Free-Living, Psychrotrophic Bacteria of the Genus Psychrobacter Are Descendants of Pathobionts

Host-adapted microorganisms are generally assumed to have evolved from free-living, environmental microorganisms, as examples of the reverse process are rare. In the phylum Gammaproteobacteria, family Moraxellaceae, the genus Psychrobacter includes strains from a broad ecological distribution including animal bodies as well as sea ice and other nonhost environments. To elucidate the relationship between these ecological niches and Psychrobacter's evolutionary history, we performed tandem genomic analyses with phenotyping of 85 Psychrobacter accessions. Phylogenomic analysis of the family Moraxellaceae reveals that basal members of the Psychrobacter clade are Moraxella spp., a group of often-pathogenic organisms. Psychrobacter exhibited two broad growth patterns in our phenotypic screen: one group that we called the "flexible ecotype" (FE) had the ability to grow between 4 and 37°C, and the other, which we called the "restricted ecotype" (RE), could grow between 4 and 25°C. The FE group includes phylogenetically basal strains, and FE strains exhibit increased transposon copy numbers, smaller genomes, and a higher likelihood to be bile salt resistant. The RE group contains only phylogenetically derived strains and has increased proportions of lipid metabolism and biofilm formation genes, functions that are adaptive to cold stress. In a 16S rRNA gene survey of polar bear fecal samples, we detect both FE and RE strains, but in in vivo colonizations of gnotobiotic mice, only FE strains persist. Our results indicate the ability to grow at 37°C, seemingly necessary for mammalian gut colonization, is an ancestral trait for Psychrobacter, which likely evolved from a pathobiont.IMPORTANCE Host-associated microbes are generally assumed to have evolved from free-living ones. The evolutionary transition of microbes in the opposite direction, from host associated toward free living, has been predicted based on phylogenetic data but not studied in depth. Here, we provide evidence that the genus Psychrobacter, particularly well known for inhabiting low-temperature, high-salt environments such as sea ice, permafrost soils, and frozen foodstuffs, has evolved from a mammalian-associated ancestor. We show that some Psychrobacter strains retain seemingly ancestral genomic and phenotypic traits that correspond with host association while others have diverged to psychrotrophic or psychrophilic lifestyles.

Psychrobacter fjordensis sp. nov., a psychrotolerant bacterium isolated from an Arctic fjord in Svalbard

A Gram-negative, non-motile, non-spore-forming, psychrotolerant and halotolerant bacterium designated BSw21516B(T), was obtained from seawater in Kongsfjorden, a glacial fjord in the Arctic Svalbard and subjected to taxonomic analysis using a polyphasic approach. This bacterium was observed to optimally grow at 25-29 °C; between at 4 and 34 °C, but not at >35 °C; and in the presence of 0-8 % (w/v) NaCl at an optimum concentration of 2-5 % (w/v) NaCl. Strain BSw21516B(T) was found to contain Ubiquinone-8 (Q-8) as a predominant respiratory lipoquinone and C18:1 ω9c and summed feature 3 (C16:1 ω7c and/or iso-C15:0 2-OH) as predominant cellular fatty acids. Phylogenetic analysis of 16S rRNA and gyrB gene sequences showed that this isolate belongs to the genus Psychrobacter and is closely related to Psychrobacter fozii LMG 21280(T), which was isolated from a sediment sample in Antarctica. DNA hybridization experiments revealed a low level of DNA-DNA relatedness (less than 58.6 %) between strain BSw21516B(T) and its closest relatives. Based on these results a new species Psychrobacter fjordensis sp. nov. is proposed (type strain BSw21516B(T) = KCTC 42279(T) = CCTCC AB 2014020(T)).

Psychrobacter oceani sp. nov., isolated from marine sediment

A novel marine bacterium, designated strain 4k5T, was isolated from a sediment sample of the Pacific Ocean. The strain was Gram-stain-negative, strictly aerobic, non-motile, oxidase-positive and catalase-positive and required Na+ for growth. Its major isoprenoid quinone was ubiquinone 8 (Q-8), and its cellular fatty acid profile consisted mainly of C18 : 1v9c (71.4 %), C16 : 1v7c (9.1 %) and C18 : 0. The DNA G+C content was 45.3 mol%. 16S rRNA gene sequence analysis suggested that strain 4k5T is a member of the genus Psychrobacter . Strain 4k5T exhibited the closely phylogenetic affinity to Psychrobacter pacificensis IFO 16270T (99.4 % 16S rRNA gene sequence similarity), P. piscatorii T-3-2T (97.7 %), P. nivimaris 88/2-7T (97.7 %), P. celer SW-238T (97.7 %), P. aestuarii SC35T (97.6 %) and P. vallis CMS39T (97.6 %). DNA–DNA hybridization between strain 4k5T and P. pacificensis NBRC 103191T, P. piscatorii JCM 15603T. P. nivimaris DSM 16093T, P. celer JCM 12601T, P. aestuarii JCM 16343T and P. vallis DSM 15337T was 42.5, 47.0, 38.1, 23.7, 9.0 and 27.4 %, respectively. Owing to the significant differences in phenotypic and chemotaxonomic characteristics, phylogenetic analysis based on the 16S rRNA gene sequence and DNA–DNA relatedness data, the isolate merits classification within a novel species, for which the name Psychrobacter oceani sp. nov. is proposed. The type strain is 4k5T ( = JCM 30235T = NCIMB 14948T).

Litorisediminicola beolgyonensis gen. nov., sp. nov., isolated from a coastal sediment

A Gram-negative, non-spore-forming, aerobic, non-flagellated and rod- or oval-shaped bacterial strain, BB-MW24T, was isolated from a coastal sediment in South Korea. Strain BB-MW24T grew optimally at 30–37 °C, at pH 7.0–7.5 and in the presence of 2.0–3.0 % (w/v) NaCl. A neighbour-joining phylogenetic tree based on 16S rRNA gene sequences revealed that strain BB-MW24T clustered with Ponticoccus litoralis CL-GR66T and Roseivivax lentus S5-5T, with which it exhibited 96.0 and 96.2 % 16S rRNA gene sequence similarity, respectively. Strain BB-MW24T exhibited 85.2 % gyrB sequence similarity with Sagittula stellata DSM 11524T and 83.3 and 83.2 % gyrB sequence similarity with P. litoralis DSM 18986T and R. lentus S5-5T, respectively. Strain BB-MW24T contained Q-10 as the predominant ubiquinone and C18 : 1ω7c as the predominant fatty acid. The polar lipid profile in strain BB-MW24T was similar to those of members of the genera Ponticoccus and Roseivivax , but one unidentified phospholipid was found as a major polar lipid only in strain BB-MW24T. The DNA G+C content was 68.9 mol%. The phylogenetic data and differential chemotaxonomic and phenotypic properties revealed that strain BB-MW24T represents a novel species in a new genus within the class Alphaproteobacteria , for which the name Litorisediminicola beolgyonensis gen. nov., sp. nov. is proposed; the type strain of Litorisediminicola beolgyonensis is BB-MW24T ( = KCTC 32139T  = CCUG 62953T).