Most microbiologists have heard of the spherically-shaped cocci and the rod-shaped bacilli cell shapes. Fewer, however have heard of the star-shaped cells associated with the genus Stella. This ubiquitous bacterial genus has cells whose cross sections look like stars (see the images below). Though they are found in many different environments, the genetics behind the unusual cell shape is not fully understood, and is one of the reasons why a scientific team recently sequenced the genomes of 3 Stella species, which are published in ASM's Microbiology Resource Announcements journal.
We spoke with first author Dr. Tomoya Maeda to discuss how he and others may use these genomes in their research.
Where are Stella bacteria found naturally?
Maeda: Vasilyeva et al. first isolated Stella bacteria from the nutrient-rich chernozem soil of the Krasnodar region of southern Russia. Stella bacteria have been found in soil, horse manure, and aquatic habitats showing various degrees of salinity and pollution. Although star-shaped bacteria are very unique, these bacteria seem to be widely distributed in different environments including soil and water.
Stella vacuolata is one of the 3 species sequenced in this study. The star-shape bacteria can be seen with brightfield microscopy (top) but the shape becomes more clear when the bacteria are stained with FM4-64 and imaged with fluorescence microscopy (bottom).
Source: T. Maeda
How can these genomes help scientists to understand the evolutionary relationships of Stella?
Maeda: Since alphaproteobacteria including Stella exhibit a diverse morphology, comparative genome analysis together with mapping of morphological phenotypes allows us to understand how the star shape of these bacteria evolved.
What can the genome sequences tell you about the unusual morphology of Stella cells?
Maeda: Our genome analysis revealed that Stella bacteria have many orthologs of E. coli genes encoding rod-shape determining proteins, such as MreB and RodA. This finding suggests that Stella bacteria share basic machinery for cell shape determination with E. coli. Closely related bacteria to Stella, such as Skermanella, have a straight-rod shape, which suggests Stella bacteria evolved from a conventional rod-shaped bacterium. However, the star-shape seems to be formed by polar elongation, while MreB and RodA are responsible for lateral elongation of the peptidoglycan layer in E. coli. Therefore, we speculate that Stella bacteria additionally have different machinery for determining its cell shape. Indeed, our genome analysis revealed that Stella bacteria have an ortholog of the Caulobacter popZ gene, which encodes a cell pole organizing protein. This popZ ortholog may be involved in the star-shape formation in Stella bacteria.
How will you use these genome sequence in your research going forward?
Maeda: We are now conducting laboratory evolution studies to obtain mutant strains of the Stella bacteria with morphological defects. After the laboratory evolution, we will determine the mutation points by resequencing the genome, using these genome sequences as reference. If we find candidate genes encoding star-shape determining proteins, we will also conduct phylogenetic analysis of these genes.
How will other scientists in the research community use this resource?
Maeda: First, this resource can be used for studies about the mechanisms underlying generation of complex morphology and evolution of shape diversity in bacteria. Since our research is the first report of the complete genome sequences of members of the genera Stella, other scientists can use the genome information for studies such as comparative genome analysis and mining industrially-relevant enzymes. Second, because Stella bacteria show distinct morphology among alphaproteobacteria, we think these genome sequences will fill in the gap in missing information of alphaproteobacterial genome.
Why Microbiology Resource Announcements?
“Scientific research has benefited from the resources developed and shared between scientists — from software published on Github to plasmids made available on Addgene. These resources facilitate our work and allow researchers to build on and extend the work of others. Indeed, some of the most impactful papers have been those describing resources — the E. coli Keio Knockout Collection manuscript, for example, has been cited nearly 5,000 times. Many resources are worth sharing. We hope your next submission will spur someone else’s work or, potentially, inspire collaboration in our community.” – Microbiology Resource Announcements Editor-in-Chief Irene Newton.
