It’s not the curves, it’s the oxygen groups: Viral attachment to produce

In an effort to understand and eventually reduce the incidence of foodborne illnesses, University of Illinois researchers studied the ability of pathogenic viruses to adhere to fresh produce surfaces.

lettuce“We chose 24 of the most common salad vegetables in the U.S. and assayed them to see if there was any relationship between the morphology and chemistry of the leaf or fruit surface and the adherence of viral particles, before and after a washing treatment,” says U of I geneticist Jack Juvik.

The researchers inoculated leafy salad greens and tomatoes with a swine virus that mimics human rotavirus, a common pathogen responsible for diarrhea, vomiting, fever, and abdominal pain. After exposing the vegetable surfaces to the virus, the researchers rinsed the vegetables twice with a standard saline solution.

“We correlated virus adherence to roughness of the surface at different scales. We also looked at the chemistry of the proteins and waxes associated with the leaf cuticle – a waxy layer that protects the plant against diseases and reduces water loss,” Juvik explains. “Before this, no one had tested the relationship between chemistry and surface texture on the adherence of virus particles.”

The researchers found a thousand-fold difference in the number of viral particles adhering to different types of leafy greens and tomatoes. Vegetables with three-dimensional crystalline wax structures on the leaf cuticle harbored significantly fewer virus particles after rinsing. This was counterintuitive, as it was expected that small virus particles could “hide” in the rough structures of these cuticles.

lettuce.nov10“I was surprised, too,” Juvik says. “But normally, viruses adhere to oxygen groups, like OH, which are associated with proteins and carbohydrates on the surface. When the wax completely covers the surface, it becomes totally hydrophobic, which renders the whole leaf surface harder for viruses to attach to. Furthermore, rinsing those leaves with water gives the viruses the OH groups they’re looking for, so they’re easier to wash away.”

Produce is exposed to viruses and other pathogens in a number of ways, including contaminated irrigation water, animal wastes, and handling by sick workers. But because salad vegetables are consumed fresh, pathogens cannot be killed by cooking or most other sterilization methods.

“Viruses are literally everywhere, causing many opportunities for infection. But the information from this study can be used down the road to select or breed for varieties that might have the capacity to reduce adherence of these particles,” Juvik explains.

The researchers have already repeated the study using the bacterium E. coli, but they plan to look at even more vegetable varieties and pathogens in future studies.

The article, “Influence of epicuticular physiochemical properties on porcine rotavirus adsorption to 24 leafy green vegetables and tomatoes” was published in PLOS One. The study was led by Lu Lu, whose co-authors included Juvik, Kang-Mo Ku, Sindy Paola Palma-Salgado, Andrew Page Storm, Hao Feng, and Thanh Nguyen, all from the University of Illinois. The project received funding from the USDA’s National Institute of Food and Agriculture.

Rare amino acid influences E. coli O157 infection

Scientists have discovered how a rare amino-acid in humans influences the behavior of the E.coli bacterium.

e.coliO157H7Most of the thousands of strains of E. coli are harmless, with many being a normal part of the gut flora in healthy people, however some strains can cause illness in humans.

Among the most well-known is E. coli O157, typically acquired via contaminated food, which causes severe diarrhea and can lead to kidney damage.

The O157 strain only infects the gut so scientists at the University of Glasgow wanted to know what stopped it from spreading to other parts of the body.

The team led by Dr Andrew Roe, and PhD student James Connolly of the Institute of Infection, Immunity & Inflammation, analyzed the genome sequence of 1,500 strains of E. coli.

They wanted to see how the genes of the bug, which enable it to attach to and infect a host, responded to varying concentrations of D-Serine, an amino-acid produced in the brain where it plays a role in nerve signaling.

They found that E. coli O157 is unable to attach itself to host tissue in high concentrations of D-Serine. Other strains, such as those that cause meningitis, thrive in the presence of the amino-acid.

The discovery, published in the ISME Journal, opens up the possibility of altering the diet to increase levels of D-Serine to prevent E. coli O157 infection or perhaps treat it.

Dr Andrew Roe, senior lecturer, said: “This work provides new insights into the infection process with the aim of developing compounds that block such bugs from attaching to the host.

“With many strains of E. coli developing resistance to traditional antibiotics, such approaches are urgently needed.

“If we can disarm such bacteria rather than killing them it puts less pressure on the bacteria to evolve into something that is resistant to treatment.”

e.coli.magnifiedE. coli O157 doesn’t normally live in humans, instead residing in the gut of cattle. Eating contaminated food is the most common cause of infection but it can also be picked up in the environment, through contact with the bacteria in fields, for example.

The genetic variety between strains of E. coli is huge, with around 2,000 ‘core’ genes and 18,000 genes that vary between strains. Different strains are able to attach themselves to different tissues, causing a range of different infections.

The bacterium can cause a wide range of infections including those of the gut, bladder, bloodstream and brain. These can be very common, for example, over half of all women suffer from E. coli associated bladder infections at some point in their lives.


The host metabolite D-serine contributes to bacterial niche specificity through gene selection

ISME Journal [ahead of print]

James PR Connolly, Robert J Goldstone, Karl Burgess, Richard J Cogdell, Scott A Beatson, Waldemar Vollmer, David GE Smith, and Andrew J Roe


Escherichia coli comprise a diverse array of both commensals and niche-specific pathotypes. The ability to cause disease results from both carriage of specific virulence factors and regulatory control of these via environmental stimuli. Moreover, host metabolites further refine the response of bacteria to their environment and can dramatically affect the outcome of the host–pathogen interaction. Here, we demonstrate that the host metabolite, D-serine, selectively affects gene expression in E. coli O157:H7. Transcriptomic profiling showed exposure to D-serine results in activation of the SOS response and suppresses expression of the Type 3 Secretion System (T3SS) used to attach to host cells. We also show that concurrent carriage of both the D-serine tolerance locus (dsdCXA) and the locus of enterocyte effacement pathogenicity island encoding a T3SS is extremely rare, a genotype that we attribute to an ‘evolutionary incompatibility’ between the two loci. This study demonstrates the importance of co-operation between both core and pathogenic genetic elements in defining niche specificity.

Salad Smackdown at Food Micro ’08

The press releases were fast and furious and the excitement non-stop  today in response to some new research about Salmonella sticking to salad greens that was presented at Food Micro ’08 in Aberdeen.

Professor Gadi “Flagella” Frankel of Imperial College London was first into the ring yesterday with a press release containing tragically cliché headline, How Salmonella bacteria contaminate salad leaves — it’s not rocket science, and produced by his own Imperial Colleague that said,

"In their efforts to eat healthily, people are eating more salad products, choosing to buy organic brands, and preferring the ease of ‘pre-washed’ bagged salads from supermarkets, then ever before. All of these factors, together with the globalisation of the food market, mean that cases of Salmonella and E. coli poisoning caused by salads are likely to rise in the future. This is why it’s important to get a head start with understanding how contamination occurs now.”

U.K. media outlets rose to the challenge, with the Horrible Herald inverting the order of the press release to lede with,

“The growing popularity of pre-packed salads is likely to lead to an increase in food poisoning cases, scientists warned yesterday.

“They said the increased uptake in the salads in particular, but also in fruit and vegetables, is likely to be reflected in a future rise in food poisoning.

Professor Gadi Frankel, from Imperial College, said a greater understanding of how salads are contaminated is important because cases of food poisoning caused by salads are "likely to rise in the future."

The Fresh Prepared Salads Producer Group – really, that’s the association name, how about Big Salad – today, “completely refutes suggestions in the press that prepared salads are unsafe to eat," and tag teamed with Prof. Bill “Critical” Keevil, professor of environmental health care at the University of Southampton, who was at the conference in Aberdeen where the salad research which sparked the stories was presented, and said,

"I was extremely disappointed by the quality of the data presented and its interpretation. We have known for a long time the various mechanisms that bacteria can use to attach itself to a range of surfaces, including plants. This is not new."

Big Salad said in a statement:,

"Our products sold as ‘washed and ready to eat’ are just that. We have long recognised that to produce a safe-to-eat salad one needs safe-to-eat produce off the field. To achieve that, we strive to ensure that dangerous microbes do not get the opportunity to contact our crops – such that hypotheses as to how they initially adhere are irrelevant. The UK prepared salads sector has an unrivalled safety record and employs stringent controls, described as ‘excellent’ by the FSA – not necessarily the case elsewhere in the world. There has not been a confirmed outbreak associated with prepared salad since 2001 in the UK. … There is absolutely no evidence to suggest that re-washing a prepared salad will do any good at all – and it’s even possible that exposing the salad leaf to the ‘kitchen sink’ will increase the food safety risk. Indeed, the Advisory Committee on the Microbiological Safety of Food (FSA) has recently determined that re-washing is unlikely to remove any contamination remaining on the produce after the manufacturing process.”

To further muddle things, Judith “Hey Now” Hilton wrote on a U.K. Food Standards Agency blog that,

“In fact, while we advise that it’s a good idea to wash salad items in general, there is no need for consumers to rewash ready-to-eat bagged salads unless it says otherwise on the packet.  You can best help yourself by following good food hygiene practice at home – it’s important to follow the 4Cs – cooking, cleaning, chilling, avoiding cross contamination.”

Smackdown. Consumers, if you get sick from ready-to-eat salads, it’s your fault.