A discovery by scientists at the Institute of Food Research (IFR) will for the first time allow food companies to generate microbial risk assessments of specific products and improve their ability to predict botulism.
Botulism is caused by Clostridium botulinum, an anaerobic, spore-forming rod that produces a potent neurotoxin. Its spores are heat-resistant and can survive in foods that are incorrectly or minimally processed.
According to the US Food and Drug Administration’s Center for Food Safety and Applied Nutrition (CFSAN), seven types of botulism are recognised. Four affect humans and two affect animals, most commonly wild fowl and poultry, cattle, horses and some species of fish. A seventh type has been isolated from soil, but no outbreaks of the disease have been associated with it.
Foodborne botulism is a severe type of food poisoning caused by the ingestion of foods containing the potent neurotoxin formed during growth of the organism. The toxin is heat labile and can be destroyed if heated at 80°C for ten minutes or longer. The incidence of the disease is low, but the disease is of considerable concern because of its high mortality rate if not treated immediately and properly.
Most of the 10 to 30 outbreaks that are reported annually in the USA, for example, are associated with inadequately processed, home-canned foods, but occasionally commercially produced foods have been involved in outbreaks. Sausages, meat products, canned vegetables and seafood products have been the most frequent vehicles for human botulism.
In nature, both the organism and its spores are widely distributed. They occur in both cultivated and forest soils, bottom sediments of streams, lakes, and coastal waters, and in the intestinal tracts of fish and mammals, and in the gills and viscera of crabs and other shellfish.
The CFSAN says that the types of foods involved in botulism vary according to food preservation and eating habits in different regions. Any food that is conducive to outgrowth and toxin production that when processed allows spore survival and is not subsequently heated before consumption can be associated with botulism. Almost any type of food that is not very acidic - pH above 4.6 – can support growth and toxin production. Botulinal toxin has been demonstrated in a considerable variety of foods such as canned corn, peppers, green beans, soups, beets, asparagus, mushrooms, ripe olives, spinach, tuna fish, chicken and chicken livers and liver pate, luncheon meats, ham, sausage, stuffed eggplant, lobster, and smoked and salted fish.
The IFR’s breakthrough is in understanding how these spores germinate and divide.
Lead author of the newly published study Sandra Stringer said: “We set out to unravel the various stages within lag time leading to the production of deadly neurotoxin. This is like looking at the time between loading a gun and actually pulling the trigger.”
Spores are the time travellers of the bacterial world. They are produced at times of environmental stress and exist in a state of suspended animation. In the protective pod of a spore coat, they resist temperature extremes and dehydration and can survive for millions of years until conditions are ripe for germination.
A single spore of Clostridium botulinum can lead to neurotoxin production in food. Previous studies have found that the lower the number of spores, the more difficult it is to predict growth patterns. However, prediction of lag time has until now been based on the belief that the first spore to germinate will be the first to produce actively dividing cells and start toxin production.
The IFR study is the first to investigate each stage within lag time and the relationship between them.
“The only way to study each stage in detail is by using microscopy and image analysis,” said Stringer. “We developed a novel imaging system and made microscopic observations of 1739 spores. We tracked their irreversible progress through germination and rehydration to shedding the spore coat, emerging as a young cell, maturing and finally beginning cell division. We found that each stage from germination to growth is variable between individual spores and none of the stages are related. Germination is therefore not a good predictor to use in risk assessment work as it underestimates the time to growth and toxin production.”
Images of individual spores were captured every five minutes for 15 hours then analysed.
“This was painstaking work, but worth it”, said Stringer.
IFR mathematical biologist Gary Barker says the findings have immediate practical benefits. “This fundamental science can be incorporated into real risk assessments for real products. Food companies can approach us for microbial risk assessments of specific products based on a model we have developed that reflects on the variability of spore lag time,” he concluded.
