Sunday, 1 March 2020

Rubinisphaera italica: A new species of Planctomycete Bacteria from the Tyrrhenian Sea.

Together with the Verrucomicrobia, Lentisphaerae, Kirimatiellaeota and Chlamydiae, the Planctomycetes form the medically and biotechnologically relevant PVC superphylum of Bacteria. In the past, the Planctomycetes were postulated as the missing link between Bacteria and Eukaryotes, and to be beyond the bacterial cell plan. This concept was based on proposed exceptional Planctomycetal features, such as, a lack of peptidoglycan (a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most Bacteria), a compartmentalised cell plan, a nucleus-like structure, and the performance of endocytosis (the taking in of matter by a living cell by invagination of its membrane to form a vacuole). Further investigation of the Planctomycetal physiology and morphology based on the advent of novel techniques changed this picture. In particular, presence of peptidoglycan in some Planctomycetes was confirmed in 2015. But still, Planctomycetes remain exceptional and enigmatic in comparison to well-characterised typical Bacteria. For example, they divide unusually, either by budding, binary fission or even a combination of both, and lack canonical divisome proteins including the otherwise universal FtsZ (a protein that assembles into a ring at the future site of bacterial cell division). Many Planctomycetes survive in oligotrophic environments, such as seawater, by utilising a range of high-molecular- weight sugars derived from Algae after attaching to these nutrient-rich surfaces. It is frequently observed that Planctomycetes are highly abundant in biofilms on nutrient-rich marine surfaces. This is astonishing when considering their moderate growth rates compared to faster-growing competitors in this ecological niche. It is thus likely that Planctomycetes are ‘talented’ producers of secondary metabolites, which could mediate (symbiotic) interactions with Algae or act as antibiotics. Taken together, Planctomycetes are amongst the most maverick of all Bacteria known thus far.

In a paper published in the journal Antonie van Leeuwenhoek on 26 November 2019, Nicolai Kallscheuer of the Department of Microbiology at Radboud Universiteit, Mareike Jogler, also of the Department of Microbiology at Radboud Universiteit, and of the Leibniz Institute German Collection of Microorganisms and Cell Cultures, Sandra Wiegand and Stijn Peeters, again of the Department of Microbiology at Radboud Universiteit, Anja Heuer and Christian Boedeker, also of the Leibniz Institute German Collection of Microorganisms and Cell Cultures, Mike Jetten, again of the Department of Microbiology at Radboud Universiteit, Manfred Rohde of the Central Facility for Microscopy at the Helmholtz Centre for Infection Research, and Christian Jogler, once again of the Department of Microbiology at Radboud Universiteit, describe a new species of Planctomycete Bacteria from the Tyrrhenian Sea. 

The specimens from which the new species are described (Strain Pan54T) was isolated on 10t September 2013 from an Algal surface in hydrothermal area (sampling site 38.6392 N, 15.1051 E) close to the island Panarea in the north of Sicily, Italy.  Panarea has an area of 3.3 km² and is the second smallest of the Aeolian islands. The island itself is only a small part of a sub-marine edifice in form of a truncated cone with an eastern protrusion with its base being 1500 m below sea level. The entire cone has a diameter of 23 km and an area of 460 km² and several thermal springs are located in proximity to the island. In the surroundings of Panarea areas with increased temperatures, higher levels of nutrients including nitrogen and sulphur sources are present, which motivated Kellscheuer et al. to choose this geographical location as a valuable source of so far unknown species of Prokaryotes. 

In Kellscheuer et al.'s phylogenetic analysis Pan54T appears in a monophyletic clade with its closest relative Rubinisphaera brasiliensis (Strain DSM 5305T). Rubinisphaera brasiliensis was originally isolated from a water sample of Lagoa Vermelha, a salt pit near Rio de Janeiro, Brasil, initially described as Planctomyces brasiliensis in 1989 and was later reclassified and assigned its own, unique genus. Rubinisphaera brasiliensis is currently the only validly described species within the genus Rubinisphaera. Pan54T and Rubinisphaera brasiliensis share a 16S rRNA gene sequence identity of 96.2%. This value is below the threshold of 98.7% for a novel species, but above the threshold for a novel Planctomycetal genus of 94.5%. 

For a morphological characterization, Pan54T cells were harvested during the exponential growth phase. Pan54T cells were found to be pear-shaped (averaging 1.6 μm by 0.8 μm) and form strong aggregates and biofilms. Cells have a textured surface and contain evenly distributed crateriform structures. Pan54T divides by polar budding. Daughter cells have the same shape as mother cells. Thin sections of Pan54T cells show a condensed nucleoid and cytoplasmic invaginations. Colonies are white indicating a lack of carotenoids as pigmenting compounds. Pan54T has a very similar morphology as Rubinisphaera brasiliensis. In direct comparison Pan54T has a slightly elongated shape and crateriform structures appear to be more pronounced. It was similar to in thatRubinisphaera brasiliensis rosette formations were observed rather than larger aggregates. The non-pigmented colonies of  Pan54T differ from the yellow to orange pigmentation of Rubinisphaera brasiliensis colonies.

Scanning electron microscopy image of a rosette of cells. Strain Pan54T. Kellscheuer et al. (2019).

In physiological experiments, Pan54T grew at a temperature range of 14–27 °C and a pH range of 6.0–10.0, but failed to grow at 30 °C or above. The optimal conditions were pH 9.0 and 26 °C. The observed temperature optimum of Pan54T is low compared to Rubinisphaera brasiliensis (30–33 °C). grows at temperatures of 37 Rubinisphaera brasiliensis °° °C  while Pan54T failed to grow at 30 °° °C or higher. Taken together, growth of Pan54T is mesophilic and slightly alkaliphilic with a generation time of 18 hours.

Based on the data collected during strain characterisation, Kellscheuer et al. conclude that Pan54T represents a novel species within the genus Rubinisphaera. They propose the name Rubinisphaera italica and present Pan54T as the type strain of the species.

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  1. Thank you for sharing

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