TWiV 406: Pow, right in the enteroids!

The TWiV team discusses eye infections caused by Zika virus, failure of Culex mosquitoes to transmit the virus, and replication of norovirus in stem cell derived enteroids.

You can find TWiV #406 at microbe.tv/twiv, or listen below.

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TWiV 318: Last year in virology

On episode #318 of the science show This Week in Virology, the TWiV gang reviews ten fascinating, compelling, and riveting virology stories from 2014.

You can find TWiV #318 at www.microbe.tv/twiv.

TWiV 313: With viruses like these, who needs enemas?

On episode #313 of the science show This Week in Virology, Vincent, Alan, and Rich discuss how norovirus, an enteric virus, can replace the functions of the gut microbiome.

You can find TWiV #313 at www.microbe.tv/twiv.

TWiV 312: She sells B cells

On episode #312 of the science show This Week in Virology, the TWiVbolans discuss the finding that human noroviruses, major causes of gastroenteritis, can for the first time be propagated in B cell cultures, with the help of enteric bacteria.

You can find TWiV #312 at www.microbe.tv/twiv.

TWiV 274: Data dump

On episode #274 of the science show This Week in Virology, the TWiV team discusses recent cases of polio-like paralysis in California, and the virome of 14th century paleofeces.

You can find TWiV #274 at www.microbe.tv/twiv.

Why do viruses cause disease?

EvolutionVirulence, the capacity to cause disease, varies markedly among viruses. Some viruses cause lethal disease while others do not. For example, nearly all humans infected with rabies virus develop a disease of the central nervous system which ultimately leads to death. In contrast, most humans are infected with circoviruses with no apparent consequence. Is there a benefit for a virus to be virulent?

One explanation for viral virulence is that it facilitates transmission. However, a comparison of infections caused by two enteric viruses, poliovirus and norovirus, does not support this general view. Both viruses infect the gastrointestinal tract and are spread efficiently among humans by fecal contamination. However, norovirus infection causes vomiting and diarrhea, while poliovirus infection of the intestine is without symptoms (the rare invasion of the nervous system, and subsequent paralysis, is an accidental dead end). Both viruses have successfully colonized humans for many years, so why does only one of them cause gastrointestinal tract disease?

Two recent studies of bacterial virulence provide some clues about the evolution of virulence. In one a commensal strain of Escherichia coli was serially propagated in the presence of macrophages, which are cells of the immune system that take up and destroy the bacteria. After many such passages, bacterial clones were isolated that escape phagocytosis and killing by macrophages. These clones had also acquired increased pathogenicity in mice. In other words, the genetic changes that allowed the bacteria to evade the immune response also lead to increased virulence.

In another example of evolution to virulence, it was found the the bacterium Pseudomonas aeruginosa can sense the presence of competing gram-positive bacteria because the latter shed the cell wall component peptidoglycan. In response to this molecule, P. aeruginosa secretes proteins that kill the other bacteria. These secreted proteins also make the bacterium more virulent in a host – in their absence, the bacteria are less virulent. In other words, P. aeruginosa damages its host in an attempt to remove nearby bacterial competitors.

In both bacterial examples, virulence can be viewed as collateral damage: the consequence of evading the immune response, or killing off competitors. Being virulent was not the primary goal. This explanation for bacterial virulence is straightforward and compelling: virulence is not directly selected for during evolution but comes along for the ride. Can it be applied to viruses?

All eukaryotic viruses must encode at least one protein that antagonizes host immune responses, otherwise they would be eliminated. These immune evasion proteins are certainly virulence factors: in general, when they are deleted or altered, the capacity of the virus to cause disease in a host is reduced. Like bacterial virulence, viral virulence might be collateral damage incurred by having to evade immune responses. This hypothesis is attractive but seems overly simplistic. If the ubiquitous and benign circoviruses did not evade host responses, then they would be eliminated from the human population.

The reasons why some viruses are virulent and others are not remain elusive. It is possible to reduce viral virulence by mutation, but this type of experiment does not reveal why viruses cause disease. The inverse experiment would be more informative: to select from a population of avirulent virus those that can cause disease. The results of such an experiment would help to identify the selection pressures that allow viruses to evolve to virulence.

TWiV 252: Who read the last email?

On episode #252 of the science show This Week in Virology, the complete TWiV team reads email from listeners about anti-vaccine activists, a career in microbiology, placentas, a virology textbook, the HeLa cell genome, norovirus, and much more.

You can find TWiV #252 at www.microbe.tv/twiv.

TWiV 243: Live from ASV at Penn State

On this episode of the science show This Week in Virology, which was recorded before a large enthusiastic audience at the annual meeting of the American Society for Virology, Vincent, Rich, and Kathy speak with Rebecca and Christiane about their work on metapneumoviruses and noroviruses.

You can find TWiV #243 at www.microbe.tv/twiv.

TWiV 212: Apocalypse TWiV 122112 212

On episode #212 of the science show This Week in Virology, the TWiVerers answer listener email about genetically modified chickens, a hendra vaccine for horses, online education, curing color blindness, Roosevelt and polio, Th cells, and much more.

You can find TWiV #212 at www.microbe.tv/twiv.

TWiV 174: Dog runs and mooing miRs

On episode #174 of the podcast This Week in Virology, Vincent, Alan, and Rich consider whether pet dogs might transmit human noroviruses, and an RNA virus microRNA that might be involved in oncogenesis.

You can find TWiV #174 at www.microbe.tv/twiv.