Microbiology

Peptidoglycan (PG) is essential for bacterial survival and maintaining cell shape. The rod-shaped model bacterium Escherichia coli has a set of seven endopeptidases that remodel the PG during cell growth. The gamma proteobacterium Candidatus Thiosymbion oneisti is also rod-shaped and attaches to the cuticle of its nematode host by one pole. It widens and divides by longitudinal fission using the canonical proteins MreB and FtsZ. The PG layer of Ca. T. oneisti has an unusually high peptide cross-linkage of 67% but relatively short glycan chains with an average length of 12 disaccharides. Curiously, it has only two predicted endopeptidases, MepA and PBP4. Cellular localization of symbiont PBP4 by fluorescently labeled antibodies reveals its polar localization and its accumulation at the constriction sites, suggesting that PBP4 is involved in PG biosynthesis during septum formation. Isolated symbiont PBP4 protein shows a different selectivity for β-lactams compared to its homologue from E. coli. Bocillin-FL binding by PBP4 is activated by some β-lactams, suggesting the presence of an allosteric binding site. Overall, our data point to a role of PBP4 in PG cleavage during the longitudinal cell division and to a PG that might have been adapted to the symbiotic lifestyle.

Summary Members of the bacterial order Rickettsiales are obligatorily associated with a wide range of eukaryotic hosts. Their evolutionary trajectories, in particular concerning the origin of shared or differential traits among distant sub‐lineages, are still poorly understood. Here, we characterized a novel Rickettsiales bacterium associated with the ciliate Paramecium tredecaurelia and phylogenetically related to the Rickettsia genus. Its genome encodes significant lineage‐specific features, chiefly the mevalonate pathway gene repertoire, involved in isoprenoid precursor biosynthesis. Not only this pathway has never been described in Rickettsiales , it also is very rare among bacteria, though typical in eukaryotes, thus likely representing a horizontally acquired trait. The presence of these genes could enable an efficient exploitation of host‐derived intermediates for isoprenoid synthesis. Moreover, we hypothesize the reversed reactions could have replaced canonical pathways for producing acetyl‐CoA, essential for phospholipid biosynthesis. Additionally, we detected phylogenetically unrelated mevalonate pathway genes in metagenome‐derived Rickettsiales sequences, likely indicating evolutionary convergent effects of independent horizontal gene transfer events. Accordingly, convergence, involving both gene acquisitions and losses, is highlighted as a relevant evolutionary phenomenon in Rickettsiales , possibly favoured by plasticity and comparable lifestyles, representing a potentially hidden origin of other more nuanced similarities among sub‐lineages.

Abstract The symbiosis between bacteria and sponges has arguably the longest evolutionary history for any extant metazoan lineage, yet little is known about bacterial evolution or adaptation in this process. An example of often dominant and widespread bacterial symbionts of sponges is a clade of uncultured and uncharacterised Proteobacteria. Here we set out to characterise this group using metagenomics, in-depth phylogenetic analyses, metatranscriptomics, and fluorescence in situ hybridisation microscopy. We obtained five metagenome-assembled-genomes (MAGs) from different sponge species that, together with a previously published MAG (AqS2), comprise two families within a new gammaproteobacterial order that we named Ca. Tethybacterales. Members of this order share a heterotrophic lifestyle but vary in their predicted ability to use various carbon, nitrogen and sulfur sources, including taurine, spermidine and dimethylsulfoniopropionate. The deep branching of the Ca. Tethybacterales within the Gammaproteobacteria and their almost exclusive presence in sponges suggests they have entered a symbiosis with their host relatively early in evolutionary time and have subsequently functionally radiated. This is reflected in quite distinct lifestyles of various species of Ca. Tethybacterales, most notably their diverse morphologies, predicted substrate preferences, and localisation within the sponge tissue. This study provides new insight into the evolution of metazoan–bacteria symbiosis.

The present study was carried out to clarify the taxonomic assignment of two closely related Amycolatopsis species. Genomic information for 48 type strains was available at the time of conducting this analysis. Our analysis showed that two species, viz. Amycolatopsis eurytherma Kim et al. 2002 and Amycolatopsis thermoflava Chun et al. 1999, are conspecific. The 16S rRNA gene sequences of the two species possess 98.85 % sequence similarity. Further, whole-genome comparisons showed that A. eurytherma DSM 44348T and A. thermoflava N1165T shared 98.75 % average nucleotide identity, 98.63 % average amino acid identity and 87.8 % digital DNA–DNA hybridization values. These values exceed the threshold values for bacterial species delineation, indicating that they belong to the same species. Further, the phylogenomic analysis based on the core genome of the strains under study confirmed that A. eurytherma DSM 44348T and A. thermoflava N1165T formed a monophyletic clade. Based on this evidence we propose the reclassification of Amycolatopsis eurytherma Kim et al. 2002 as a later heterotypic synonym of Amycolatopsis thermoflava Chun et al. 1999.