Mixed continuous culture experiments with an antibiotic-producing streptomycete andEscherichia coli

Four strains ofStreptomyces aureofaciens capable of producing different amounts of tetracycline have been grown in continuous culture with either of two strains ofEscherichia coli orBacillus pumillus. Each of the bacteria had faster specific growth rates than any of the streptomycetes.E. coli NTCT 5993 had a higher affinity for sucrose than didS. aureofaciens SR 11. In mixed culture experiments, the bacteria displaced the streptomycete under the following conditions: (a) when the streptomycete produced no tetracycline; (b) in tetracycline-producing cultures after a fraction of the bacteria had died—the bacterial population that subsequently developed was resistant to the drug; and (c) when using tetracycline-resistant bacteria. Under nutrient conditions leading to high antibiotic levels, it was possible to kill all the bacteria, and the streptomycete survived after a transient fall in mycelium and tetracycline levels. No stable mixed population was ever seen. Once tetracycline-resistant bacteria had displaced the streptomycete, and tetracycline concentrations had fallen, a sensitive bacterial population reappeared. Competition between sensitive and resistantE. coli NCTC 5993 in the chemostat confirmed the selective advantage of the sensitive strain. Results were discussed in terms of the role of antibiotics in nature.     

Levels of Nicotinamide Adenine Dinucleotide and Reduced Nicotinamide Adenine Dinucleotide in Facultative Bacteria and the Effect of Oxygen

Nicotinamide adenine dinucleotide (NAD) and reduced NAD (NADH) levels have been measured in bacterial cultures. The cofactors were assayed by using the very sensitive cycling assay described previously by Cartier. Control experiments showed that the level of total NAD(H) falls during harvesting, and so samples were taken quickly from growing cultures and extracted immediately without separating the cells from the medium. Total NAD(H) ranged from 4.0 to 11.7 mumoles/g of dry cells for three facultative organisms, Klebsiella aerogenes, Escherichia coli, and Staphylococcus albus. NADH was remarkably constant in these bacteria; only one out of ten series of determinations was outside the range 1.4 to 1.9 mumoles/g of dry cells. NAD(+) showed much greater variation. An anaerobe (Clostridium welchii) had significantly more total NAD(H) whereas an aerobe Pseudomonas aeruginosa had about as much NAD(H) as the facultative organisms. NAD and NADH were measured during growth: once more NADH was much more constant than NAD. During change-over between aerobiosis and anaerobiosis, NADH showed a temporary increase but then returned to a constant level, whereas NAD changed from high aerobically to low anaerobically. These results are discussed in terms of the control mechanisms that may be involved.     

Molecular phylogeny of the major arthropod groups indicates polyphyly of crustaceans and a new hypothesis for the origin of hexapods

A phylogeny of the arthropods was inferred from analyses of amino acid sequences derived from the nuclear genes encoding elongation factor-1 alpha and the largest subunit of RNA polymerase II using maximum- parsimony, neighbor-joining, and maximum-likelihood methods. Analyses of elongation factor-1 alpha from 17 arthropods and 4 outgroup taxa recovered many arthropod clades supported by previous morphological studies, including Diplopoda, Myriapoda, Insecta, Hexapoda, Branchiopoda (Crustacea), Araneae, Tetrapulmonata, Arachnida, Chelicerata, and Malacostraca (Crustacea). However, counter to previous studies, elongation factor-1 alpha placed Malacostraca as sister group to the other arthropods. Branchiopod crustaceans were found to be more closely related to hexapods and myriapods than to malacostracan crustaceans. Sequences for RNA polymerase II were obtained from 11 arthropod taxa and were analyzed separately and in combination with elongation factor-1 alpha. Results from these analyses were concordant with those derived from elongation factor-1 alpha alone and provided support for a Hexapoda/Branchiopoda clade, thus arguing against the monophyly of the traditionally defined Atelocerata (Hexapoda + Myriapoda).      

Heterogeneity in biofilms

Biofilms, accumulations of microorganisms at interfaces, have been described for every aqueous system supporting life. The structure of these microbial communities ranges from monolayers of scattered single cells to thick, mucous structures of macroscopic dimensions (microbial mats; algal-microbial associations; trickling filter biofilms). During recent years the structure of biofilms from many different environments has been documented and evaluated by use of a broad variety of microscopic, physico-chemical and molecular biological techniques, revealing a generally complex 3D structure. Parallel to these investigations more and more complex mathematical models and simulations were developed to explain the development, structures, and interactions of biofilms. The forces determining the spatial structure of biofilms, including microcolonies, extracellular polymeric substances (EPS), and channels, are still the subject of controversy. To achieve conclusive explanations for the structures observed in biofilms the cooperation of both fields of investigation, modelling and experimental research, is necessary. The expanding field of molecular techniques not only allows more and more detailed documentation of the spatial distribution of species, but also of functional activities of single cells in their biofilm environment. These new methods will certainly reveal new insights in the mechanisms involved in the developmental processes involved in the formation and behavior of biofilms.     

In vivo analysis of fracture toughness of thyroid gland tumors

Background

Human solid tumors that are hard or firm on physical palpation are likely to be cancerous, a clinical maxim that has been successfully applied to cancer screening programs, such as breast self-examination. However, the biological relevance or prognostic significance of tumor hardness remains poorly understood. Here we present a fracture mechanics based in vivo approach for characterizing the fracture toughness of biological tissue of human thyroid gland tumors.

Methods

In a prospective study, 609 solid thyroid gland tumors were percutaneously probed using standard 25 gauge fine needles, their tissue toughness ranked on the basis of the nature and strength of the haptic force feedback cues, and subjected to standard fine needle biopsy. The tumors' toughness rankings and final cytological diagnoses were combined and analyzed. The interpreting cytopathologist was blinded to the tumors' toughness rankings.

Results

Our data showed that cancerous and noncancerous tumors displayed remarkable haptically distinguishable differences in their material toughness.

Conclusion

The qualitative method described here, though subject to some operator bias, identifies a previously unreported in vivo approach to classify fracture toughness of a solid tumor that can be correlated with malignancy, and paves the way for the development of a mechanical device that can accurately quantify the tissue toughness of a human tumor.     

Structural origins of fibrin clot rheology

The origins of clot rheological behavior associated with network morphology and factor XIIIa-induced cross-linking were studied in fibrin clots. Network morphology was manipulated by varying the concentrations of fibrinogen, thrombin, and calcium ion, and cross-linking was controlled by a synthetic, active-center inhibitor of FXIIIa. Quantitative measurements of network features (fiber lengths, fiber diameters, and fiber and branching densities) were made by analyzing computerized three-dimensional models constructed from stereo pairs of scanning electron micrographs. Large fiber diameters and lengths were established only when branching was minimal, and increases in fiber length were generally associated with increases in fiber diameter. Junctions at which three fibers joined were the dominant branchpoint type. Viscoelastic properties of the clots were measured with a rheometer and were correlated with structural features of the networks. At constant fibrinogen but varying thrombin and calcium concentrations, maximal rigidities were established in samples (both cross-linked and noncross-linked) which displayed a balance between large fiber sizes and great branching. Clot rigidity was also enhanced by increasing fiber and branchpoint densities at greater fibrinogen concentrations. Network morphology is only minimally altered by the FXIIIa-catalyzed cross-linking reaction, which seems to augment clot rigidity most likely by the stiffening of existing fibers.