Combined Effects Of Brevibacillus Laterosporus Antimicrobial Peptides

In this study,antimicrobial peptides produced by Brevibacillus laterosporus S62-9(AMP)were used in combination with food preservatives and feed antibiotics respectively on pathogenic bacteria in order to screen synergists of AMP to reduce its dosage and cut down costs.Meanwhile,the combined use can decrease doses of some food preservatives and feed antibiotics,provide a more secure method for the control of pathogens.Minimum inhibitory concentrations(MICs)of AMP,food preservatives and feed antibiotics to S.aureus and E.coli were determined by broth micro-dilution method.The MIC of AMP to S.aureus was far lower than that of E.coli which indicated the antibacterial activity of AMP to S.aureus was higher than E.coli.?-Polylysine,Nisin,Phenyllactic acid,Citric acid,EDTA-2Na and Ttrisodium phosphate showed antibacterial activities on S.aureus ATCC 14458 and E.coli ATCC 44752(except Nisin to E.coli ATCC 44752),among these,?-Polylysine was the strongest and Ttrisodium phosphate was the weakest.Apramycin sulfate,Neomycin sulfate and Tylosin phosphate showed activities on S.aureus ATCC 6538 and E.coli ATCC 25922,Neomycin sulfate was most active;Tilmicosin phosphate,Lincomycin hydrochloride,Bacitracin zinc and Nosiheptide showed activities only on S.aureus ATCC 6538 and Spectinomycin sulfate and Colistin sulfate showed activities only on E.coli ATCC 25922.Combined effects of AMP with food preservatives were determined by micro-dilution checkerboard test.From the checkerboard test: to S.aureus ATCC 14458,the FICI values of AMP with ?-Polylysine/Nisin were 0.282 and 0.532,which suggested a synergistic and additive effect between AMP and the two agents,respectively;to E.coli ATCC 44752,the FICI values of AMP with EDTA-2Na,Ttrisodium phosphate and Citric acid were less than0.5,indicating synergic relationship was existed between AMP and these preservatives.Time kill curves showed that,when AMP and food preservatives were at the concentration of 1/2MIC,to S.aureus ATCC 14458,combination of AMP with ?-polylysine slightly better than combination of AMP and Nisin;to E.coli ATCC 44752,combinations of AMP with EDTA-2Na/Trisodium phosphate/Citric acid showed fast antibacterial effects and AMP-Trisodium phosphate was a little better than the rest.Combined effects of AMP with feed antibiotics were detected by micro-dilution checkerboard test,too.It can be seen from the checkerboard test: to S.aureus ATCC 6538,the FICI values of AMP with Neomycin sulfate/ Lincomycin hydrochloride were 0.532,the FICI values of AMP with Tylosin phosphate /Tilmicosin phosphate/ Bacitracin zinc were 1,implying there were additive effects between AMP and these 5 feed antibiotics;to E.coli ATCC 25922,combinations of AMP with Apramycin sulfate/Colistin sulfate performed intensive synergies with the FICI values were 0.375 and 0.313,combinations of AMP with Neomycin sulfate/Spectinomycin sulfate illustrated additive actions with the FICI values were 1 and 0.75.From time kill tests,when AMP and feed antibiotics were at the concentration of 1/2MIC,to S.aureus ATCC 6538,AMP-Neomycin sulfate and AMP-Lincomycin hydrochloride showed better effects;to E.coli ATCC 25922,combinations of AMP with Colistin sulfate/Apramycin sulfate/Spectinomycin sulfate showed strong bacterial effects,AMP-Neomycin sulfate were less active than these three combinations.