Graduate Thesis Or Dissertation

Studies on growth metabolites produced by Propionibacterium shermanii

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  • The ability of Propionibacterium shermanii to produce growth metabolites which were inhibitory to food spoilage indicator organisms was investigated. Asperqillus niqer, a Pseudomonas species and Kluyveromyces fraqilis were the spoilage indicators chosen to represent molds, gram negative psychrotrophic bacteria and yeasts, respectively. Supplementation of milk or whey based media was attempted to optimize synthesis of the inhibitory growth metabolites by P. shermanii. Production of the propionibacterial growth metabolites (PGM) was stimulated by addition of cobalt (II) nitrate (16 mg/L) or disodium fumarate (0.4%) to milk or whey growth media. PGM was heat sensitive, bacteriostatic and fungistatic. The antimicrobial effect of PGM was utilized in preservation of commercially produced cottage cheese and strawberry-flavored yogurt. The incorporation of freeze dried PGM (FDPM) into yogurt and liquid PGM (LPM) into cottage cheese caused significant reductions in spoilage organisms and extension of shelf life. Cottage cheese containing LPM (1% in the dressing) exhibited 6 to 9 days extension in the cheese shelf life over the controls. Ten percent FDPM completely preserved a commercial brand of yogurt against spoilage organisms for 82 days at 5°C. PGM components were separated by gel filteration, ultrafilteration and cationic exchange. Activity of fractions was assayed against the indicator bacterium (Pseudomonas sp.). Fractions showing high activity were assayed for fatty acid content and uv absorption (200-400 nm). Several lines of evidence suggested the existence of open structured tetrapyrroles (bactobilin) and their degradation products. Bactobilin was thought to function as an ionophore in the inhibitory action of PGM and also as a natural buffering agent protecting the organism against its own toxic metabolites. The ability of P. shermanii to utilize and accumulate elemental selenium was studied. Utilization was influenced by the presence of additional sugars in a tryptic soy broth medium. Swiss cheese starter cultures formed a muddy pink color in growth media containing sodium selenite. Pink color defects in cheeses were thought to depend on the ability of starter bacteria to accumulate elemental selenium. Extracellular rennet-like protease(s) was observed to occur in the presence of selenite in P. shermanii strains and Lactobacillus bulgaricus strain R-5. The enzyme appeared to be an oxygen sensitive selenoprotein. Differences in selenite and tellurite tolerance by L. bulgaricus and Streptococcus thermophilus were utilized as a basis for development of a selective differential medium for isolation and enumeration of the two organisms. Growth of L. bulgaricus was selectively promoted at high selenite concentrations (above 160 μ g/L). Streptococcus thermophilus growth was inhibited by the same selenite concentrations. Conversely, S. thermophilus was stimulated to grow at high tellurite concentrations (above 100 μ g/L) in contrast to the inability of L. bulqaricus to grow under the same conditions.
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