Graduate Thesis Or Dissertation
 

Mechanisms of lactose metabolism by strains of Streptococcus lactis

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  • Rapid acid production by lactic streptococci used in the manufacture of fermented dairy products is essential to the economy of the industry and to the public health of consumers. There remains to developed, however, an adequate laboratory procedure to detect and distinguish between slow and fast strains of these bacteria. This investigation involved a study of β-galactosidase, the enzyme responsible for hydrolysis of lactose, in an attempt to determine the cause for failure of Streptococcus lactis strains to reveal enzyme activity when assayed using conventional techniques. Lactose dehydrogenase and β-galactoside permease were other enzymes studied. From the results, it was clear that two mechanisms for lactose utilization were operative in strains of S. lactis. Flouride ion caused a marked decrease in the lactose uptake and o-nitrophenyl-β-D-galactopyranoside (ONPG) hydrolysis by S. lactis C₂F. On the other hand, S. lactis 7962 was not inhibited by flouride but was effected by arsenate. The presence of lactose prevented ONPG hydrolysis only in strain C₂F. Toluene treated cells of S. lactis C₂F had a phosphoenol-pyruvate requirement for β-galactosidase activity as measured by ONPG hydrolysis. The addition of ATP to toluene treated cells of strain 7962 provided stimulation of the significant activity already present. Optimal temperature and hydrogen ion conditions for whole cell enzyme assay were pH 7.0 and 45 C for S. lactis C₂F and pH 6.5 and 52 C for the other strain. Glucose and galactose were found in cell free extracts of strain 7962 after incubation with lactose whereas only galactose was present in extracts of strain C₂F. The assay for lactose dehydrogenase was found to be non-specific for lactose and was not studied further. However, it was found that strain C₂F had stronger reducing power than strain 7962, being able to reduce substrates with E₀' values as low as -0.252. Uptake of lactose by the faster acid-producing C₂F strain was approximately twice the rate of the other strain and therefore uptake of substrate provided a more accurate measurement of activity than did hydrolysis of ONPG. S. lactis 7962 contained the classical Escherichia coli type of lactose utilization in which the disaccharide was split by β-galacto-sidase into glucose and galactose. The inactivity of the other strain, S. lactis C₂F, was therefore due to a PEP dependent system involving another mechanism not utilizing ONPG.
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