Enhanced Acid Sensitivity of Pressure-Damaged Escherichia coli O157 Cells

Pressure-damaged Escherichia coli O157 cells were more acid sensitive than native cells and were impaired in pH homeostasis. However differences in acid sensitivity were not related to differences in cytoplasmic pH (pH_i). Cellular β-galactosidase was more acid labile in damaged cells. Sensitization to acid may thus involve loss of protective or repair functions.     

Sub-lethal temperature thresholds indicate acclimation and physiological limits in brook trout Salvelinus fontinalis

The upper thermal tolerance of brook trout Salvelinus fontinalis was estimated using critical thermal maxima (CT_max) experiments on fish acclimated to temperatures that span the species' thermal range (5–25°C). The CT_max increased with acclimation temperature but plateaued in fish acclimated to 20, 23 and 25°C. Plasma lactate was highest, and the hepato-somatic index (I_H) was lowest at 23 and 25°C, which suggests additional metabolic costs at those acclimation temperatures. The results suggest that there is a sub-lethal threshold between 20 and 23°C, beyond which the fish experience reduced physiological performance.     

New Mathematical Modeling Approach for Predicting Microbial Inactivation by High Hydrostatic Pressure

A new primary model based on a thermodynamically consistent first-order kinetic approach was constructed to describe non-log-linear inactivation kinetics of pressure-treated bacteria. The model assumes a first-order process in which the specific inactivation rate changes inversely with the square root of time. The model gave reasonable fits to experimental data over six to seven orders of magnitude. It was also tested on 138 published data sets and provided good fits in about 70% of cases in which the shape of the curve followed the typical convex upward form. In the remainder of published examples, curves contained additional shoulder regions or extended tail regions. Curves with shoulders could be accommodated by including an additional time delay parameter and curves with tails shoulders could be accommodated by omitting points in the tail beyond the point at which survival levels remained more or less constant. The model parameters varied regularly with pressure, which may reflect a genuine mechanistic basis for the model. This property also allowed the calculation of (a) parameters analogous to the decimal reduction time D and z, the temperature increase needed to change the D value by a factor of 10, in thermal processing, and hence the processing conditions needed to attain a desired level of inactivation; and (b) the apparent thermodynamic volumes of activation associated with the lethal events. The hypothesis that inactivation rates changed as a function of the square root of time would be consistent with a diffusion-limited process.     

Principles of separating micro-organisms from suspensions using ultrasound

C.A. MILES, M.J. MORLEY, W.R. HUDSON AND B.M. MACKEY. 1995. When particles in suspension are placed in a stationary ultrasonic field, they form bands of high concentration at half-wavelength intervals. Several theoretical papers have recently contributed to an understanding of the process but, for particles as small as bacteria, there are substantial discrepancies between the different theories, and a lack of published observations.
The threshold amplitudes required to band latex spheres (0.5-5.0 μm diameter) were measured in the frequency range 1–3 MHz and used to establish conditions, reported here for the first time, suitable for banding and moving vegetative bacterial cells in pure culture. Ultrasonic means for separating and concentrating cream and bacteria at opposite ends of a tube containing a mixture of Escherichia coli and diluted milk are also described and the breakdown of theoretical equations at low particle size is discussed.