Master of Science (MS)
School of Animal Science
Lactococcus lactis is a dairy culture bacterium widely used in dairy products that contain salt (NaCl) such as cheese and salted butter. Osmotic conditions generally hinder the growth of both pathogen and desirable bacteria. It has been observed that many stress-induced proteins are produced after exposure to an environmental stress protecting the cell against other stresses since the first exposure starts the defense mechanisms of the cells creating an effect of cross-protection. If salt tolerance is enhanced in desirable bacteria they would survive better in salty environments. The objective of this study was to evaluate the salt tolerance of Lactococcus lactis R-604 after exposure of various stress conditions. The culture was subjected to 10% v/v ethanol for 30 minutes, 15 mM of hydrogen peroxide for 30 minutes, mild heat at 52°C for 30 minutes and UV light (245 nm) for 5 minutes. The culture was also subjected to starvation (no lactose in M17 broth) for 24 hours or prior osmotic adaption (3% w/v NaCl in M17 broth) for 24 h aerobically at 30°C. A control was run without any stress under the same conditions of each experiment. An initial concentration of 7 log CFU’s/mL was used for all treatments. Growth was determined under 5 concentrations of NaCl (0, 1, 3, 5 and 7% w/v). Plating was done every 24 h for 5 days in M17 agar with 0.5% w/v of lactose and incubated aerobically at 30°C for 48 hours. Salt tolerance was enhanced after mild heat or ethanol exposure at 5% w/v NaCl on days 3, 4 and 5. Salt tolerance was also enhance after hydrogen peroxide stress at 5% w/v NaCl on days 4 and 5 and after 24 hours of lactose starvation at 3% w/v on day 3. L. lactis R-604 was not negatively affected by any of the stress conditions applied at salt concentrations of 0, 1 and 3% w/v NaCl. The culture was able to grow in 3% NaCl or no lactose after 24 hours and these stress conditions did not affect its salt tolerance. Growth was maintained at 7% w/v NaCl regardless the stress conditions, however combination of ethanol or hydrogen peroxide at 7% w/v NaCl reduced growth of L. lactis R-604.
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Gonzalez Duran, Ernesto Emilio, "Salt Tolerance of Lactococcus lactis R-604 as Influenced by Exposure to Various Stress Conditions" (2015). LSU Master's Theses. 1936.