B. cereus from a national collection of 564 strains associated with 140 strong-evidence foodborne outbreaks (FBOs) in France during 2007 to 2014 were characterised.
The aim was to improve monitoring and investigation of B. cereus-induced FBOs, assess the risk of emerging clusters of strains and identify strain variability.
In 66 of the FBOs, B. cereus was isolated with other bacterial species (including S. aureus and C. perfringens) making it impossible to affirm if it was the cause.
How common is foodborne B. cereus?
The study focused on 339 B. cereus strains isolated from food samples analysed during 74 FBOs where no other pathogenic bacteria were detected.
For 42 of the 74 FBOs, a unique strain pattern was identified, providing a strain collection for further analysis of the correlation between B. cereus genotypic features and associated diseases.
The pathogenic spectrum of B. cereus ranges from strains used as probiotics to those lethal to humans and it remains difficult to predict pathogenic potential, said the researchers.
If B. cereus is suspected, several identification tests can be performed: morphology tests on selective media, resistance to polymyxin B, lecithinase synthesis, haemolytic capacity, mannitol fermentation and starch hydrolysis.
B. cereus is the second most frequently found causative agent of confirmed and suspected foodborne outbreaks in France after Staphylococcus aureus.
Starchy food and vegetables were the most frequent food vehicles identified and only 14% were associated with foodstuffs of animal origin.
A total of 747 of 911 human cases were in institutional catering contexts.
Weak or strong evidence
Between 400 and 108 B. cereus CFU/g were found in incriminated foods. Levels lower than 105 CFU/g were found in 48/57 FBOs due to diarrhoeal strains and in 11/17 due to emetic strains.
An FBO is ‘strong-evidence’ when the food vehicle, source, link between outbreak cases and food vehicle, place of exposure and contributory factors are known and reported.
When several parts are missing the outbreak is considered as ‘weak-evidence’.
Between 2006 and 2014, B. cereus was the second or third major cause in weak evidence FBOs.
In 2014, B. cereus represented the second cause in weak-evidence FBOs, with 1,902 human cases for 224 FBOs, and the second cause of strong-evidence FBOs, with 23 FBOs accounting for 447 human cases and 18 hospitalisations.
B. cereus strains isolated from foodstuff suspected of being involved in an FBO are usually collected by the laboratory for food safety in ANSES (Agency for Food, Environmental and Occupational Health and Safety).
During 1996 to 2005 only 94 strong-evidence and 196 weak-evidence FBOs were reported, whereas in 2014, 23 and 241 strong- and weak-evidence FBOs were notified.
The emetic syndrome is characterised by vomiting and nausea, usually 30 minutes to six hours after ingestion and can be confused with Staphylococcus aureus.
It is due to ingestion of a thermostable toxin known as cereulide, pre-formed before ingestion of contaminated foods.
Diarrhoeic symptoms are characterised by abdominal cramps and watery diarrhoea within eight to 16 hours after eating contaminated foods.
These symptoms and incubation periods can be confused with Clostridium perfringens poisoning.
Researchers proposed an approach based on reported symptoms and incubation period.
“We recommend collecting at least five colonies from each food sample potentially contaminated with B. cereus, with different morphologies, as several B. cereus with different genetic characteristics may be present in the same food product.”
“Bacillus Cereus-induced foodborne outbreaks in France, 2007 to 2014: Epidemiology and genetic characterisation”
Authors: B Glasset, S Herbin, L Guillier, S Cadel-Six, M Vignaud, J Grout, S Pairaud, V Michel, J Hennekinne, N Ramarao and A Brisabois