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Resistance to streptomycin in a producing strain of Streptomyces griseus
No full textStreptomyces griseus S104 was sensitive to streptomycin during exponential growth in a medium which, in the subsequent stationary phase, supported production of the antibiotic in yields above 200 μg/ml. When antibiotic production began cultures developed a tolerance toward their lethal metabolite. This was not due to an increase in pH associated with antibiotic production, since pH effects on streptomycin sensitivity in S. griseus were in the reverse direction.
However, the degree of tolerance was directly related to the amount of cell materia1 present. Streptomycin production caused no change in the proportion of resistant variants in the population, nor did it cause the severe inhibition of protein synthesis observed in non-producing cultures exposed to the antibiotic. The lack of an effect on protein synthesis is attributed to the absence of streptomycin within the cytoplasm since soluble extracts from myceliurn harvested in the
production phase were inactive when bio-assayed immediately after cell disuptionH. however, they developed antibacterial activity rapidly when heated, and more slowly when incubated at 25°C. The addition of phosphatase inhibitors during incubation prevented the appearance of antibiotic activitv, and it was concluded that a small amount of streotomvcin phosphate is present in the rnycelium during antibiotic production. Differences in (14C) streotomvcin uptake suggested
that the myceliurn was appreciably less permeable to the antibiotic in the production phase than during exponential growth. However, a small amount was taken up and much of it was in the soluble fraction of disrupted cells. Bioassays showed that this 14C-labeled antibiotic within the cells had been partially inactivated, suggesting that conversion of streptomycin to an inactive derivative is involved in the mechanism which protects the organism from its metabolite
Action of streptomycin on the growth of Streptomyces griseus
No full textA sterptomycin-producing culture of Streptomyces griseus was sensitive to streptomycin, but inhibition was temporary, and cultures supplemented with the antibiotic up to 100 microgram/ml grew after a lag. Streptomycyn tolerance developed, not by inactivation of the antibiotic but by selection of resistant variants in the natural population. Addition of streptomycin to growing cultures caused a drastic reduction in protein synthesis and an accumulation of "stuck" (70 S) ribosomes within the mycelium. Although the effect on protein synthesis could not be confirmed in vitro because cell extracts fro S. griseus contained an inhibitor of plyuridilic-acid-directed polymerization of L-phenylalanine, it is concluded that streptomycin prevents the growth of this organism by a mechanism similar to that observed with other bacteria and that the tolerance of producing cultures cannot be attributed to the presence of streptomycin-resistant ribosome
Streptomycin resistance in strains of Streptomyces species 3022a
No full textResistance to low (5 μg/ml) concentrations of streptomycin in agar media was not inherited by all of the surviving population. Outgrowth of cultures in liquid media supplemented with the antibiotic depended upon inoculum size. Antibiotic titers in the supplemented cultures ecreased during incubation, and an inactive radioactive product was detected when [I4C] streptomycin was used. This low-level resistance is, therefore, attributed to enzymic inactivation of the antibiotic.
Growth in 10 pgld or higher concentrations of streptomycin on agar media was due to selection of resistant variants present in the parent strain. A range of such variants existed, decreasing in frequency as their degree of resistance increased. Examination of one that was resistant to moderate concentrations of streptomycin, (25 μg/ml) and a second that was resistant to high (100 μg/ml) concentrations of streptomycin suggested that both possessed ribosomes which had lower affinity for the antibiotic than those of the parent strain, and that tolerance to high levels of streptomycin was due to a resistant ribosomal system for protein biosynthesis
Genetic recombination in Streptomyces species 3022a
No full textMutation with ultraviolet light and chemical mutagens yielded strains with auxotrophic and streptomycin-resistance markers. These were used to show the existence of genetic recombination in Streptomyces species 3022a and to construct a linkage map for 17 marker loci
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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