The emergence of bacterial antibiotic resistance is a growing problem, yet the variables that influence the rate of emergence of resistance are not well understood. In a microfluidic device designed to mimic naturally occurring bacterial niches, resistance of Escherichia coli to the antibiotic ciprofloxacin developed within 10 hours. Resistance emerged with as few as 100 bacteria in the initial inoculation. Whole-genome sequencing of the resistant organisms revealed that four functional single-nucleotide polymorphisms attained fixation. Knowledge about the rapid emergence of antibiotic resistance in the heterogeneous conditions within the mammalian body may be helpful in understanding the emergence of drug resistance during cancer chemotherapy.

We show here that upconversion phosphors can be imaged both by infrared excitation and in a scanning electron microscope. We have synthesized and characterized for this work up-converting phosphor nanoparticles nonaggregated nanocrystals of size range 50-200 nm. We have investigated the optical properties of 50-200 nm nanoparticles and found a square dependence of the emitted visible fluorescence on the infrared excitation and verified that under electron excitation similar narrow band emission spectra can be obtained as is seen with IR upconversion. The viability of the nanoparticles for biological imaging was confirmed by imaging the digestive system of the nematode worm Caenorhabditis elegans, and we have confirmed using energy-dispersive X-ray analysis that the up-conversion nanoparticles can be identified in a scanning electron microscope at high spatial resolution.

The copy number of any protein fluctuates among cells in a population; characterizing and understanding these fluctuations is a fundamental problem in biophysics. We show here that protein distributions measured under a broad range of biological realizations collapse to a single non-Gaussian curve under scaling by the first two moments. Moreover in all experiments the variance is found to depend quadratically on the mean, showing that a single degree of freedom determines the entire distribution. Our results imply that protein fluctuations do not reflect any specific molecular or cellular mechanism, and suggest that some buffering process masks these details and induces universality.The protein content of a cell is a primary determinant of its phenotype. However, protein copy number is subject to large cell-to-cell variation even among genetically identical cells grown under uniform conditions ([1-3] and references therein). This variation has been the subject of intensive research in recent years ([4-7] and references therein). Much of this previous work was devoted to characterizing the stochastic properties of various processes underlying gene expression, such as transcription and translation [8], or different stages of the cell cycle [9], and understanding their effect on protein variation. However, gene expression is generally coupled to all aspects of cell physiology, such as growth [10], metabolism [11], aging [12], division [13,14] and epigenetic processes [15,16], as well as gene location and function [17], all of which were shown to affect protein variation. The emerging picture is of a plethora of correlated mechanisms at different levels of organization; how they integrate to shape the total protein variation in a dividing population remains an open question [11,14].In this work we addressed this question by a phenomenological approach. We measured distributions of highly expressed proteins in proliferating clonal populations of bacteria and yeast under natural conditions, where gene expression is coupled to other cellular processes. By designing an array of different metabolic and regulatory conditions as well as growth environments, we collected a compendium of measurements which systematically covers the major processes of gene expression and 2 cell division, and compared the measured distributions in a wide range of biological realizations. More specifically, our comparisons included: (a) Two archetypical microorganisms, bacteria and yeast, with two well-studied regulatory systems of essential metabolic pathways: the LAC operon in E. coli [18] and the GAL system in S. cerevisiae [19]. Both systems were studied under environmental conditions in which expression is strongly coupled to metabolism, namely they control the utilization of an essential sugar (lactose and galactose, respectively) as the sole carbon The spectrum of our experiments spans an array of "control parameters" p which covers many of the essential processes affecting protein content in cells. The two organisms used, E. coli an...

From flocking birds to swarming insects, interactions of organisms large and small lead to the emergence of collective dynamics. Here, we report striking collective swimming of bovine sperm in dynamic clusters, enabled by the viscoelasticity of the fluid. Sperm oriented in the same direction within each cluster, and cluster size and cell-cell alignment strength increased with viscoelasticity of the fluid. In contrast, sperm swam randomly and individually in Newtonian (nonelastic) fluids of low and high viscosity. Analysis of the fluid motion surrounding individual swimming sperm indicated that sperm-fluid interaction was facilitated by the elastic component of the fluid. In humans, as well as cattle, sperm are naturally deposited at the entrance to the cervix and must swim through viscoelastic cervical mucus and other mucoid secretions to reach the site of fertilization. Collective swimming induced by elasticity may thus facilitate sperm migration and contribute to successful fertilization. We note that almost all biological fluids (e.g. mucus and blood) are viscoelastic in nature, and this finding highlights the importance of fluid elasticity in biological function.