Graduate Thesis Or Dissertation
 

Control of acid production and β-galactosidase synthesis in lactic streptococci

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  • The public health and economic significance of rapid acid production by lactic streptococci in controlled dairy fermentations is well known. However, the fast acid-producing characteristic of these organisms is not stable, and cultures of fast organisms have been shown to contain slow cells. These studies were carried out to characterize slow and fast acid-producing cells isolated from Streptococcus lactis. Since growth and acid production by lactic streptococci are dependent on their ability to metabolize lactose, the enzymes β-galactosidase, which hydrolyizes lactose, and galactoside-permease, which is responsible for the entry and intracellular accumulation of lactose, were also studied. From Streptococcus lactis C2, a fast strain which produced sufficient acid to coagulate sterile nonfat milk in 18 hours at 21 C, a slow mutant was isolated which required incubation for at least 48 hours to effect coagulation. Nonfat milk cultures of these two organisms were compared to find causes for the reduced acid production by the slow strain. These two bacteria had the same generation time (1.4 hours) in the exponential growth phase, and produced the same amount of acid per cell. The average viable population of the fast culture after 18 hours at 21 C, however, was about four times as great as the slow. Also, the fast culture was four times more proteolytic in nonfat milk than the slow. The slow organism appeared to have lost its capacity to synthesize proteolytic enzymes. Tryptic and pancreatic hydroysates of casein stimulated acid production in milk by the slow and fast strains; acid hydrolyized casein was not an effective stimulant. There was, therefore, a direct relationship between available nitrogen, in the form of peptides, and total growth and acid production in milk. Acid development in milk by the fast strain was also enhanced by adenine, hypoxanthine, adenosine, or inosine. Therefore, there appeared also to be a relationship between the synthesis of proteolytic enzymes in the fast strain and the availability of purine bases or nucleosides. The slow strain, however, having lost its ability to synthesize proteolytic enzymes, derived no benefit from these nucleic acid derivatives. Synthesis of β-galactosidase by several strains of lactic streptococci was induced by lactose. The rate of hydrolysis of orthonitrophenyl-β-D-galactopyranoside was used to measure enzyme activity. The enzyme of all but one strain was unstable when whole cells were sonicated or treated with toluene; the enzyme of one strain, Streptococcus lactis 7962, was stable to these treatments, which resulted in at least a five-fold increase in activity over that found in whole cells. The optimal assay conditions for toluene-treated cells of this strain were 37 C and pH 7.0 in sodium phosphate buffer. Lactose was the most effective inducer of enzyme synthesis. Methyl-β-D-thiogalactopyranoside, isopropyl-β-D-thiogalactopyranoside, and galactose were also inducers of the enzyme, but were not as effective as lactose. Melibiose, maltose, calcium lactobionate, and glucose were poor inducers of enzyme synthesis. Exogenously supplied glucose repressed induced β-galactosidase synthesis; galactose only slightly inhibited induced enzyme synthesis. Lactose-C¹⁴ uptake by S. lactis 7962 was mediated by a β-galactoside-permease. Synthesis of this enzyme was induced by lactose. Maltose induced synthesis of this enzyme, but less effectively than lactose. Glucose was not an inducer. Glucose also repressed synthesis of induced β-galactoside-permease. The means of control of induced β-galactosidase and β-galactoside-permease synthesis in S. lactis was similar to that in Escherichia coli.
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