Graduate Thesis Or Dissertation

Strain dominance in frozen multiple strain lactic starters

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  • Two multiple strain lactic streptococcal starter concentrates were examined to determine whether or not elimination of one of the three component strains, by addition of its specific phage, resulted in disproportionate inhibition of acid production (strain dominance). Both starters showed dominance when tested three to four weeks after freezing and storage at -20 C. In one starter, dominance persisted throughout ten months of storage. In the second starter, dominance decreased over eight months of storage, and the lag phase of growth increased from one to four hours. Plate counts showed only a twofold difference in cell numbers between uninfected and phage-infected dominant strain cultures. Differential freezing survival and recovery are suggested as factors in the production and alteration of strain dominance patterns. The single strain components of the two starters were separately concentrated and frozen. They were then thawed at 30-day intervals and survival, lactate sensitivity, acid-producing activity, sensitivity to lactate vs. sensitivity to pH, antibiotic production, and phage sensitivity were examined. Findings were compared to the results of tests done before freezing in an effort to determine the causes of the observed strain dominance. Dominance by strain one in starter A was explained by its high activity when peptide fragments were supplied it by the proteolytic enzymes of the other starter A component strains, and by its ability to maintain its activity during frozen storage. Occurrence of the dominant strain in Starter B was explained by its high survival, its tolerance to salt (indicating a lower degree of freezing damage), and its high activity after frozen storage. It was concluded that there is no single factor identifiable as the cause of strain dominance. Phage sensitivity is suggested to be a good indicator. The concept of strain dominance was shown to have practical value, and ways in which the techniques for its discovery might be improved are suggested.
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