Graduate Thesis Or Dissertation
 

Studies on mutation and lactose fermentation in lactic streptococci

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  • This study involved an investigation of lactic streptococci which had irreversibly lost ability to produce the high levels of acid in milk necessary during the manufacture of certain fermented dairy products. The slow acid producing mutants studied were obtained from a normal acid producing strain by natural selection following induction by ultraviolet light irradiation. Comparisons were made between the parent strain and the various slow acid producing mutants. Streptococcus lactis C2F, the parent, fast acid-producing strain, was stimulated by the addition of nitrogenous compounds to milk, possessed a high level of proteolytic ability, grew well in a minimal synthetic medium, grew poorly in a complete medium, was non-viscous in milk, had the ability to hydrolyze arginine, was catalase negative, did not form a pellicle in broth, had a generation time of about 60 minutes in milk at 21°C. and was sensitive to its specific C2 phage. The naturally occurring slow mutant grew well in minimal synthetic medium, grew poorly in a complete medium, was non-viscous in milk, had the ability to hydrolyze arginine, was catalase negative, did not form a pellicle in broth, and had a generation time the same as the parent strain. The natural slow cultures which received one irradiation treatment were not stimulated by nitrogenous additives, possessed a low level of proteolytic ability, grew poorly in a minimal synthetic media and complete medium, had the ability to hydrolyze arginine, had an extended generation time, and were resistant to the C2 phage. Mutants isolated following one irradiation treatment were stimulated by the addition of nitrogenous compounds, possessed a low level of proteolytic ability, grew poorly in a minimal and a complete synthetic medium, were non-viscous in milk, had a generation time the same as the parent strain, and were sensitive to the C2 phage. Second irradiation mutants were not stimulated by the addition of nitrogenous compounds, possessed a low level of proteolytic ability, grew poorly in the minimal and complete synthetic medium, had an extended generation time, and were resistant to the C2 phage. A study also was made of the primary lactose degrading enzymes of lactic streptococci. Two systems were found present, β-galactosidase and lactose dehydrogenase. The latter enzyme was found in greatest amount in S. lactis, next in Streptococcus diacetilactis, and thirdly in Streptococcus cremoris.
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