HERE鈥橲 something to contemplate as you recover from year-end feasting: your lower intestine is packed full of plant viruses, hitching a ride to their next victims.
Microbiologists know little about the viruses that live in the human gut because most viruses from faeces refuse to grow in lab cultures. Indeed, scientists studying the Norwalk virus, a notorious cause of gastroenteritis, had to feed human volunteers faecal extracts from infected patients to grow the virus.
But using the tools of modern genomics, it is now possible to screen faecal samples directly for whatever viral genes they may contain. When Forest Rohwer of San Diego State University in California used this technique in 2003 to look for viruses with DNA genomes, most of the genes he found were either from completely novel viruses or from phages 鈥� viruses which attack the gut鈥檚 bacterial inhabitants.
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So when Rohwer鈥檚 team and researchers led by Yijun Ruan of the Genome Institute of Singapore screened faeces for viruses with genomes made from RNA, they expected to find a similar collection of phages and unknown viruses. Instead, 97 per cent of the sequences they recovered came from viruses that infect plants (Public Library of Science Biology, DOI: 10.1371/journal.pbio.0040003).
Other microbiologists are surprised by this finding. 鈥淚 never considered this to be a possibility,鈥� admits Michael Blaut, who studies gut microflora at the German Institute for Human Nutrition in Potsdam. 鈥淏ut then, no one really looked before.鈥�
The viruses present varied from person to person, and between samples taken from the same person at different times. Of the 35 plant viruses Ruan and Rohwer detected, 24 were known crop pathogens.
鈥淧lants inoculated with human faecal extracts got infected, so using sewage sludge as fertiliser could spread crop disease鈥�
The most common was pepper mild mottle virus, or PMMV, which infects capsicums including chilli peppers, a prominent feature of the diet in both Singapore and southern California. The teams could not find the virus in fresh peppers, but it was plentiful in hot sauces and chilli powder, suggesting that diseased peppers tend to end up in these products.
The researchers then inoculated healthy capsicum leaves with human faecal extracts. The plants subsequently came down with PMMV disease, which suggests that using human faeces or sewage sludge as fertiliser could spread crop disease. Ordinary composting doesn鈥檛 kill viruses such as PMMV, says Ruan: 鈥淭hese are very tough viruses.鈥�
Plant viruses should not attack animal cells, although they may be able to reproduce in our gut bacteria. Ruan and Rohwer found that levels of plant viruses in faeces were 20 times as high as in food, but this could simply be because most of what we eat is digested and absorbed. 鈥淚 would not expect the viruses to be replicating, but the experiments must be done,鈥� says Blaut.
Either way, gut-dwelling plant viruses might be put to good use. Ruan and Rohwer speculate that they could be genetically engineered to carry molecules from viruses such as the polio virus or rotaviruses, which cause diarrhoea, and so act as vaccines.
