A growing body of circumstantial evidence suggests that ice nucleation active (Ice) bacteria contribute to the initiation of precipitation by heterologous freezing of super-cooled water in clouds. However, little is known about the concentration of Ice bacteria in precipitation, their genetic and phenotypic diversity, and their relationship to air mass trajectories and precipitation chemistry. In this study, 23 precipitation events were collected over 15 months in Virginia, USA. Air mass trajectories and water chemistry were determined and 33 134 isolates were screened for ice nucleation activity (INA) at -8 °C. Of 1144 isolates that tested positive during initial screening, 593 had confirmed INA at -8 °C in repeated tests. Concentrations of Ice strains in precipitation were found to range from 0 to 13 219 colony forming units per liter, with a mean of 384±147. Most Ice bacteria were identified as members of known and unknown Ice species in the Pseudomonadaceae, Enterobacteriaceae and Xanthomonadaceae families, which nucleate ice employing the well-characterized membrane-bound INA protein. Two Ice strains, however, were identified as Lysinibacillus, a Gram-positive genus not previously known to include Ice bacteria. INA of the Lysinibacillus strains is due to a nanometer-sized molecule that is heat resistant, lysozyme and proteinase resistant, and secreted. Ice bacteria and the INA mechanisms they employ are thus more diverse than expected. We discuss to what extent the concentration of culturable Ice bacteria in precipitation and the identification of a new heat-resistant biological INA mechanism support a role for Ice bacteria in the initiation of precipitation.

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LysR-type transcriptional regulators (LTTRs) are the most common type of transcriptional regulators in prokaryotes and function by altering gene expression in response to environmental stimuli. In the class Alphaproteobacteria, a conserved LTTR named VtlR is critical to the establishment of host-microbe interactions. In the mammalian pathogen Brucella abortus, VtlR is required for full virulence in a mouse model of infection, and VtlR activates the expression of abcR2, which encodes a small regulatory RNA (sRNA). In the plant symbiont Sinorhizobium meliloti, the ortholog of VtlR, named LsrB, is involved in the symbiosis of the bacterium with alfalfa. Agrobacterium tumefaciens is a close relative of both B. abortus and S. meliloti, and this bacterium is the causative agent of crown gall disease in plants. In the present study, we demonstrate that VtlR is involved in the ability of A. tumefaciens to grow appropriately in artificial medium, and an A. tumefaciens vtlR deletion strain is defective in motility, biofilm formation, and tumorigenesis of potato discs. RNA-sequencing analyses revealed that more than 250 genes are dysregulated in the ∆vtlR strain, and importantly, VtlR directly controls the expression of three sRNAs in A. tumefaciens. Taken together, these data support a model in which VtlR indirectly regulates hundreds of genes via manipulation of sRNA pathways in A. tumefaciens, and moreover, while the VtlR/LsrB protein is present and structurally conserved in many members of the Alphaproteobacteria, the VtlR/LsrB regulatory circuitry has diverged in order to accommodate the unique environmental niche of each organism.

As part of an undergraduate microbiology course, a yellow-orange pigmented, Gram-staining negative, rod-shaped, non-motile bacterial strain, designated CTM(T), was isolated from a creek in North-central Pennsylvania during the winter of 2006. The 16S rRNA gene sequence of the strain showed ~97 % similarity to that of Chryseobacterium soldanellicola PSD1-4(T) and Chryseobacterium soli JS6-6(T), while the protein-coding gyrB gene sequence of strain CTM(T) showed <87 % similarity to those of its two closest relatives. Using a polyphasic approach, strain CTM(T) was characterized and compared to these and other closely related species of the genus Chryseobacterium. Strain CTM(T) was similar to other strains of the genus Chryseobacterium in that it contained MK-6 as its major respiratory quinone, produced flexirubin-type pigments, oxidase and catalase, hydrolysed DNA, gelatin and aesculin and contained the fatty acids iso-C₁₅:₀, iso-C₁₇:₁ω9c, iso-C₁₇:₀ 3-OH and summed feature 3 (C₁₆:₁ω6c, C₁₆:₁ω7c and/or iso-C₁₅:₀ 2-OH). Based on the results of this study, strain CTM(T) represents a novel species of the genus Chryseobacterium, for which the name Chryseobacterium piperi sp. nov. is proposed. The type strain is CTM(T) ( = ATCC BAA-1782(T)  = CCUG 57707(T)  = JCM 15960(T)  = DSM 22249(T)  = KCTC 23267(T)).

Microbes in the atmosphere have broad ecological impacts, including the potential to trigger precipitation through species and strains that act as ice nucleation particles. To characterize spatiotemporal trends of microbial assemblages in precipitation we sequenced 16S (bacterial) and 18S (fungal) rRNA gene amplicon libraries collected from 72 precipitation events in three U.S. states (Idaho, Louisiana, and Virginia) over four seasons. We considered these data from the perspective of a novel metacommunity framework. In agreement with our heuristic, we found evidence for distinct mechanisms underlying the composition and diversity of bacterial and fungal assemblages in precipitation. Specifically, we determined that (1) bacterial operational taxonomic unit (OTU) composition of precipitation was strongly associated with macroscale drivers including season and high-altitude characteristics of storms; (2) fungal OTU composition was strongly correlated with mesoscale drivers including particular spatial locations; (3) b-diversity (heterogeneity of taxa among samples) for both bacteria and fungi was largely maintained by turnover of taxa; however, (4) bacterial assemblages had higher contributions to total b-diversity from nestedness (i.e., lower richness assemblages were largely taxonomic subsets of richer assemblages), due to losses of taxa during dispersal, particularly among potential ice nucleation active bacteria; and (5) fungal assemblages had higher contributions to total b-diversity from turnover due to OTU replacement. Spatiotemporal trends in precipitation-borne metacommunities allowed delineation of a large number of statistically significant indicator taxa for particular sites and seasons, including trends for bacteria that are potentially ice nucleation active. Our findings advance understanding regarding the dispersion of aerosolized microbes via wet deposition, and the development of theory concerning potential assembly rules for bioaerosol assemblages.

As part of an undergraduate microbiology course, a yellow-orange-pigmented, Gram-staining negative, rod-shaped, non-motile bacterial strain was isolated from a glass tank housing several red-spotted newts (Notophthalmus viridescens). The sequence of the 16S rRNA gene of this strain, designated KM T , was 97.4-98.0 % similar to those of the type strains of Chryseobacterium luteum, C. shigense and C. vrystaatense, while the similarity levels for protein-coding genes were less than 94.7 % for rpoB, less than 92.1 % for groEL and less than 87.1 % for gyrB. These values are lower than for many other established distinct species. Polyphasic characterization and comparison to these relatives revealed that strain KM T was similar to other Chryseobacterium strains in that it contained MK-6 as its major respiratory quinone and phosphatidylethanolamine as the most abundant polar lipid, produced flexirubin-type pigments, oxidase and catalase and primarily contained the fatty acids iso-C 15 : 0 , iso-C 17 : 1 v9c, iso-C 17 : 0 3-OH and summed feature 3 (comprising C 16 : 1 v6c and/or C 16 : 1 v7c). Based on the results of this study, strain KM T represents a novel species, for which the name Chryseobacterium angstadtii sp. nov. is proposed. The type strain is KM T (5ATCC

On a subfreezing surface, nucleating embryos usually form as supercooled condensate that later freezes into ice, as opposed to desublimation. Ice nucleating proteins (INPs) have been widely used to quickly freeze existing supercooled water; however, nobody has studied how they might affect the initial mode of embryo formation. We show that INPs deposited on a substrate can switch the mode of embryo nucleation to desublimation, rather than supercooled condensation, beneath a critical temperature. By patterning a hydrophobic surface with an array of hydrophilic stripes, the INPs can be selectively deposited by evaporating water that exclusively spreads along the hydrophilic regions. The resulting array of desublimating ice stripes created dry zones free of condensation or frost in the intermediate areas, as the hygroscopic ice stripes served as overlapping humidity sinks.

Earth’s radiation budget and frequency and intensity of precipitation are influenced by aerosols with ice nucleation activity (INA), i.e., particles that catalyze the formation of ice. Some bacteria, fungi, and pollen are among the most efficient ice nucleators but the molecular basis of INA is poorly understood in most of them. Lysinibacillus parviboronicapiens (Lp) was previously identified as the first Gram-positive bacterium with INA. INA of Lp is associated with a secreted, nanometer-sized, non-proteinaceous macromolecule or particle. Here a combination of comparative genomics, transcriptomics, and a mutant screen showed that INA in Lp depends on a type I iterative polyketide synthase and a non-ribosomal peptide synthetase (PKS-NRPS). Differential filtration in combination with gradient ultracentrifugation revealed that the product of the PKS-NRPS is associated with secreted particles of a density typical of extracellular vesicles and electron microscopy showed that these particles consist in “pearl chain”-like structures not resembling any other known bacterial structures. These findings expand our knowledge of biological INA, may be a model for INA in other organisms for which the molecular basis of INA is unknown, and present another step towards unraveling the role of microbes in atmospheric processes.