Bacillus cereus is common in the environment but emetic toxin producing isolates are rare

Abstract

Aims: To determine the incidence of emetic toxin producing Bacillus cereus in soil, animal faeces and selected vegetable produce to compare the results with the previously reported high incidence in rice paddy fields. To examine whether the emetic toxin has antibiotic activity.

Methods and results: The incidence of emetic toxin producing B. cereus was evaluated by plating on selective agar 271 samples of soils, animal faeces, raw and processed vegetables. Overall, 45.8% of samples were positive for B. cereus. One hundred and seventy-seven B. cereus isolates were recovered at 30 degrees C with the grand mean spore count being 2.6 +/- 1.7 log(10) CFU g(-1) and 148 B. cereus isolates were recovered at 7 degrees C with the grand mean spore count being 2.2 +/- 1.2 log(10) CFU g(-1) of the 177 B. cereus isolated at 30 degrees C, only 3 were positive for emetic toxin production at a titre of 1/64, 1/32, 1/16, respectively. Also, 1 of 148 B. cereus isolated at 7 degrees C was positive for emetic toxin production to a titre of 1/128. All positive isolates came from washed or unwashed potato skins, one was psychrotrophic as determined by PCR and growth at 7 degrees C on subculture. The emetic toxin was not shown to have any antibiotic effects in growth inhibition studies.

Conclusions: While B. cereus was a common isolate, the incidence of the emetic strain was rare. This is in contrast to previous findings of the high incidence in rice paddy fields and the processing environment, which may suggest rice is a selective area for growth of the emetic strain of B. cereus.

Significance and impact of study: The finding that a psychrotrophic isolate of B. cereus can produce emetic toxin is the first ever such observation and suggests the possibility that psychrotrophic isolates could grow in refrigerated fresh foods and cause emesis. The incidence of emetic B. cereus strains in rice paddy fields now requires further study for comparison with the low incidence found in other soils. The emetic toxin failed to inhibit the growth of other bacterial, fungal and yeast species. Whether the toxin (which is similar in structure to the antibiotic valinomycin) plays a competitive role in the environment therefore remains unclear.