Susceptibility of fecal anaerobic bacteria from pigs and chickens to five polyether antibiotics for growth promotion.

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Abstract
The minimum inhibitory concentrations (MIC) of 100 strains of anaerobic bacteria, isolated from the intestines of healthy pigs and chickens, for five polyether antibiotics including cationomycin (CNM), lysocellin (LSC), monension (MNS), portmicin (PRM) and salinomycin (SNM) were determined by an agar-dilution technique. CNM, MNS, PRM, and SNM, at a concentration of 6.25 .mu.g/ml, were active against 41 to 56% of the Bacteroides fragilis group, whereas LSC showed less activity at the same concentration. All strains of genera Fusobacterium, Selenomonas, Mitsuokella and Megasphasera were resistant to the five polyether antibiotics. Eubacterium spp. were susceptible to the antimicrobial agents (MICs of 1.56 to 3.12 .mu.g/ml) except for MNS. CNM and PRM inhibited all strains of Bifidobacterium spp. at lower concentrations, but LSC, MNS and SNM did not. CNM, PRM and SNM, at a concentration of 1.56 .mu.g/ml, were active against all strains of Lactobacillus spp. whereas LSC and NMS did not inhibit them at the same concentration. Clostridium perfringens was also susceptible to five polyether antibiotics tested (MICs of 1.56 to 12.5 .mu.g/ml). All strains of B. fragilis from pigs were resistant to the five polyether antibiotics but those from chickens were inhibited at lower concentrations.