ANTIMICROBIAL SUSCEPTIBILITY OF BIFIDOBACTERIA

The antimicrobial susceptibility of 37 strains of bifidobacteria to 18 antimicrobial agents was determined by a macrodilution broth method. Most of the strains used were isolated from commercial yogurts and starters. Tested organisms were usually sensitive to Gram-positive spectrum antibiotics (bacitracin, erythromycin, lincomycin, and vancomycin), and most of the organisms were inhibited by a concentration <1.56 mug/ml. Erythromycin was the most active agent; all strains were inhibited by <.19 mug/ml. Beta-Lactam antibiotics (penicillin G, ampicillin, methicillin, and cephalothin), showing a wide range of minimum inhibitory concentration, were less effective than Gram-positive spectrum antibiotics. Most strains were somewhat resistant to cephalothin, exhibiting inhibition at concentrations of 6.25 to 25.0 mug/ml. Test organisms were most resistant to kanamycin, neomycin, paromomycin sulfate, nalidixic acid, and polymyxin B sulfate; inhibition occurred only at greater-than-or-equal-to 50 mug/ml, and strains were somewhat less resistant to gentamicin and streptomycin. Susceptibility to nitrofurantoin and tetracycline was variable; minimum inhibitory concentrations ranged from 1.56 to 50.0 and .39 to 50.0 mug/ml, respectively, but chloramphenicol had a narrow range from 1.56 to 6.25 mug/ml.