In just a few short weeks I will reintroduce the Microbe Series and discuss two of the worlds best things: bacteria and BEER!!!! Just as a reminder, the goal of this series is to learn a bit about the natural history of various aquatic microbes. These organism can either be picked from my current research data or they could be an interesting organism I heard about at a conference/talk. So stay tuned to learn more.
This week I want to highlight Prosthecobacter fusiformis and related species from the genus Prosthecobacter. P. fusiformis is an interesting organism; however I must admit that I discovered it randomly by picking a taxon found in every sample (eight sites total) in one of my datasets. Yes I realize that many may regard this as an odd way to pick a bacterium to write about, but I feel that this was an appropriate way to discover an organism which I knew very little about.
Let’s again start with a little taxonomy (for those into that type of thing):
Phylum – Verrucomicrobia
Order – Verrucomicrobiales
Family – Verrucomicrobaiceae
Genus – Prosthecobacter
The Prosthecobacter genus has very few cultured representatives, but it is one of the more interesting groupings I have come across recently. This genus helped to distinguish Verrucromicrobia as a valid phylum of Bacteria. Before the characterization of this group there was little evidence to separate Verrocomicrobia from the other phyla due to the lack of cultured representatives. The inclusion of this second genus (along with molecular evidence of course) which included cultured organisms (even though not many) provided better evidence for the distinction of the phylum. Furthermore, with the use of molecular tools we now know that this phylum is distinct and we have evidence of other (non cultured) organisms that group with Verrucomicrobia. Secondly, all of the cultured representitives of Prosthecobacter are morphologically distinct. This is a rare phenomenon in any group of bacteria. Four species of Prosthecobacters have been carefully characterized and all have distinct standard length and widths as well as curviness and the presence or absence of vibrioid morphology (you may need to look this up for yourself: one of my favorites though) (Figure 1, from Hedlund et al., 1997). Furthermore, in my opinion, P. dejongei should be called the mustache bacteria. Seriously, you can’t look at panel D below and not think of a French mustache. Awesome!!!! I find that this morphology is quite interesting, but I wonder why. Why is there a mustache bacteria? Why are some sister taxa just slightly longer than others? Why are some narrow and straight while others have adapted the vibrioid morphology
So where does this organism get its name? Prosthecobacter refers to the terminal prosthecae found on all of the cells. Prosthecae are a cellular appendage which includes a holdfast that is used to attach to surfaces. Due to the presence of the prosthecae these organisms were originally placed with the Caulobacter (alpha proteobacteria) which are the best known group of prosthecate bacteria. However, it was later discovered that Prosthecobacter do not divide asymmetrically to produce motile (with no prosthecae) and non-motile (with prosthecae) cells. Instead they divide laterally, and thus all daughter cells have prosthecae and remain non-motile. Due to this type of division, Prosthecobacter produce clumps of cells that radiate out along the attached surface. I’m sure that if we were able to image, without disruption, surfaces containing this organism we would be able to find intricate patterns on surfaces. I wonder what the patterns of mustache bacteria would look like?
Ok, so maybe I didn’t only highlight P. fusiformis. Obviously I am more interested in P. dojongei but you still get the point. The genus Prosthecobacter is a great example of the extreme diversity and beauty of aquatic microbes and microbes in general. I still only knew little about this group of organisms, but I find them highly fascinating and I hope you now do as well. Questions, Comments: All welcome!!!!!
Brian P. Hedlund, John J. Gosink & James T. Staley. 1997. Verrucomicrobia div. nov., a new division of the Bacteria containing three
new species of Prosthecobacter. Antonie van Leeuwenhoek 72: 29–38.
If you were to try thinking of a model ‘lake’ bacterium you may consider using a Polynucleobacter spp.
Some taxonomy (if you are into that type of thing):
Phylum – Proteobacteria
Order – Burkholderiales
Family – Burkholderiaceae
Genus – Polynucleobacter
Members of the genus Polynucleobacter are considered ‘cosmopolitan’, at least in freshwater ecosystems. Common species in this genus are P. necessarius and P. cosmopolitanu. These organisms are globally widely distributed and chances are that next time you jump into your favorite lake or wade across your local stream you will cross paths with a member of this group. Along with their distribution, these organisms are also temporally variable and one study showed that their relative abundance can fluctuate from 4 – >50% of the total microbial community (Hahn et al., 2005 AEM). However, in some Michigan lakes the abundance of this group is <1% showing that there is large spatial variability too (unpublished). Despite this, none would disagree that this group of organisms is fascinating.
In my opinion, the species P. necessariusmay be one of the more interesting in this group (see image below from Bergey’s Manual® of Systematic Bacteriology: Volume Two The Proteobacteria Part C The Alpha-, Beta-, Delta-, and Epsilonproteobacteria). For some time we have known that P. necessarius is an endosymbiont of ciliates (specifically Euplotes spp.). In addition, this interaction is known to be an obligate symbiosis and ciliates that are cleared of their endosymbiont die soon there after. Furthermore, P. necessarius has been implicated as the agent responsible for mate killing among these ciliates. Why does this interaction exist? How did it come to be? If you weren’t captivated by the spatial and temporally variability, I bet you are now.
Feel free to share your thoughts or do a little research of your own (via the web of course).
Ok. Starting next week I am going to make weekly posts about aquatic microbes. My idea is that each week I can highlight an organism commonly found in either freshwater or marine ecosystems. This should be fun.