Enumeration Comparison Study of BACARA® 2 and RAPID'B. cereus® Agars to Mannitol-Egg Yolk-Polymyxin (MYP) Agar for Detection of Bacillus cereus Group Strains.

Abstract

Background: The Bacillus cereus group (B. cereus sensu lato) is a group of spore-forming strains. B. cereus group is a human pathogen associated with foodborne outbreaks with symptoms of diarrhea and emesis.

Objective: This study evaluates the performance of BACARA® 2 (BAC2) and RAPID'B. cereus® (RAPID) agars compared to mannitol-egg yolk-polymyxin (MYP) agars and the original BACARA® (BAC) media to detect and enumerate the B. cereus group in pure culture and foods.

Methods: We incorporated five food types to include liquid milk, whey powder, mashed potatoes, rice, and tea bags. For each food matrix, there were five test portions per level inoculum prepared (low, medium, and high), and a negative control. Individual test portions were diluted 1:10 in Butterfield's phosphate buffer (BPB) and 0.1 mL of the dilutions were inoculated on BAC2, RAPID, and MYP agars. In-house R script was applied to conduct statistical analysis to compare medium performance in food adulteration tests.

Results: Inclusivity testing determined that all six B. cytotoxicus strains grew on BAC2, RAPID, and MYP but only one strain grew on BAC after 24 h incubation. RAPID had similar results as BAC2 in single-colony tests. In food tests, relative level of detection (RLOD) values indicated MYP was more sensitive for detecting B. cereus strains than BAC2 and RAPID, which is a result of B. cytotoxicus strains in mashed potato and whey powder needing an additional 24-hour incubation to exhibit typical morphology on BAC2 and RAPID.

Conclusion: Our study demonstrated the use of BAC2 and/or RAPID agar will improve the performance of the U.S. Food and Drug Administration Bacteriological Analytical Manual (BAM) B. cereus method, particularly for the detection of B. cytotoxicus. Importantly, the use of BAC2 and/or RAPID will decrease the time to detect viable B. cereus strains without additional confirmation steps compared to MYP.

Highlights: For improved isolation and identification of B. cereus group in foods, we propose the use of BAC2 or RAPID as optional agars, along with MYP.