| TM7 bacteria,also known as Saccharibacteria,are a kind of bacteria widely found in some mammalian associated and natural habitats,and have been termed as "the microbial dark matter"due to their recalcitrance to cultivation.Being a candidate phylum,they belong to the candidate phyla radiation(CPR)group,and have been recognized the importance in CPR group due to the recent success in isolation and cultivation of some TM7 strains.At present,all the cultured TM7 strains are parasitic bacteria,and mostly from human oral samples.Only one TM7 strain originated from natural habitat was accidentally isolated from the wastewater activated sludge samples.Considering a large number of uncultured TM7 bacteria existed in the natural environment,the systematic analysis of their population and habitat diversity,and the culture of the strains are of great significance for the study of both TM7 and other CPR bacteria.First,the 16S rRNA gene information in SILVA database and 41 genomes sequences in NCBI database were used to systematically analyze the population diversity of TM7 bacteria.The results showed that the composition of TM7 group was very complex,and there was a certain correlation between the habitat and phylogeny.TM7 bacteria from different habitats had great differences in the composition of genome and functional genes,and they may differentiate to varying evelutionary branches in the process of environmental adaptation.TM7 bacteria from plant rhizosphere or soil were in different evolutionary branches with mammalian related groups,and the former had larger genome and more functional genes,making it more adaptable to the complex natural environment.At the same time,the difference of TM7 bacterial genomes in different habitats revealed the diversified metabolic potential of this bacterial group,suggesting that they may interact with other bacterias in a unique way to maintain its survival.Next,the distribution of TM7 bacteria in different habitats around the world was analyzed by using the earth microbiome project database.The results showed that although TM7 bacteria were of relatively low abundance in prokaryotic community,they were widely distributed all over the world and identified in all 16 typical habitats except hypersaline environment.Among them,the fresh water,soil,plant rhizosphere and animal secretion are the major living habitats for TM7 bacteria.At the same time,linear discriminant analysis was conducted to predict the the Actinomycetes related to the high abundance TM7 bacteria in different habitats,which provided a certain reference for subsequent strain isolation.Finally,we tried to isolate and cultivate TM7 strains from different environmental samples.According to the bioinformatical results,several samples were collected from plant rhizosphere soil and fresh water,and analyzed the existance of TM7 bacteria by 16S rRNA gene full length sequencing,and positive samples were selected for the subsequent isolation trial.Different methods,e.g.,the enrichment protocol and host bait isolation,were employed,and some microbacterial cells were observed in SEM4 coculture system containing Acinetobacter johnsonii and Stenotrophomonas maltophilia.They were micrococci in shape with a diameter of 200 to 400 nm,which was similar with the size of TM7 bacteria.Unfortunately,these micrococcous cells were not able to exist stably under the current culture conditions,and nor to be isolated and cultured.To sum up,the current thesis makes a comprehensive analysis of the population and habitat diversity of TM7 bacteria,and explores the isolation and culture of TM7 strains from the natural environment,which provides important information and reference for the subsequent research. |
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