Microbiologists have characterised the DNA in bacteria responsible for vomiting

Apr 16 2004

James Cook University microbiologists have characterised the DNA in bacteria responsible for one of the world's most common forms of food poisoning.

And the bacteria, which cause millions of violent vomiting cases each year, are only one genetic step removed from anthrax.

Using a powerful molecular method, JCU postgraduate student Paul Horwood has identified the gene in Bacillus cereus that enables this tiny microbe to manufacture the toxin cereulide using the starch in rice.

Research suggests up to one-fifth of all food poisoning outbreaks worldwide may be caused by Bacillus cereus. It is responsible for two distinct food poisoning syndromes: one causes acute nausea and vomiting (the emetic syndrome) and the other diarrhoea.

The emetic syndrome is commonly associated with rice and now, thanks to the JCU project, scientists can detect the cause and carry out structured surveys to accurately define the prevalence of this form of food poisoning.

It all comes down to a gene that ultimately may enable rice companies to identify contaminated rice before it even goes to market.

Dr Graham Burgess, who co-supervised Mr Horwood's PhD studies in JCU's microbiology and immunology program, says Bacillus cereus produce enzymes that pull amino-acids out of the starchy rice substrate and link them together to build cereulide.

"The toxin is pre-formed in the rice before ingested by humans, making it extremely resistant to heat or acids. It survives passage through the stomach and into the gut, where it then causes illness. Scientists regard the illness as a form of intoxication, unlike most food poisoning syndromes that are treated as an infection," Dr Burgess said.

The JCU research has also established that Bacillus cereus have similar genes to Bacillus anthracis, which makes anthrax, and Bacillus thuringiensis, used as a pesticide on food crops because it kills insects.

"The only difference between these three species is the presence of plasmids, or extra loops of DNA," Mr Horwood said.

Bacillus cereus food poisoning is common in Asian countries due to the large amounts of rice consumed, but toxic strains of the bacteria are found worldwide.

"We think the toxin is a primary cause of food poisoning. The evidence for this is largely circumstantial, but it seems that the chances of food poisoning are very strong if the gene is present in food," Mr Horwood said.

"A common cause of the problem is the practice of flash-frying rice in Chinese restaurants.

"In some restaurants rice is flash-fried and then left to cool down. Just before it is served, the rice is again flash-fried to make it warm. Sometimes the rice will be stored overnight and used the next day.

"When this happens, contamination arises because the toxin and the spores of the organism are resistant to cooking. This means that the spores can germinate in stored food and produce large amounts of toxin that will not be destroyed by subsequent cooking.

"Within 30 minutes to two hours of eating the contaminated rice, you start vomiting and this lasts for around 24 hours."

There is potential for the JCU findings to be commercialised by companies wanting to test rice batches for toxic bacteria.

Dr Burgess said the extremely small size of the toxin cereulide has made it difficult up until now to produce a reliable toxin detection method. Cell culture methods have been labour-intensive, inaccurate and too subjective.

"However, we have used a molecular method called polymerase chain reaction or PCR to detect the emetic genes. This should assist in developing stronger quality controls for the rice industry," Dr Burgess said.

The project was funded by the Rural Industries Research and Development Corporation (RIRDC) and the Ricegrowers Cooperative.

Media contact: Theresa Millard, JCU Media Liaison Office, 07 47814822 or 0409 596 271