We have developed a method, using laser, optical tweezers and direct microscopic analysis of reproductive potential and membrane integrity, to assess single-cell viability in a stationary-phase Escherichia coli population. It is demonstrated here that a reduction in cell integrity, determined by using the fluorescent nucleic acid stain propidium iodide, correlated well with a reduction in cell proliferating potential during the stationaryphase period studied. Moreover, the same cells that exhibited reduced integrity were found to be the ones that failed to divide upon nutrient addition. A small but significant number of the intact cells (496 of 7,466 [6.6%]) failed to replicate. In other words, we did not find evidence for the existence of a large population of intact but nonculturable cells during the stationary-phase period studied but it is clear that reproductive ability can be lost prior to the loss of membrane integrity. In addition, about 1% of the stationary-phase cells were able to divide only once upon nutrient addition, and in a few cases, only one of the two cells produced by division was able to divide a second time, indicating that localized cell deterioration, inherited by only one of the daughters, may occur. The usefulness of the optical trapping methodology in elucidating the mechanisms involved in stationary-phase-induced bacterial death and population heterogeneity is discussed.
In this paper, the authors present a survey of published research in mobile learning. The authors investigate 114 papers from mLearn 2005, 2007, and 2008, and classify them according to two dimensions: research method and research purpose. Research methods and purposes are important parts of how research is conducted. Opinions and approaches toward research differ greatly. The classified papers are evenly distributed among the research methods investigated, with one exception, there are few in basic research. In terms of research purpose, papers that describe research are well represented but there is a lack of papers targeting evaluation. Papers recounting both basic research and research evaluation are imperative, as they help a research field to mature and researchers to avoid repeating known pitfalls. This maturity, in turn, leads to better scalability and sustainability for future research efforts in the mobile learning community.
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