Graduate Thesis Or Dissertation
 

Influence of temperature on the associative growth of Streptococcus thermophilus and Lactobacillus bulgaricus

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  • Optimum growth temperatures were determined for nine strains of Streptococcus thermophilus and ten strains of Lactobacillus bulgaricus, all commercial yogurt or yogurt starter isolates. Optimum growth temperatures ranged from 35 C to 42 C for S.thermophilus strains, averaging 38.6. Optimum growth temperatures for L.bulgaricus strains ranged from 43 C to 46 C, averaging 44.4 C. Based on these results, L.bulgaricus and S.thermophilus strains exhibiting similar, average, or divergent optimal growth temperatures were mixed(in a 1:1 v/v inoculum) and incubated in pasteurized whole milk(plus 4.8% added nonfat milk solids) at 37 C, 42 C, and 45 C, until a pH of 4.2 was reached. Initial and post-incubation rod-coccus counts(cfu/ml) were determined by plating on LB medium. S.thermophilus reached higher cell numbers than L.bulgaricus at 37,42 and 45 C in 93.3. of the mixed culture trials. Xverage rod/coccus ratios obtained at 37, 42 and 45 C were 1:2.2, 1:8, and 1:2.4 respectively. All S.thermophilus strains exhibited an uncoupling of growth from acid production. Optimum temperatures for acid production in S.thermophilus strains ranged from 2 to 8 degrees above optimum growth temperatures, averaging 43.6 C. Average optimum temperature for acid production in L.bulgaricus strains was 44.9 C. All strains were tested for starch and urea hydrolysis. Only one of nine strains of S.thermophilus was found to hydrolyze starch. All strains of S.thermophilus were found to hydrolyze urea. High incidence of non-ureolytic cells found within a ureolytic strain suggest that the genetic information for urease might be located on a plasmid. LB agar medium was determined to be optimal for rod/coccus differentiation relative to 5 other media. Rod/coccus ratios as determined by microscopic examination tended to underestimate numbers of cocci by an average of 10.6%, as compared to rod/coccus ratios determined by plating methods(cf u/ml). Six commercial yogurts were examined. Total counts(cfu/ml) ranged from 3.4 x 10^9 to 1.4 x 10^8. Four of six samples revealed a higher number of cocci relative to rods. One yogurt sample, labeled to contain viable Lactobacillus acidophilus, had an estimated count of less than 1000 bile tolerant organisms per ml.
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