Characterization of Bacillus cereus isolates from fresh vegetables and refrigerated minimally processed foods by biochemical and physiological tests

• Published in: Food microbiology Vol. 19; no. 5; pp. 491 - 499
• Main Authors: Valero, M., Hernández-Herrero, L.A., Fernández, P.S., Salmerón, M.C.
• Format: Journal Article
• Language: English
• Published: London Elsevier Ltd 01-10-2002; Elsevier
• Subjects: Biological and medical sciences; Food industries; Food microbiology; Fruit and vegetable industries; Fundamental and applied biological sciences. Psychology; Europe; Spain; Biochemical analysis; Vegetables; Bacillaceae; Characterization; Bacillus cereus; Refrigerated storage; Bacillales; Surveillance; Quality control; Bacteria; Physiology; Biological contamination; Quantitative analysis
• ISSN: 0740-0020; 1095-9998
• DOI: 10.1006/fmic.2002.0507

Bacillus cereus contamination has been evaluated in a total of 56 samples of fresh peppers, cucumbers, tomatoes, carrots, zucchini, garlic and onions, used commonly as ingredients to manufacture refrigerated minimally processed foods. It has been also evaluated in some of these products, such as gazpacho (Mediterranean vegetable cold soup made from fresh vegetables), salmorejo (vegetable cold purée containing bread and egg), ajoblanco (cold cream containing bread, almond, and olive oil as main ingredients) and zanaranja (carrot and orange juice mixture). The average counts did not exceed 10
4 cfu g
−1 in any case and all the vegetables and processed products except garlic and zanaranja presented contamination of this foodborne pathogen. Cucumbers presented the highest count among all the vegetable products analysed, followed by carrots. According to the ISO 7932:1993(F) confirmation procedure, a total of 36 presumptive
B. cereus strains were isolated from the 85 samples of various vegetable products analysed. Isolates were further characterized and identified by API 50CH/20E phenotypic system using APILAB Plus software, combined with additional tests of motility, oxidase activity and anaerobic growth. Of the 36 presumptive
B. cereus isolates, 32 (88·9%) were confirmed to be
B. cereus, two strains (5·5%) were identified as
B. licheniformis, and other two strains (5·5%) were identified as
B. firmus. About 81·3% (26 strains) of 32
B. cereus tested isolates hydrolysed starch and 71·9% (23 strains) produced diarrheal enterotoxin in TSB culture as detected by the BCET-RPLA test. The heat resistance at 90°C for spores of the 32
B. cereus strains ranged from 1·4 to 21·2 min. Strains unable to hydrolyse starch were the most heat-resistant, with
D
90 values higher than 10·8 min. All the enterotoxin-positive strains were able to hydrolyse starch.