The effects of heat and NaCl on the activity of superoxide dismutase from Staphylococcus aureus were examined. A linear decrease in superoxide dismutase activity occurred when S. aureus MF-31 cells were thermally stressed for 90 min at 52°C in 100 mM potassium phosphate buffer (pH 7.2). After 20 min of heating, only 5% of the superoxide dismutase activity was lost. Heating for 60, 90, and 120 min resulted in decreases of approximately 10, 22, and 68%, respectively. The rates of thermal inactivation of superoxide dismutase from S. aureus strains 196E and 210 were similar and slightly greater than those of strains MF-31, S-6, and 181. The addition of NaCl before or after heating resulted in increased losses of superoxide dismutase activity.
Evidence is presented suggesting that the decreased enumeration of heat-stressed Staphylococcus aureus cells on selective media is the result of accumulation of metabolic H2O2. It accumulates due to the decreased activity of catalase caused by the synergistic effects of heat and NaCl. Heated cells enumerated anaerobically on tryptic soy agar (TSA) containing 6.5% NaCl (TSAS 6.5) exhibited a 200-fold increase compared to cells enumerated aerobically on the same medium. The anaerobic counts on TSAS 6.5 were similar to the aerobic counts on TSA. Increases in both death and injury occurred when S. aureus was propagated in tryptic soy broth (TSB) plus 10% NaCl (TSBS) instead of TSB before thermal injury. Addition of catalase to TSA and TSA containing 7.5% NaCl (TSAS) increased the count to approximately the same levels on TSA and TSAS as that found following thermal injury after propagation in TSB. Catalase activity was 12-fold higher in stationary phase cells propagated in TSB than in TSBS. Indirect evidence indicates that toxic levels of H2O2 accumulated rapidly, causing one to two log decreases in enumeration after 30 to 60 min incubation on TSAS.
Exposure of crude cell lysates of Staphylococcus aureus MF-31 to 5 or 10 mM hydrogen peroxide resulted in a linear decrease in superoxide dismutase activity. Approximately 13% of the superoxide dismutase activity was lost after 16 min. Thermally stressed and nonstressed cells were exposed to a photochemically generated exogenous flux of superoxide radicals (02'). The death of thermally stressed cells was linear with time. Addition of superoxide dismutase or catalase to the O2-generating system resulted in protection of thermally stressed and nonstressed cells, with the protective effect being greater for thermally stressed cells. Incorporation of 02-, hydroxyl radical, or singlet oxygen scavengers or antioxidants to tryptic soy agar containing 7.5% NaCl did not increase the enumeration of thermally stressed cells.
Superoxide dismutase (SOD) activity was determined during the growth cycle of unheated and heat-injured cells of Staphylococcus aureus MF-31. SOD activity levels dropped in unheated cells during the lag phase, increased during logarithmic phase, and became constant in the stationary phase. Cells which were sublethally heated (520C, 20 min) in 100 mM phosphate buffer and subsequently allowed to recover in tryptic soy broth demonstrated an 85% decrease in SOD activity upon inoculation into recovery medium. As the injured cells repaired the heat-induced lesions and entered logarithmic growth, SOD levels rapidly increased. Heatinjured cells allowed to recover in tryptic soy broth plus 10% NaCl showed similar decreases in SOD activity levels. However, no subsequent increase was observed when specific activity was calculated based on milligrams of protein.
The ability of cultures of 19 lactic acid bacteria to ferment a peanut milk (PM) prepared from blanched, full‐fat Runner Variety peanut kernels was examined. Lactobacillus acidophilus, L. brevis, L. xylosus, L. plantarum, Pediococcus acidifactici and Streptococcus thermophilus after 48 hr produced the highest levels of acid in plain PM, producing respectively 0.25, 0.20, 0.42, 0.42, 0.17 and 0.27% titratable acidity (TA), expressed as lactic acid. Chemical analysis indicated that sucrose at 0.60% (w/v) was the major fermentable carbohydrate present in the PM. Supplementation of the PM with 1% levels of glucose, sucrose, whey, tryptose or yeast extract increased acid production by most cultures. Glucose supplementation of PM enabled L. delbrueckii, L. plantarum, L. helveticus and L. casei to produce 0.69, 0.17, 0.75, and 0.73% TA, respectively. Invertase treatment of the PM led to increases in acid production of 0.23–0.30 TA units by cultures previously unable to ferment the untreated PM.‘In a model system, the addition of 2.3% (w/v) oleic, 1.3% linoleic and 0.4% palmitic acid to a basal Elliker's broth with 1% sucrose (EB‐S) as the sole carbohydrate source, led to 100% inhibition of acid production after 48 hr, compared to controls, for cultures of L. acidophilus, L. cellobiosus and S. thermophilus. For cultures of L. brevis, L. xylosus, L. plantarum and P. acidilactici, the inhibition of acid production by the fatty acids after 48 hr was much less, only about 5%.
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