Diverse broad-host-range plasmids from freshwater carry few accessory genes.

Broad-host-range self-transferable plasmids are known to facilitate bacterial adaptation by spreading genes between phylogenetically distinct hosts. These plasmids typically have a conserved backbone region and a variable accessory region that encodes host-beneficial traits. We do not know, however, how well plasmids that do not encode accessory functions can survive in nature. The goal of this study was to characterize the backbone and accessory gene content of plasmids that were captured from freshwater sources without selecting for a particular phenotype or cultivating their host. To do this, triparental matings were used such that the only required phenotype was the plasmid's ability to mobilize a nonconjugative plasmid. Based on complete genome sequences of 10 plasmids, only 5 carried identifiable accessory gene regions, and none carried antibiotic resistance genes. The plasmids belong to four known incompatibility groups (IncN, IncP-1, IncU, and IncW) and two potentially new groups. Eight of the plasmids were shown to have a broad host range, being able to transfer into alpha-, beta-, and gammaproteobacteria. Because of the absence of antibiotic resistance genes, we resampled one of the sites and compared the proportion of captured plasmids that conferred antibiotic resistance to their hosts with the proportion of such plasmids captured from the effluent of a local wastewater treatment plant. Few of the captured plasmids from either site encoded antibiotic resistance. A high diversity of plasmids that encode no or unknown accessory functions is thus readily found in freshwater habitats. The question remains how the plasmids persist in these microbial communities.

General bidirectional thermal characterization via the 3ω technique.

The 3ω technique has become a popular method for determining the thermophysical properties of microscale and bulk materials. The prerequisite fabrication of a highly linear metal line a few hundred nanometers thick on the sample can be a failing point in specific material systems. This difficulty can be overcome by utilizing a bidirectional experimental geometry that employs a contact resistance between the sample and heating wire, which also allows for data collection under varying axial pressure loads. In this work, such a system is demonstrated with an emphasis on developing a thermal mount that will optimize sensitivity to the thermophysical parameters of interest: the sample's thermal conductivity, volumetric heat capacity, and the contact resistance between the sample and mount. A general thermal model is presented that can be simplified to analyze nearly any similar system. This model is then employed to analyze a sample in the mounting scheme described with varying applied pressures to demonstrate the general feasibility of the system.