Effect of polimixin B and neomycin sulfate on growth of selected probiotics

Effect of polimixin B and neomycin sulfate on growth of Lactobacillus acidophilus, Bifidobacterium bifidum, Lactobacillus bulgaricus, Lactobacillus rhamnosus and Lactobacillus casei was studied by measuring the optical density at 600nm and pH using De Man, Rogosa and Sharpe (MRS) broth as the control. The addition of polimixin B was 33.3U, 66.6U, 100U and 133.3U and the addition of nalidixic acid (%, w/v) was 0.0005, 0.003, 0.006 and 0.010. Results were as follows: addition of polimixin B at the concentration above 33.3U has a significant inhibition on the growth of B. bifidum and has no influence on the other four probiotics at incubation 24h. The optimum selective concentration of polimixin B in MRS media was 33.3U for selective enumeration of B. bifidum. Addition of neomycin sulfate has a significant inhibition on the growth of the five selected probiotics at incubation 24h. Addition of neomycin sulfate in MRS can not use to realize selective enumeration in yogurt containing probiotics.