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Master of Science (M.S.)
Year Degree Awarded
Month Degree Awarded
B.cereus, Spores, levels, Enterotoxigenicity, Growth, Rice
Bacillus cereus is a ubiquitously found foodborne pathogen that is frequently associated with two types of illness: emesis and diarrhea. Two heat labile enterotoxins have been associated with the diarrheal syndrome whereas a heat stable acid stable peptide toxin has been associated with the emetic syndrome. In the U.S, B. cereus is responsible for 1-2% of the total outbreaks from bacteria. Although there are reports of isolation and characterization of this pathogen from various food stuffs all around the world, there are no reports on the levels, toxin producing ability, or growth characteristics from U.S retail rice. Considering that rice is grown mostly in developing countries and most of the rice in the U.S is imported, there is a high chance of the rice being contaminated with B. cereus spores. Therefore, the major objective of this thesis was to characterize B. cereus spores from U.S retail rice. The levels were determined and further the enterotoxigenic ability and the growth characteristics along with the physical characteristics of the isolates were studied.
Among the 178 samples analyzed, Spores of Bacillus species were found in 94 (52.8%) of the rice samples with an average concentration of 32.6 CFU/g (3.6-460 CFU/g). Eighty nine of the 94 isolates were tested positive for one of the two enterotoxins produced by B. cereus. none of the 94 isolates tested positive for the emetic gene.
All the isolates generally grew well in cooked rice. Levels of 106/g were detected in cooked rice after 22h at 200C and after 34h at 170C whereas at 120C the counts did not go above 104/g even after 48h. A significant difference in the heat resistance of the emetic and the diarrheal strains was found. The emetic but not the diarrheal type grew well at inoculum levels of 102/g and 103/g level following cooking. So these results suggest although the diarrheal type are more predominant in U.S retail rice, the chances of foodborne illness arising from the diarrheal strains is low.
B. cereus, B, thuringiensis and B. mycoides were investigated for their physical characteristics. Appendages were not found on B. mycoides. By contrast, all the isolates had exosporia. The isolates were characterized to be moderately to highly hydrophobic and all the isolates had a net negative charge. Judging by their physical characteristics, it can be concluded that these spores may have a high affinity for adhering to inert surfaces.
Ronald G Labbe