Spore formation, especially by molds and some bacterial species, enables them to survive for a long time and provides a basis for the continuation of the species. It also provides a means of their easy dissemination by dust and air in the environment.1,2 In this manner, foods can be contaminated
by their spores rather easily from various sources. In a suitable food environment, spores germi- nate, grow, and produce undesirable (or desirable) effects. Mold and yeast spores are relatively sensitive to heat, and their growth can also be prevented by storing foods in the absence of air.
Many species of Bacillus, Clostridium, and Desulfotomaculum are associated with food spoil- age and foodborne diseases. As a result of high heat resistance, the spores are of special interest and importance in food processing. Special attention must be given to processing and preserving the foods so that the spores are either destroyed or prevented from undergoing germination and outgrowth because ungerminated spores cannot cause spoilage or foodborne disease. Another possibility is to induce the spores to germinate and outgrow and then expose them to an antibacte- rial treatment to destroy them. Superdormant spores of the spoilage and pathogenic species pose another problem. As they are not detected normally with the other spores, a processing condi- tion could be wrongly adopted with the idea that it will eliminate all spores. Subsequently, these surviving superdormant spores can germinate, outgrow, and grow, and can either cause the food to spoil or make it unsafe. As it is impossible to destroy all spores in many foods, several specific methods or a combination of processing and preservation methods have been developed to over- come problems of spores in food.1–3
In the canning of low-acid foods, very high heat treatment is employed to achieve commercial sterility that kills spores of all pathogenic bacteria and most spoilage bacteria (except some thermo- philic spoilage bacteria). To prevent germination of spores, depending on the food type, nitrites (in processed meat), low pH (acid products), low AW, or high salt are used. High hydrostatic pressure is
currently being studied to determine its spore destruction potential. Although spores of molds are destroyed at relatively low pressure (<400 MPa), spores of many pathogenic and spoilage bacteria need a combination of very high pressure (≥700 MPa) and high temperature (≥90°C) to obtain commercial sterility.7 It has been known for long that spores of many foodborne bacteria can be acti-
vated to germination and outgrowth at a lower pressure range. Following such a pressure treatment, another antibacterial treatment, such as another pressure cycle, heat, or antimicrobial preservative can be given to destroy the germinated and outgrown spores before cell growth starts. The influence of several parameters on germination, such as pressure range, time of pressurization, pressuriza- tion temperature, and holding time following pressurization, is currently being studied.8 Table 9.1
presents the results of a study. It can be seen that under any given condition, germination induction varies greatly with species. In general, germination increases with pressure and temperature within the range studied. More such studies with many strains of the important species will provide more meaningful data to determine the potential of such treatment to control bacterial spores in food.
Microbial Sporulation and Germination ◾ 97
Conclusion
Spore formation by certain yeasts, molds, and bacterial species is a means of survival and continu- ation of the life process. In yeasts and molds, sporulation occurs by asexual and sexual processes; in bacteria, it occurs through differentiation, regulated and expressed by many genes.
Spore formation enables the dissemination of the species widely in the environment as well as contamination of foods. Their growth in food can be undesirable when they cause spoilage and produce toxins (except yeasts) in food and can be desirable in the processing of some foods. Methods of destruction of spores and inhibition of germination of spores (of bacteria) by different means are used to control their growth in food.
Some bacterial species sporulate as a means of survival strategy under conditions of physical, chemical, or environmental stresses by genetically regulated processes. Cells of many bacterial spe- cies, when exposed to conditions suboptimal for growth or sublethal, manifest different character- istics, the control mechanisms of which are not properly understood currently. These conditions are observed with many foodborne bacteria and have important implications in food microbiol- ogy, which are discussed in Chapter 10.
QUESTIONS
1. List the differences among mold, yeast, and bacterial spores. 2. List five genera of foodborne bacteria that form spores.
3. Draw and label the structure of a bacterial spore and discuss the functions or characteristics of each structural component.
table 9.1 Germination induction of Bacterial endospores by Hydrostatic Pressure at 25°C and 50°C
Bacterial Strain
Pressurization for Five Minutes at Temperature (°C) % Germination Inductiona at MPab 138 345 483 Bacillus cereus ATCC10876 25 99 99 99 50 99 99 99 Bacillus stearothermophilus ATCC 12980 25 45 88 88 50 99 98 98 Clostridium sporogenes PA 3679 25 0 0 4 50 40 50 82 Clostridium perfringens ATCC 1027 25 17 21 29 50 12 40 44
a The spore suspensions were pressurized for five minutes either at 25°C or 50ºC, then
stored at 4ºC for one hour and heated at 75ºC for 15 minutes to destroy the germi- nated spores. Germination induction was determined by enumerating the surviving spores and subtracting the number from the unpressurized (control) spore suspensions.
98 ◾ Fundamental Food Microbiology
4. List the stages between the formation of a bacterial spore and its emergence as a vegetative cell. Also, list the major events that occur in each stage.
5. Discuss the triggering mechanisms in sporulation and spore germination in bacteria. 6. Discuss the importance of bacterial spores in food.
7. Briefly discuss the methods used to control problems associated with bacterial spores in food.
8. Explain how low-range hydrostatic pressure can be combined with other antibacterial treat- ment to destroy bacterial spores in food.
References
1. Feofilova, E.P., Ivashechkin, A.A., Alekhin, A.I., and Sergeeva, Y.E., Fungal spores: Dormancy, germi- nation, chemical composition, and role in biotechnology (review), Appl. Biochem. Microbiol., 48, 1–11, 2012.
2. Gould, G.W., Germination. In The Bacterial Spores, Gould, G.W., Ed., Academic Press, New York, 1969, p. 397.
3. Gombas, D.A., Bacterial sporulation and germination. In Food Microbiology, Vol. 1, Montville, T.J., Ed., CRC Press, Boca Raton, FL, 1985, p. 131.
4. Sneath, P.H.A., Endospore-forming Gram-positive rods and cocci. In Bergey’s Manual of Systematic Bacteriology, Vol. 2, Sneath, P.H.A., Ed., Williams & Wilkins, Baltimore, 1986, p. 1104.
5. Baril, E., Coroller, L., Couvert, O., El Jabri, M., Leguerinel, I., Postollec, F., Mafart, P. et al., Sporulation boundaries and spore formation kinetics of Bacillus spp. as a function of temperature, pH and AW, Food
Microbiol., 32, 79–86, 2012.
6. Lengeler, J.W., Drews, G., and Schlegel, H.G., Biology of the Prokaryotes, Blackwell Science, New York, 1999, p. 586.
7. Farkas, D.F. and Hoover, D.G., High pressure processing: Kinetics of microbial inactivation for alterna- tive food processing technologies. J. Food Sci., 65, 47, 2000.
8. Ray, B., High hydrostatic pressure: Microbial inactivation and food preservation. In The Encyclopedia of Environmental Microbiology, Britton, G., Ed., John Wiley & Sons, New York, 2002, p. 1552.
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