The latest giant virus discovery is Tetraselmis virus 1, which infects green algae. It is unusual because it encodes enzymes involved in fermentation. Green beer, anyone?
On episode #315 of the science show This Week in Virology, Vincent, Alan, Rich and Kathy discuss the association of a virus with sea star melting disease, and the finding of a phycodnavirus in the oropharynx of humans with altered cognitive functions.
You can find TWiV #315 at www.microbe.tv/twiv.
Many well-known human viruses, including poliovirus, rabies virus, West Nile virus, can infect cells of the nervous system, leading to alterations in the function of that organ. Could a virus that infects algae also cause human neurological alterations?
Chloroviruses are large DNA-containing viruses that infect unicellular algae called zoochlorellae (pictured: image credit, ViralZone). Unexpectedly, chlorovirus DNA sequences were found in the oropharynx of 40 of 92 individuals (43.5%) who had no known physical or psychiatric illness. The clinical specimens had been obtained as part of a study of cognitive function, and it was possible to determine that presence of chlorovirus DNA was associated with a slight but statistically significant decreased performance in tests for visual motor speed, delayed memory, and attention.
When mice were fed chlorovirus-infected algae, they showed decreased performance in tests of cognitive function, such as recognition memory and sensory-motor gating. Some of these animals developed antibodies against the virus, suggesting that viral replication took place. Furthermore, feeding of chlorovirus to mice was associated with changes in gene expression in the hippocampus, the part of the brain essential for learning, memory, and behavior.
It is not known if the chlorovirus replicates in humans or in mice; only viral nucleic acids were detected. No mention is made of attempts to isolate infectious chloroviruses from humans or mice. The amount of chlorovirus in the oropharynx is not known. However the results of sequence analysis, in which low numbers of sequences were found in each person suggest very low numbers of genomes. Of course, it is possible that virus replication took place some time ago, and its effects linger after replication has subsided.
Chloroviruses are commonly found in inland waters, and the subjects could have acquired the virus via inhalation or drinking contaminated water. It is entirely possible that the virus does not replicate in humans, but is present in the oropharynx as a common environmental contaminant. Many plant and insect virus sequences can be isolated from the human intestinal tract as a consequence of the food we ingest, but there is no evidence that they can replicate at that site. Consequently, chlorovirus might not have any role in the reduced cognitive functions observed in this study. It is possible that exposure to another factor together with chloroviruses, such as heavy metals, is responsible for the observed cognitive differences.
The suggestion that a virus infection might cause subtle cognitive defects is not outlandish. For example, lymphocytic choriomeningitis virus infects rodents congenitally or immediately after birth and establishes a persistent infection of virtually all tissues. These mice show no outward signs of illness, but careful study of infected animals reveals that they are less ‘smart’ than their uninfected peers.
The results are intriguing and warrant more study, including a determination of whether an infectious chlorovirus can be isolated from humans, whether this virus can replicate in human cells in culture, and how they differ from environmental isolates. It would also be important to determine if antibodies to chloroviruses are present in humans, and if they are associated with any diseases. It is too early to conclude that a virus of algae causes altered human neurological functions.
On episode #34 of the science show This Week in Microbiology, Vincent, Michael, and Elio discuss changing populations of Emiliania huxleyi and their viruses in the North and Black Seas.
You can find TWiM #34 at microbeworld.org/twim.
Hosts: Vincent Racaniello, Alan Dove, Dickson Despommier, and Welkin Johnson
Vincent, Alan, Dickson and Welkin review how a virus regulates the severity of mucocutaneous leishmaniasis, virophage control of antarctic algal host-virus dynamics, and human metapneumovirus infection in gorillas.
Click the arrow above to play, or right-click to download TWiV #128 (67 MB .mp3, 92 minutes).
Links for this episode:
- Leishmania RNA virus controls severity of mucocutaneous leishmaniasis (Science)
- Leishmaniavirus at ViralZone
- Listen to TWiP #14 on Leishmania
- Virophages may control algal virus-host dynamics (PNAS)
- Human metapneumovirus infection of gorillas (EID)
- Reverse zoonoses (virology blog)
- Copy-number variation and co-evolution of viral an primate genomes (PLoS Genetics)
- NIH Study Section rosters
- TWiV on Facebook
- Letters read on TWiV 128
Weekly Science Picks
Welkin – Walter Reed Collection, University of Virginia
Dickson – Bacteria-phage antagonistic coevolution in soil (Science)
Alan – The Artful Amoeba
Vincent – Potential bacteriophage applications (Microbe)
Send your virology questions and comments (email or mp3 file) to firstname.lastname@example.org, or call them in to 908-312-0760. You can also post articles that you would like us to discuss at microbeworld.org and tag them with twiv.