MicrobiologyBytes

A new study has created a model of what scientists think the first multicellular cooperation might have looked like, showing that yeast cells – in an environment that requires them to work for their food – grow and reproduce better in multicellular clumps than singly.

Researchers found that cells of brewer’s yeast that clumped together were able more effectively to manipulate and absorb sugars in their environment than were similar cells that lived singly. The experiments showed that in environments where the yeast’s sugar food source is dilute and the number of cells is small, the ability to clump together allowed cells that otherwise would have remained hungry and static to grow and divide.

The work used the yeast Saccharomyces cerevisiae, which is commonly used in brewing and bread-making and has long been used by scientists as a model organism for understanding single-celled life. The researchers devised a series of experiments that presented two problems for the yeast cells to solve if they were to take in enough food to grow and divide: the first was how to change their food from an unusable form to a usable form; the second was how to actually take in this food. The researchers put the yeast in a solution of sucrose – table sugar – which is composed of two simpler sugars, glucose and fructose. Yeast lives on sugar, but the sucrose can’t get through the membrane that surrounds the cell. So the yeast makes an enzyme called invertase to chop the sucrose into glucose and fructose, each of which can enter the cell using gate-keeping molecules, called transporters, that form part of the membrane.

Working alone, a single yeast cell in a dilute solution of sucrose would never take in enough glucose and fructose to be able to grow and divide. But by cooperating, clumps of yeast in that same solution might have a chance. With several cells in proximity, all releasing invertase to create smaller sugars, these cooperating yeast cells would increase the density of those sugars near the clump, increasing the chances that each cell could take in enough to grow and divide. Sure enough, when the researchers tested these hypotheses on two strains of yeast, they found that the strain which clumped cells together was growing and dividing, while the yeast cells living alone were not.

Sucrose Utilization in Budding Yeast as a Model for the Origin of Undifferentiated Multicellularity. (2011) PLoS Biol 9(8): e1001122. doi:10.1371/journal.pbio.1001122
We use the budding yeast, Saccharomyces cerevisiae, to investigate one model for the initial emergence of multicellularity: the formation of multicellular aggregates as a result of incomplete cell separation. We combine simulations with experiments to show how the use of secreted public goods favors the formation of multicellular aggregates. Yeast cells can cooperate by secreting invertase, an enzyme that digests sucrose into monosaccharides, and many wild isolates are multicellular because cell walls remain attached to each other after the cells divide. We manipulate invertase secretion and cell attachment, and show that multicellular clumps have two advantages over single cells: they grow under conditions where single cells cannot and they compete better against cheaters, cells that do not make invertase. We propose that the prior use of public goods led to selection for the incomplete cell separation that first produced multicellularity.

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Ecologist Jessica Green has found that mechanical ventilation does get rid of many types of microbes, but the wrong kinds: the ones left in the hospital are much more likely to be pathogens:

Wolbachia pipientis is a maternally inherited symbiotic bacteria that is widespread among most insects including laboratory stocks of Drosophila melanogaster, as well as filarial nematodes and crustaceans. Wolbachia belong to the Richettsial family responsible for the deadly human diseases such as typhus, Rocky Mountain spotted fever, and Q fever, but themselves are not involved in any known human diseases. Wolbachia are best known for their ability to induce reproductive alterations in hosts such as male killing, feminization, parthenogenesis, and cytoplasmic incompatibility, all of which result in increased number of infected female offspring and thereby helping vertical transfer of Wolbachia. These reproductive alterations may promote speciation in extreme cases. Because of these intriguing properties, Wolbachia have been extensively studied for entomology, agriculture and evolution.

Despite Wolbachia’s unique role in host reproduction and physiology, their underlying cellular mechanisms are yet to be addressed. Studies with electron microscopy have revealed that Wolbachia bacteria are strictly present in vesicular structures in the cytoplasm of host cells. These Wolbachia vesicles are attached to astral microtubules near centrosomes by short electron-dense bridges, and their centrosomal localization is dependent on microtubules but not actin. Wolbachia bacteria are enclosed within three layers of membranes: the outer layer is host origin and two inner layers are bacterial cell wall and bacterial plasma membrane. Since parasitic bacteria and enveloped mammalian viruses often utilize a variety of subcellular organelles such as endoplasmic reticulum and Golgi apparatus during their life cycles, Wolbachia may also be present in a host organelle that can aid the replication and propagation of Wolbachia. Identification of this host organelle is critical for understanding the Wolbachia‘s ability in changing host physiology.

This paper reports that Wolbachia reside in a group of Golgi-related vesicles. This raises an interesting possibility that Wolbachia may mark the unique group of Golgi vesicles linked to membrane biogenesis. The additional finding that localization of Wolbachia vesicles is regulated by genes involved in cell/tissue polarity also provided a surprising new potential activity for these polarity genes in Golgi localization.

Wolbachia Bacteria Reside in Host Golgi-Related Vesicles Whose Position Is Regulated by Polarity Proteins. (2011) PLoS ONE 6(7): e22703. doi:10.1371/journal.pone.0022703
Wolbachia pipientis are intracellular symbiotic bacteria extremely common in various organisms including Drosophila melanogaster, and are known for their ability to induce changes in host reproduction. These bacteria are present in astral microtubule-associated vesicular structures in host cytoplasm, but little is known about the identity of these vesicles. We report here that Wolbachia are restricted only to a group of Golgi-related vesicles concentrated near the site of membrane biogenesis and minus-ends of microtubules. The Wolbachia vesicles were significantly mislocalized in mutant embryos defective in cell/planar polarity genes suggesting that cell/tissue polarity genes are required for apical localization of these Golgi-related vesicles. Furthermore, two of the polarity proteins, Van Gogh/Strabismus and Scribble, appeared to be present in these Golgi-related vesicles. Thus, establishment of polarity may be closely linked to the precise insertion of Golgi vesicles into the new membrane addition site.

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Recent studies have unearthed a treasure trove of prehistoric virus ‘fossils’, viral genomes or genome segments frozen millions of years ago as integrated copies in the genomes of diverse animal hosts. The fact that these integrated viral fossils can be easily recognized as belonging to modern virus families is stunning given the fact that modern exogenous viruses have replicated and evolved for many millions of years since these viral fossils were captured. Despite high rates of mutation, the evolution of virus sequence is clearly constrained. This constraint comes partially from intrinsic selective forces that limit virus evolution, such as selection for modulation of pathogenicity to the host and the structural constraints of the virus itself. Other major constraints on virus evolution come from the diverse immune strategies imposed by hosts. Cumulatively, these constraints act together to limit all aspects of virus evolution from the swarm of variants produced in a single host to the evolution of expanded host range. These newly identified fossils indicate that constraint on virus evolution might be far greater than has previously been appreciated.

Two-stepping through time: mammals and viruses. (2011) Trends Microbiol. 19(6): 286-294
Recent studies have identified ancient virus genomes preserved as fossils within diverse animal genomes. These fossils have led to the revelation that a broad range of mammalian virus families are older and more ubiquitous than previously appreciated. Long-term interactions between viruses and their hosts often develop into genetic arms races where both parties continually jockey for evolutionary dominance. It is difficult to imagine how mammalian hosts have kept pace in the evolutionary race against rapidly evolving viruses over large expanses of time, given their much slower evolutionary rates. However, recent data has begun to reveal the evolutionary strategy of slowly-evolving hosts. We review these data and suggest a modified arms race model where the evolutionary possibilities of viruses are relatively constrained. Such a model could allow more accurate forecasting of virus evolution.

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The new version of Principles of Molecular Virology is being printed right now :-)

You can order your copy now from Amazon or Amazon.co.uk

I find that there’s still a lot of confusion around the ideas behind open educational resources, so I wrote a short piece for Microbiology Today about the OeRBITAL project:

For a long time, bacteria were considered insensitive to light, with the exception of phototrophs that use sunlight as an energy source. The photosensory proteins in these phototrophs were believed to help them find optimally illuminated environments for solar energy harvesting. Six classes of photoreceptors exist, and representatives of five of these classes were first identified in phototrophic bacteria. In these organisms, photoreceptors control production of the photosynthetic apparatus, coordinate composition of light-absorbing antennas to better capture available light, and trigger phototaxis responses.

Recent studies have established that pathogenic bacteria rely on light as one of the means to assess their location and increase virulence in preparation for entering a new host. Given that a number of animal and plant pathogens contain photoactivated proteins, it is intriguing to imagine that the transition between environmental and host-associated lifestyles is commonly regulated by light. Several experimental examples and bioinformatic analysis support the view that light helps bacteria make another important lifestyle decision, i.e. between a single-cellular planktonic state and a surface-attached multicellular community (biofilm). Many outstanding questions regarding these light-regulated lifestyle choices remain. If we want to better control bacterial virulence and biofilms, we need to literally shed light on bacteria more often.

Light helps bacteria make important lifestyle decisions. Trends Microbiol. Jun 9 2011
Until recently, bacterial responses to changes in light environments were regarded as specialized adaptations in a small number of phototrophs. However, the genomes of many photosynthetic and chemotrophic bacteria not known to have photophysiological responses also encode photoreceptor proteins. What new trends in the biological responses triggered by these photoreceptors are emerging? Here, we review several instances where members of different blue-light receptor classes (LOV, BLUF and PYP) photoregulate a lifestyle choice between the motile single-cellular state and the multicellular surface-attached community state (biofilm) by a range of mechanisms including bacterial two-component systems, the second messenger cyclic di-GMP and direct interactions of photoreceptors with transcription factors. We also discuss how ‘seeing’ helps some pathogenic bacteria make another important choice, i.e. between environmental and host-associated lifestyles.

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