Effect of lysozyme or nisin on survival of some bacteria treated with high pressure at subzero temperature

The aim of this work was to examine the inactivation of some Gram-positive and Gram-negative bacteria exposed to the pressure of 193 MPa at -20 ºC in the presence of lysozyme or nisin at concentration of 400 mg/ml. The highest effect of pressure at subzero temperature and lysozyme was found with pressure sensitive Pseudomonas fluorescens; viable cells of this strain were not detected in 1 ml of sample after combined treatment. The action of pressure at subzero temperature and lysozyme or nisin against Escherichia coli led to synergistic reduction by 0.7 or 1.6 log cycles, respectively, while it was practically insignificant for two Staphylococcus aureus strains. Viability loss of E. coli and S. aureus occurred during storage for 20 h of the samples at 37 and 5 ºC, which were previously pressurized with lysozyme or nisin. The synergistic effect of pressure and nisin at pH 5 against E. coli cells just after the pressure treatment was lower than that at pH 7, however, the extent of the lethal effect after storage was higher.

Improvement of nitrate and nitrite reduction rates prediction

Reported models of denitrification rates integrate in an unique parameter the pH-dependent inhibition by HNO2 and the pH effect on the bacterial metabolic activity; furthermore, they do not quantify separately the pH effect on the nitrate and on the nitrite reduction rates. The goal of this work was to quantify both effects on the kinetics of nitrate and nitrite reduction to improve the models’ predictive value. Assays were performed at a pH range of 6.5-9.0 in batch reactors at 37ºC with an activated sludge. At the studied pH range and at below the HNO2 inhibitory concentration (0.004 mg L-1), the maximum nitrate reduction rate diminished 23 percent and 50 percent by decreasing or increasing, respectively, one pH unit from 8.0. The maximum nitrite reduction at pH 8.0 diminished 15 percent at pH 7.0 and 40 percent at pH 9.0. At HNO2 concentrations over the inhibitory concentration, except at pH > 8.0, the maximum nitrate reduction rate diminished 50 percent upon decreasing the pH from 8.0 to 7.0 or increasing it from 8.0 to 9.0. Inclusion of the pH effect in the reported models improved their predictive value; average deviations from the experimental data were reduced from 53 percent to 10.7 percent or 33.8 percent to 10.5 percent for nitrite and nitrate reduction rates, respectively.