Heat resistance of Listeria: strain differences and effects of meat type and curing salts

The heat resistances of 27 strains of Listeria monocytogenes and two strains of L. innocua were compared in broth heated at 57°C. No strain was exceptionally resistant. The heat resistance of a representative isolate of L. monocytogenes was compared in fresh and cured beef and chicken, and an equation was derived to predict the time necessary to achieve a '7D' inactivation at temperatures between 50 and 70°C.     

Elevation of the heat resistance of Salmonella typhimurium by sublethal heat shock

The survival of Salmonella typhimurium after a standard heat challenge at 55°C for 25 min increased by several orders of magnitude when cells grown at 37°C were pre-incubated at 42°, 45° or 48°C before heating at the higher temperature. Heat resistance increased rapidly after the temperature shift, reaching near maximum levels within 30 min. Elevated heat resistance persisted for at least 10 h. Preincubation of cells at 48°C for 30 min increased their resistance to subsequent heating at 50°, 52°, 55°, 57° or 59°C. Survival curves of resistant cells were curvilinear. Estimated times for a '7D' inactivation increased by 2.6- to 20-fold compared with cells not pre-incubated before heat challenge.     

Determinants of time-dependent membrane conductance. The nonrole of classical ion-membrane molecule interactions

We have examined the steady-state and time-dependent electrical properties of a model membrane system. The model assumes that the directed velocity and energy of ions moving through the membrane are determined by the applied electric field, ionic diffusion forces, and central elastic collisions between ions and membrane molecules. A simple analysis of the steady-state electrical properties of the model yields results identical with ones obtained previously using a more complex analysis procedure. The time-dependent conductance changes of the model in response to a step change in electric field strength when there is solution symmetry display three qualitative patterns dependent on the nature of the ion-membrane molecule interaction. One of the patterns of conductance change is quite similar to that observed in the sodium conductance system of a number of excitable tissues: an initial conductance rise to a maximum (activation) followed by a decay to a final steady-state value (inactivation). However, the correspondence between the time-dependent model behavior and known experimental behavior of excitable systems is only qualitative. We conclude that the classical ion-membrane molecule interactions we consider are not involved in determining time-dependent conductance processes in the excitable systems for which comparison is possible.     

The effect of high hydrostatic pressure on Salmonella thompson and Listeria monocytogenes examined by electron microscopy

Cells of Listeria monocytogenes that had been exposed to pressure contained vacuolar regions in the cytoplasm. Pressure-treated cells of Salmonella thompson contained no vacuoles but had fewer ribosomes than untreated cells and their appearance suggested that some cell lysis had occurred. In both organisms changes in the appearance of the nuclear material were observed.     

Evolution of the ATP-binding-cassette transmembrane transporters of vertebrates

The ATP-binding-cassette transmembrane transporters (ABC transporters) known from vertebrates belong to four major subfamilies: (1) the P- glycoproteins (Pgp); (2) the cystic fibrosis transmembrane conductance regulators (CFTR); (3) the Tap proteins encoded with the major histocompatibility complex of mammals; and (4) the peroxisomal membrane proteins. Both Pgp and CFTR have a structure suggesting a past internal gene duplication; a phylogenetic analysis indicated that these duplications occurred independently, while an independent tandem gene duplication occurred in the case of the Tap family. Both the Pgp and Tap proteins show evidence of relationship to bacterial ABC transporters lacking internal duplication, and both are significantly more closely related to the HlyB and MsbA families of transporters from purple bacteria than they are to ABC transporters from nonpurple bacteria. The simplest hypothesis to explain this observation is that eukaryotic Pgp and Tap genes are descended from a mitochondrial gene or genes that were subsequently translocated to the nuclear genome. The Pgp genes of eukaryotes are characterized by a remarkable degree of convergent evolution between the ATP-binding cassettes of their N- terminal and C-terminal halves, whereas no such convergence is seen between the two halves of CFTR genes or between the duplicated Tap genes. Exon 13 of the CFTR gene, which encodes a putative regulatory domain not found in other ABC transporters apart from CFTR, showed high levels of both synonymous and nonsynonymous difference in comparisons among different mammalian species, suggesting that this region is a mutational hot spot.      

Thermal denaturation and loss of viability in Escherichia coli and Bacillus stearothermophilus

When Escherichia coli was heated at 10°C/min in a differential scanning calorimeter, the onset of irreversible thermal denaturation occurred at 51°C, about 5°C above the maximum growth temperature. The temperature at which death rate was maximal (63°C) coincided with the thermogram peak caused by denaturation of the 30S ribosomal subunit. The maximum death rate in vegetative cells of Bacillus stearothermophilus occurred at the higher temperature of 71°C which also coincided with the leading edge of the main thermogram peak.     

Evidence for the presence of β-lactamase inStreptomyces glaucescens and its inhibition by sodium clavulanate

Streptomyces glaucescens is shown to possess -lactamase activity which is inhibitable by clavulanate. This is important in regard to its use as a cloning host for enzymes of \-lactam biosynthesis.