When I first entered the field of virology, in the 1970s, the definition of virus included the then-correct observation that no viral genome encoded any part of the translational apparatus. This dictum was shattered by the discovery of giant viruses which were found to encode tRNAs, aminoacyl tRNA syntheses, and many proteins involved in translation. The phrase ‘only the ribosome is lacking’ was famously used to describe the giant Tupanvirus. While catchy, it is no longer entirely true: viral genomes have been identified that encode ribosomal proteins.
By Gertrud U. Rey
Quorum sensing is a form of cell to cell communication in bacteria in which individual cells coordinate their behavior based on population density. In human terms, the word “quorum” means “the minimum number of people required to conduct business.”
The TWiV team travels to Texas A&M University, home of the Center for Phage Technology, where they speak with Ry Young and Jason Gill about their work on viruses that infect bacteria.
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Show notes at microbe.tv/twiv
Virologist Roger W. Hendrix died on 15 August 2017. I only met Roger once, at the 2011 ASM meeting in New Orleans where we recorded an episode of This Week in Virology. The video of that episode is below, starting at my conversation with Roger at 30:34. Harmit Malik and Rachel Katzenellenbogen were my other guests on TWiV 135.
Not long after their discovery, viruses that infect bacteria – bacteriophages – were considered as therapeutic agents for treating infections. Despite many years of research on so-called phage therapy, clinical trials have produced conflicting results. They might be explained in part by the results of a new study which show that the host innate immune system is crucial for the efficacy of phage therapy.
When mice are infected intranasally with Pseudomonas aeruginosa (which causes pneumonia in patients with weak immune systems), the bacterium multiplies in the lungs and kills the animals in less than two days. When a P. aeruginosa lytic phage (i.e. that kills the bacteria) is instilled in the nose of the mice two hours after bacterial infection, all the mice survive and there are no detectable bacteria in the lungs. The phage can even be used prophylactically: it can prevent pneumonia when given up to four days before bacterial challenge.
The ability of phage to clear P. aeruginosa infection in the mouse lungs depends on the innate immune response. When bacteria infect a host, they are rapidly detected by pattern recognition receptors such as toll-like receptors. These receptors detect pathogen-specific molecular patterns and initiate a signaling cascade that leads to the production of cytokines, which may stop the infection. Phage cannot clear P. aeruginosa infection in mice lacking the myd88 gene, which is central to the activity of toll like receptors. This result shows that the innate immune response is crucial for the ability of phages to clear bacterial infections. In contrast, neither T cells, B cells, or innate lymphoid cells such as NK cells are needed for phage therapy to work.
The neutrophil is a cell of the immune system that is important in curtailing bacterial infections. Phage therapy does not work in mice depleted of neutrophils. This result suggests that humans with neutropenia, or low neutrophil counts, might not respond well to phage therapy.
A concern with phage therapy is that bacterial mutants resistant to infection might arise, leading to treatment failure. In silico modeling indicated that phage-resistant bacteria are eliminated by the innate immune response. In contrast, phage resistant bacteria dominate the population in mice lacking the myd88 gene.
These results demonstrate that in mice, successful phage therapy depends on a both the innate immune response of the host, which the authors call ‘immunophage synergy’. Whether such synergy also occurs in humans is not known, but should be studied. Even if observed in humans, immunophage synergy might not be a feature of infections in other anatomical locations, or those caused by other bacteria. Nevertheless, should immunophage synergy occur in people, then clearly only those with appropriate host immunity – which needs to be defined – should be given phage therapy.
Viruses infect every living organism on the planet, but not every habitat has been explored for their presence. The igneous ocean crust had not yet been examined for viruses, but seek and ye shall find: there are plenty of viruses under the seas.
The oceanic basement is an enormous ecosystem that lies at the bottom of the seas, beneath a thick layer of sediment. It is composed of igneous rock through which percolates 20 million cubic kilometers of water. Previous study of cores from this region has revelealed the presence of prokaryotes, but no one had looked for viruses.
Facilitating the study of the oceanic basement are seafloor observatories that have been placed into existing boreholes. Two have been placed in 3.5 million year old rock in the northeastern Pacific Ocean. They penetrate hundreds of meters through sediment and into the basement (illustrated; image credit) and are fitted with plumbing that allows sampling of uncontaminated fluids from different depths in the basement rock.
Analysis of fluids recovered from these sites revealed both prokaryotes (8,000 per ml) and virus particles (90,000 per ml). Ribosomal RNA sequence analysis showed that bacteria dominated these communities, with some Archaea but virtually no eukaryotes.
Examination of the fluids by electron microscopy showed virus particles of different kinds: tailed and untailed icosahedral particles, untailed globular particles, and rod and spindle shaped. My favorite is the lemon shaped particle, for its form and implied taste.
To provide information on the viral genomes in the oceanic basement, sequences were determined from total cellular DNA (material retained on a 0.2 micron filter) extracted from the samples. Most viral sequences likely had archaeal hosts. Some prophage sequences were identified – viral genomes integrated into host DNA – which allowed more certain identification of the infected cell.
Most of the identified archaeal and bacterial virus sequences came from the families Myoviridae and Siphoviridae (think tailed, icosahedral viruses). One complete circular DNA genome identified is 55,906 nucleotides in length with 81 open reading frames. Twenty of these encode proteins with recognizable functions, such as capsid proteins, a primase and a DNA polymerase. No genes encoding tRNAs, such as those found in giant viruses, were identified.
Some sequences were similar to those of giant viruses like mimiviruses and phycodnaviruses. These viruses are known to only infect eukaryotes. Eukaryotic genomes were rare in the basement metagenome collections (1% of the community in one location).
I am not surprised that viruses have been found in ocean’s basement. Still, I’m amazed when I think about how far down they are, in warm water (65 degrees C) in 3.5 million year old rock.
To paraphrase Samuel E. Wright (Under The Sea):
The viruses are always greenerIn somebody else’s lakeYou dream about going up thereBut that is a big mistakeJust look at the viruses around youRight here under the ocean floorSuch wonderful viruses surround youWhat more is you lookin’ for?
The lovely TWiV team explore evolution of our fecal virome, and the antiviral RNA interference response in the nematode C. elegans.
You can find TWiV #433 at microbe.tv/twiv, or listen below.
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The TWiVirions reveal bacteriophage genes that control eukaryotic reproduction, and the biochemical basis for increased Ebolavirus glycoprotein activity during the recent outbreak.
You can find TWiV #431 at microbe.tv/twiv, or listen below.
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