1,111 AUmL-1) and EDTA as chelating agent, compared to a Gram-positive (Staphylococcus aureus ATCC25923). Thesusceptibility evaluations were performed in broth microdilution with Mueller-Hinton medium and inoculum with106 CFU mL-1 of indicator bacterium, and the results were obtained by optical density readings (630 nm) at regularintervals during the incubation period. It was observed that EDTA is capable of reducing cell viability of P.aeruginosa. The bacteriocin nisin demonstrated effective antimicrobial action to four indicator bacteria, underdifferent concentrations. The combined treatments of nisin+EDTA obtained high inhibition rates, and the presenceof EDTA was significant only to Gram-negative bacteria. Thus, treatments accounted effective strategies to promoteinhibition of cell growth during microdilution broth assays.