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This study was undertaken to find optimum conditions of tryptone, yeast extract, glucose, Tween 80, and incubation temperature for the growth of Lactobacillus casei YIT 9018 and to assess the effects of these factors by use of response surface methodology. A central composite design was used as an experimental design for allocation of treatment combinations. A second-order polynomial regression model, which was used at first for analysis of the experiment, had a significant lack of fit. Therefore, cubic and quartic terms were incorporated into the regression model through variable selection procedures. Effects involving incubation temperature, yeast extract, glucose, and tryptone were significant, whereas the only significant effect involving Tween 80 was the interaction effect between temperature and Tween 80. It turned out that growth of L. casei YIT 9018 was most strongly affected by the incubation temperature. Estimated optimum conditions of the factors for growth of L. casei YIT 9018 are as follows:

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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 micrograms/ml. Erythromycin was the most active agent; all strains were inhibited by < .19 microgram/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 micrograms/ml. Test organisms were most resistant to kanamycin, neomycin, paromomycin sulfate, nalidixic acid, and polymyxin B sulfate; inhibition occurred only at > or = 50 micrograms/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 micrograms/ml, respectively, but chloramphenicol had a narrow range from 1.56 to 6.25 micrograms/ml.