Induction of Hsp104 synthesis in Saccharomyces cerevisiae in the stationary growth phase is inhibited by the petite mutation

The elevation of Hsp104 (heat shock protein) content under heat stress plays a key role in the development of thermotolerance in yeast (Saccharomyces cerevisiae) cells. Hsp104 synthesis is increased under heat stress and in the stationary growth phase. The loss of mitochondrial DNA (petite mutation) was shown to inhibit the induction of Hsp104 synthesis under heat stress (39°C) and during the transition to the stationary growth phase. Also, the petite mutation suppressed the increase in activity of antioxidant enzymes in the stationary phase, which accompanied by decrease in thermotolerance. At the same time, mutation inhibited production of reactive oxygen species and prevented cell death under heat shock in the logarithmic growth phase. The results of this study suggest that disruption of the mitochondrial functional state suppresses the expression of yeast nuclear genes upon upon entry into the stationary growth phase.     

Relationship between the culturability of stressed Listeria monocytogenes cells in non-selective and selective culture media and the cellular esterase activity measured by solid phase cytometry

Solid phase cytometry in conjunction with fluorescent probe was applied to rapidly quantify cellular esterase activity of Listeria monocytogenes cells. Viability of cells stressed by several treatments (starvation, NaCl, lactic acid and peracetic acid) was assessed simultaneously by their esterase activity estimated by fluorescence intensity and by their ability to multiply in liquid and solid, non-selective and selective, culture media. It was determined that cell physiological state has a significant impact on the cellular fluorescence intensity which was very dependent on the stress suffered by cells. No general relationship was observed between the bacterial populations observed by cytometry and the populations able to grow on culture media. The link between the cell culturability in non-selective and selective media and the esterase activity was always dependent on the stress suffered. Nevertheless, it was also established that solid phase cytometry is an efficient, sensitive and accurate tool to characterize the ability of non-selective and selective enrichment broths to allow the repair of stressed L. monocytogenes cells by examining the increase in the fraction of the most esterase active cells during the course of resuscitation.     

Metabolome analysis during the morphological transition of Candida albicans

Candida albicans is an opportunistic pathogen of humans with significant mortality in severely immunocompromised patients. The ability to switch from yeast to hyphal morphology and vice versa, in response to various environmental cues, is believed to be a critical virulence factor of this fungus. However, the mechanisms that recognize such environmental signals and trigger the morphological change at a system level are still not clearly understood. Therefore, we have compared the metabolite profiles of C. albicans cells growing under different hyphae-inducing conditions to the metabolite profiles of growing yeast cells. Surprisingly our results suggest an overall downregulation of cellular metabolism during the yeast to hyphal morphological transition. Among the metabolic pathways involved in the central carbon metabolism, we have found seventeen that were significantly downregulated in all three hyphae-inducing conditions. This indicates that these central carbon metabolic pathways are likely to be intrinsically involved in the downstream effects of the morphogenetic process.     

Effects of Above-Optimum Growth Temperature and Cell Morphology on Thermotolerance of Listeria monocytogenes Cells Suspended in Bovine Milk

The thermotolerances of two different cell forms of Listeria monocytogenes (serotype 4b) grown at 37 and 42.8°C in commercially pasteurized and laboratory-tyndallized whole milk (WM) were investigated. Test strains, after growth at 37 or 42.8°C, were suspended in WM at concentrations of approximately 1.5 × 10⁸ to 3.0 × 10⁸ cells/ml and were then heated at 56, 60, and 63°C for various exposure times. Survival was determined by enumeration on tryptone-soya-yeast extract agar and Listeria selective agar, and D values (decimal reduction times) and Z values (numbers of degrees Celsius required to cause a 10-fold change in the D value) were calculated. Higher average recovery and higher D values (i.e., seen as a 2.5- to 3-fold increase in thermotolerance) were obtained when cells were grown at 42.8°C prior to heat treatment. A relationship was observed between thermotolerance and cell morphology of L. monocytogenes. Atypical Listeria cell types (consisting predominantly of long cell chains measuring up to 60 μm in length) associated with rough (R) culture variants were shown to be 1.2-fold more thermotolerant than the typical dispersed cell form associated with normal smooth (S) cultures (P ≤ 0.001). The thermal death-time (TDT) curves of R-cell forms contained a tail section in addition to the shoulder section characteristic of TDT curves of normal single to paired cells (i.e., S form). The factors shown to influence the thermoresistance of suspended Listeria cells (P ≤ 0.001) were as follows: growth and heating temperatures, type of plating medium, recovery method, and cell morphology. Regression analysis of nonlinear data can underestimate survival of L. monocytogenes; the end point recovery method was shown to be a better method for determining thermotolerance because it takes both shoulders and tails into consideration. Despite their enhanced heat resistance, atypical R-cell forms of L. monocytogenes were unable to survive the low-temperature, long-time pasteurization process when freely suspended and heated in WM.