The August 2016 issue of Environmental Microbiology is a treasure trove of articles on symbiotic relationships. I happen to have a soft spot for the subject (am I symbiotic with symbioses?), so this issue is a handsome gift. I will comment on just a few of the articles, chosen with some effort from all this wealth.
“This review describes the reproductive oddities displayed by bacteria associated – more or less intimately – with multicellular eukaryotes.” So starts a most readable article by Sylvia Bulgheresi, an expert on variations on the theme of (humdrum?) E. coli-type cell division. It turns out that non-canonical modes of cell division are actually plentiful, their visibility hidden by the “streetlight effect” (looking under the streetlight for the lost key). Considered here are such gems as bacteria that adhere to the skin of some marine worms by their ends and divide longitudinally (the Z-ring is appropriately longitudinal to the cell’s long axis, the better to stick to their host’s cuticle; discussed here). Also included are the long crescent-shaped bacterial “braided ropes” that cover the surface of other worms; the gargantuan Epulopiscium’s engulfment of its young by the mother cell; rhizobia in root nodules; the segmented filamentous bacterium of the mouse gut; various insect endosymbionts, plus plus.
Chlamydia is an ever expanding phylum, consisting of some eight families and growing. No longer just the agents of the most common STD worldwide and the eye infection trachoma, chlamydiae include a large number of symbionts of protists, marine worms, and arthropods. These non-human chlamydiae are called "environmental" (a term I dislike because for the chlamydiae, humans are an environment, too). It is thought that this is just the tip of the chlamydial iceberg and that a huge number of other taxa are waiting to be discovered (that’s metagenomics for you!). They aren’t easy to study, as their host often exists out of the experimenter’s reach.
Here’s a report by Pizzetti, Fazi, Fuchs, and Amann on the co-isolation of a chlamydia (actually, two related clusters) and its host, a coastal lake-dwelling ameba called Vexillifera sp. The two can be maintained together in laboratory culture. These new chlamydiae are outliers, belonging to a separate branch of the chlamydial tree. Using the highly sensitive catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH), the researchers could see protists with intracellular pleomorphic chlamydiae. These novel chlamydiae are plentiful, up to 5x104 per ml. The abundance of both host and symbiont fluctuate hand in hand with the seasons. Given that “environmental” chlamydiae may cause disease in humans, these findings have public health implications.
The paper is accompanied by an erudite commentary by Viana and Buchrieser that calls the findings "environmental treasures." One reason for this is that chlamydiae will likely be found to be even more widespread and to play important and varied roles in the lives of their hosts. Another reason is their unique developmental cycle consisting of two forms – the inert highly infectious "elementary bodies," and the non-infectious metabolically active and dividing "reticulate bodies." All pathogens and symbionts face distinctive sets of challenges when in transit between hosts as opposed to reproducing within them. Here, the distinction is especially clear, being reflected in the structural and physiological dissimilarities of the two forms. Perhaps the closest known example to this is Bacillus anthracis, which is found as a spore in soils but grows vegetatively in humans and other animals. You’ll agree, chlamydiae deserve more attention.
According to this paper, a rickettsia that lives symbiotically in an Acanthamoeba carries genes that encode for proteins with eukaryotic domains, such as ankyrins, leucine-rich repeats, and tetratricopeptide repeats. Proteins with these domains participate in structural protein-protein interactions in eukaryotes and are probably involved here in yet unsolved interactions of the bacteria with the host. The rickettsiae live within a host vacuole, and thus have ample chance to interact with host constituents (see figure). The researchers, from the Ettema and Horn labs, point out that rickettsiae are especially interesting when considering eukaryotic evolution, not just because they are abundanct intracellular parasites, but also for being the closest extant relatives of mitochondria. The authors called the rickettsia ‘Candidatus Jidaibacter acanthamoeba’ (alluding to the Jedi in the Star Wars saga). This finding mirrors that of the recently discovered archaeon called Lokiarchaeum (discussed here and locally here), which possesses eukaryotic genes involved in membrane trafficking. Eukaryogenesis redux?
Detection And Isolation Of Plant-Associated Bacteria Scavenging Atmospheric Molecular Hydrogen
I confess, it never occurred to me to worry about the uptake of hydrogen (H2) from the atmosphere, thinking that it is unlikely to be an abundant gas. But a paper from the labs of Y. Kamagata and P. Constant says that although relatively scarce (0.530 parts per million volume of air), H2 is the second most abundant reduced gas, after methane, so pay attention! Atmospheric H2 is derived from the burning of biomass and fossil fuels and from the photochemical oxidation of hydrocarbons. It turns out that H2 is taken up by both soil and plant-dwelling actinomycetes. To quote the authors: "Recently, it has been proposed that atmospheric H2 may serve as maintenance energy during starvation and sporulation of high-affinity H2-oxidizing Actinobacteria. H2 treatment of soil was known to promote plant growth of both legume and non-leguminous crops."
The authors found the hhyL gene (a subunit of a [NiFe]-hydrogenase) in clone libraries, suggesting that high-affinity H2-oxidizing bacteria are ubiquitous in herbaceous plants. They say that this is the first report “focusing on plant-associated bacteria possessing high-affinity H2 uptake activity.”
Genomic Characterization of Symbiotic Mycoplasmas from the Stomach of Deep-Sea Isopod Bathynomus sp.
I have long been mystified by the mycoplasmas. Cell wall-less as they are, they would appear to be microbial picogram weaklings. But they are mighty resilient and environmentally highly successful. I can’t say that I ever thought of them as mutualistic symbionts. But they appear to be useful members of the microbiota of the stomachs of deep-sea scavengers, the isopods. These large cousins of the terrestrial woodlice don’t get much to eat, so they can use all the help they can get. Mycoplasmas may well be involved in digesting plant cell walls that drift down to the ocean floor and constitute the main food source for the isopods. They are also thought to be involved in the digestive activities of the terrestrial isopods.
The evidence? A paper by the labs of L-S He and A. Dancin presents the genomes of two phylogenetically similar mycoplasmas found in marine isopodal stomachs, differing in GC content and gene number. The size of their genomes is estimated to be about 780 kb and 698 kb. The 16S RNA of the bacteria is distant from that of mycoplasma symbionts of terrestrial isopods and deep sea shellfish. Both of those discussed here carry genes for sugar transport and uptake, plus genes for proteolytic enzymes and oligosaccharide degradation. Having genes for sialic acid degradation and transport suggests that they may be involved in the attachment of the organisms to their host’s stomach wall. So, add mutualism to the ecological skills of mycoplasmas.