Characterization of superdormant spores of Bacillus cereus and Bacillus weihenstephanensis
Date
2011
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Publisher
University of Delaware
Abstract
Superdormant spores are those which germinate extremely slowly compared to the rest of the spore population. Although the exact cause of spore superdormancy is not known, evidence suggests that superdormat spores have a fewer nutrient germinant receptors compared to dormant spores. The recent development of a method to isolate superdormant spores in a laboratory setting has made it possible to study the phenomenon of spore superdormancy. It is important to investigate the causes of spore superdormancy and how we can reduce the number of these spores in foods. Differences in the isolation of superdormant and heterogeneous spore populations of psychrotolerant strains of Bacillus cereus and Bacillus weihenstephanensis and one mesophilic strain of B. cereus were studied. The effects of sporulation temperature on the isolation of superdormant spores using various concentrations of individual nutrient germinants were investigated. The average recovery of superdormant spores was 17.79 % with the greatest average recovery with 250 μM L-asparagine and the lowest with 10 mM L-alanine. Spores of psychrotolerant strains showed the same germination rates as the mesophilic strain (p = 0.7001). Overall superdormant spore recovery was not significantly different with spores initially sporulated at 30°C compared to those initially sporulated at 37°C (p=0.4976).
Aqueous ozone has been demonstrated to have high potential for the inactivation of bacterial spores in foods. The effects of 190 ppm and 6.25 ppm of ozone on the recovery of superdormant spores compared to ozone-treated heterogeneous spore populations of all three strains was also studied. The percent recovery of spores for all strains and spore type was significantly reduced after 20 min of ozone treatment (p = 0.0001). Superdormant spores were identified to be approximately 20 % more resistant to ozone treatment than heterogeneous spore populations (p = 0.0234). In addition, psychrotolerant strains were significantly more resistant to ozone treatment than the mesophilic strain. A trend for the percent recovery of spores to increase above the control (above 100 % recovery) occurred commonly for superdormant spores exposed to lower levels of ozone for shorter-time intervals. The potential of an ozone-high hydrostatic pressure (HHP) hurdle technology model was explored. Superdormant spores that were or were not pretreated with 190-ppm aqueous ozone for 10 min were subsequently treated at 500 MPa and 40°C for 10 min. The recovery of ozone-pretreated superdormant spores subsequently treated with HHP was not lower than the recovery of spores that were not pretreated with ozone (p = 0.7380). There was no significant difference in the percent recovery of superdormant spores after ozone and HHP treatment between psychrotolerant and mesophilic strains (p = 0.4865).
The results of this study help to further characterize superdormant spores and the cause of spore superdormancy. It is important to understand spore superdormancy in order to better control superdormant spores in the food industry. Hurdle technologies, including the proposed ozone-HHP technology, have great implications for the control of dormant and superdormant spores in foods and should continue to be explored.
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Keywords
Psychrotolerant