Pepper virus serves as proxy for human pollution

By Gary Raham
Nature Writer and Illustrator

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Ever see those signs on the road: Accident Ahead. Expect Delays? By the time the message is posted, the tow trucks are halfway to the garage and the lawyers have already spent their fees.

It's a bit like that now with "Polluted Water" signs at beaches and estuaries. The organism typically used to determine risk is Escherichia coli, that friendly vitamin K-producing bacterium in our colons that considers human waste a vital resource.

To measure pollution, health agencies have to inoculate growth media with water samples and count the number of E. coli. colonies that grow – a process that can take days. By then the threat to swimmers at a beach may have passed.

Scientists now believe a virus may be hanging around our colons that can make testing faster: the pepper mild mottle virus, a pathogen of those hot and bell peppers we love to eat in salsas and burritos.

The study that fingers the pepper virus as a pollution proxy originated with researchers at the College of Marine Science, University of South Florida in St. Petersburg. They found that the pepper mild mottle virus "is widespread and abundant in wastewater from the United States."

The virus is associated specifically with human waste, not animal waste (with the exception of some chicken and seagull samples). It occurs "with several other pathogens and indicators of fecal pollution," but not in unpolluted seawater samples. Because the pepper virus is much more abundant in raw sewage than are viruses that can make people sick, it may make a good indicator of a recent pollution event.

Why can these viruses be detected faster? To answer this question, it helps to visualize the world of the very small.

A human cell may seem small, since it has to be magnified 100 times or more to see it, but human cells are like double-trailer semis compared with Volkswagen-sized bacterial cells like E. coli. Both semis and Volkswagens can travel the open road together with capable engines and sturdy tires. Likewise, human cells and bacterial cells are independent entities that grow and reproduce using genetic instructions from DNA to produce the thousands of proteins needed to build infrastructure and facilitate critical chemical reactions in the body.

Viruses are something else. They possess RNA that codes for (in the case of Tobamoviruses like the pepper mild mottle virus) four molecules: a protein shell, two proteins necessary for reproduction, and one protein that allows them to hitch a ride with a complex cell or bacterial cell. They are kind of like lawnmowers with special validating tickets to hitch rides with either Volkswagens or semis.

OK, the metaphor begins to creak with the strain.

At any rate, because viruses possess a handful of polypeptide molecules, scientists can screen for them fairly easily as they run water samples through hair-like tubes filled with gel. Such molecules possess a net electrical charge and can be attracted by the appropriate electrode. One portable device developed by the Sandia National Laboratories for detecting water-borne pollutants has been compared to the "tricorder" used by the intrepid crew of the Enterprise on "Star Trek."

Of course nothing is quite as simple as it might seem. Pepper viruses are plant pathogens with molecular tickets that should allow them to pester only plant cells. They should have as much chance of creating problems for humans as a gambler has of breaking the bank at the Luxor in Vegas.

Other scientists, however, believe they have shown that the pepper mild mottle virus may be responsible for producing some immune responses in humans that lead to fever and abdominal discomfort. If corroborated, it's unclear whether this would affect the use of this virus as a pollution indicator.

Viruses are still pretty mysterious creatures. They were inknown until 118 years ago when Dmitri Iwanowsk (1864-1920) discovered that agents infectious to tobacco plants could pass through filters that would trap the smallest bacteria. Those tobacco mosaic viruses (close relatives of the pepper virus) and all the viruses discovered since then (including more sinister yellow fever and flu viruses) represent just a fraction of a vast "unknown genomic pool," as one scientist describes it.

Some of those wandering genomic lawnmowers will cause us problems for sure, but others will become quite useful tools. Hopefully, the pepper mild mottle virus falls in the latter category.