Source solution for 'hamburger disease'

Prefer your hamburger rare? Big beef-eating countries have been experiencing an epidemic of 'hamburger disease' in recent years, caused by pathogenic strains of the gut microbe Escherichia coli that originate in animals.

Toxins secreted by E. coli strain 0157:H, which can contaminate mince beef, can cause severe food poisoning -- 0157 infections caused by inadequately cooked meat have killed children in the US and Argentina.

The problem strain in Australia is E. coli 0111:H, found in pork mince -- it has caused several major food-poisoning outbreaks, including the infamous Garibaldi salami episode.

The 'H' in the code-names for both microbes stands for 'haemorrhagic' -- technically, the pathogens are known as enterohaemorrhagic strains. The symptoms include internal bleeding, diarrhoea, severe nausea and vomiting, and in some cases, permanent damage to major organs like the kidneys.

Pathogenic Salmonella bacteria contaminating poultry have also caused numerous episodes of food poisoning.

Delegates to this week's Australian Society for Microbiology (ASM) annual conference in Melbourne's Convention Centre have heard of two potential solutions to the pathogenic E. coli problem.

A Canadian research group is developing a vaccine, while CSIRO researchers at the Australian Animal Health Laboratory (AAHL) in Geelong are developing friendly strains of 'probiotic' gut bacteria that would be custom-designed to suppress or destroy pathogenic bacteria through biochemical warfare.

In both cases, the remedy seeks to address the problem at its source: in the animals whose own benign gut bacteria are so hazardous to humans.

Dr Mark Tizard's research group at AAHL is experimenting with natural compounds produced by bacteria that allow them to survive and thrive in competition with rivals in the rich microbial communities of the gut.

The compounds, called bacteriocins, are peptides -- small fragments of proteins -- that can literally punch holes in the opposition.

Tizard's group is develop genetic constructs that will be spliced into benign E. coli strains found in the digestive tract of cattle, or weakened Salmonella strains developed for vaccine production.

By daisy-chaining genes to produce "designer" cocktails of bacteriocins directed specifically at pathogenic E. coli strains, or other food-poisoning bacteria.

"If we're putting these engineered strains into cattle, and they're designed specifically to survive in cattle, they will be contained - they won't get into meat or dairy products," Tizard said. . Tizard said the project is being driven by the fact that when antibiotics are used as growth promoters in livestock, they select for resistance genes.

Food-contamination episodes can result in these resistant bacteria swapping genes with their counterparts in the human gut, creating resistant strains that are no longer susceptible to these antibiotics.

"There are still some substantial hurdles, but the attraction of the approach is that bacteriocins are not currently used as antibiotics in humans, so if bacteria in animals become resistant, they will not become a problem in humans."

Tizard said the advantage of daisy-chaining several bacteriocin genes is that bacteria would need to acquire multiple resistance genes, simultaneously, to survive in the gut flora -- the odds against this happening are extremely remote.

At the University of British Columbia in Canada, Prof Brett Finlay is leading a project to develop a vaccine against E. coli 0157:H.

The North American cattle industry is sponsoring the project -- Finlay and his colleague, Dr Andy Potter, of the Veterinary Institute Disease Organisation in Saskatoon, Saskatchewan, are already field-testing a prototype vaccine in 36,000 cattle in Canada.

The vaccine targets proteins that are essential to the microbe's ability to infect mammalian cells -- the Canadian team, like its Australian counterparts, felt it would be much easier and cheaper to vaccinate cattle than to vaccinate millions of humans against a rare, yet potentially lethal infection.