QUANTITATIVE MICROBIOLOGY: A BASIS FOR FOOD SAFETY
Source: Emerging Infectious Diseases, Oct-Dec97, Vol. 3 Issue 4, p541, 9p, 2 graphs
Author(s): McMeekin, T.A.; Brown, J.
Abstract: Focuses on the behavior of microorganisms in foods. Effect of the food type on the behavior of microbial populations; Predicting the effect of the food properties by mathematical models; Contribution of temperature to foodborne diseases; Suggestion of the basis for a universal temperature indicator; Need for the combination of kinetic and probability approaches to modeling.
Because microorganisms are easily dispersed, display physiologic diversity, and tolerate extreme conditions, they are ubiquitous and may contaminate and grow in many food products. The behavior of microbial populations in foods (growth, survival, or death) is determined by the properties of the food (e.g., water activity and pH) and the storage conditions (e.g., temperature, relative humidity, and atmosphere). The effect of these properties can be predicted by mathematical models derived from quantitative studies on microbial populations. Temperature abuse is a major factor contributing to foodborne disease; monitoring temperature history during food processing, distribution, and storage is a simple, effective means to reduce the incidence of food poisoning. Interpretation of temperature profiles by computer programs based on predictive models allows informed decisions on the shelf life and safety of foods. In- or on-package temperature indicators require further development to accurately predict microbial behavior. We suggest a basis for a "universal" temperature indicator. This
article emphasizes the need to combine kinetic and probability approaches to modeling and suggests a method to define the bacterial growth/no growth interface. Advances in controlling foodborne pathogens depend on understanding the pathogens' physiologic responses to growth constraints, including constraints conferring increased survival capacity.
Ensuring the microbial safety and shelf life of foods depends on minimizing the initial level of microbial contamination, preventing or limiting the rate of microbial growth, or destroying microbial populations. With many foods, these strategies have been practiced successfully for thousands of years. However, in the last decade, the incidence of foodborne disease has increased in the industrialized world (1), despite the introduction of the Hazard Analysis and Critical Control Points (HACCP) concept and the promulgation of regulations in food safety. The increased incidence of foodborne disease is caused by changes in agricultural and food processing practices, increasing international trade in foods, and social changes (which include changed eating habits and increased population mobility) (2).
This article develops the propositions that available quantitative information, properly applied, is a basis for improved food safety; the information available, largely an empiric description of microbial behavior in foods, highlights a lack of understanding of the physiology of foodborne pathogens; and knowledge of the physiology may lead to more precise control of foodborne bacteria and novel protocols to ensure the microbiologic safety of foods.
Characteristics of Bacteria
Bacteria have inhabited the earth for approximately three and a half billion years and have colonized almost every conceivable habitat (3). In fact, the development of microbial populations is probably precluded only when liquid water is absent or conditions are so extreme that rapid denaturation of proteins outpaces their rate of replacement. The major characteristics that underpin the success of prokaryotes are small size and ease of dispersal, physiologic diversity, and tolerance of extreme conditions (4). The temperature range over which microbial populations develop is -12 degrees Celcius (the temperature at which intracellular water freezes) to +112 degrees Celcius (the temperature at which liquid water is maintained only under elevated pressure). The pH range is pHI to pH 12, and the salinity range is zero to saturated (4).
Langeveld et al. (5), who studied microbial development in biofilms in a tubular heat exchanger used to pasteurize milk, report, the exploitation of different ecologic niches by bacteria. Through the ascending temperature range of the tube (-20 degrees Celcius to -90 degrees Celcius), the dominant microbiota changed from gram-negative bacteria such as Acinetobacter, to coliforms to Streptococcus thermophilus to thermophilic bacilli. At the highest temperature, the wall of the exchanger was colonized by Thermus thermophilus. Thus, it appears that contaminants deposited along the length of the tube were selected by the in situ temperature, with the fastest-growing organism dominating.
Factors Affecting Microbial Behavior in Foods
Most studies in food microbiology are concerned with the rapid growth of populations, but in many ecosystems, the survival characteristics of the population also need to be considered. The longevity of bacterial spores and their resistance to harsh conditions are well documented. However, the ability of vegetative cells to resist stressful conditions is increasingly recognized as an important ecologic trait (6) Attention also needs to be given to relatively slow-growing populations in
various situations, e.g., when the shelf life of a product is extended by control of rapidly growing spoilage organisms The behavior of foodborne microorganisms, be it the growth or death of microbial populations, is based on the time of exposure to environmental factors affecting population development; for example, equivalent kills of bacteria in milk are achieved by low temperature-long time pasteurization (60 degrees Celcius/30 min) and high temperatureshort time pasteurization (72 degrees Celcius/15 sec). When populations are in the biokinetic range, the rate at which they develop is determined by factors such as temperature, water availability, and pH applied in food preservation procedures. The extent of microbial growth is a function of the time the population is exposed to combinations of intrinsic food properties (e.g., salt concentration and acidity) and extrinsic storage conditions (e.g. temperature, relative humidity, and gaseous, atmosphere). Different factors assume dominance in different foods and preservation strategies. In many foods, the full preservation potential of single
property is restricted because of consider ations related to the esthetic, organoleptic, anc nutritional properties of the product. However several properties or conditions may be combinec to provide a desired level of stability (7).
In situations where the preservation strategy is, designed to slow the rate of population growth the effect will always be increased by storage temperature.
Temperature control in processing distribution, and storage (the cold chain) is crucial to ensure the adequate shelf life and safety of many common foods, including meat fish, poultry, and milk. Newer technologies including modified atmosphere packaging ant sophisticated products such as sous-vide meals do not obviate the need for strict temperature control. Indeed, the requirement for vigilance increases with increased shelf life and the possibility of growth of psychrotrophic pathogens, over an extended period.
Microbial Responses to Stress and Microbial Physiology
Bacteria have physiologic mechanisms enabling them to survive in environments that preclude their growth. While some tolerance to environmental insults is adaptive, a wide range of protective mechanisms is induced when cells enter a stationary phase or become starved. These phenomena are under the control of the rpoS gene, which codes for a stationary-phasespecific sigma factor, expression of which triggers the development of a semidormant state in which bacteria
can better resist multiple physical challenges (36). This factor and the gene products whose expression it controls are of vital significance to food microbiology; they form the basis for a global stress response in which one stress can confer protection to a wide range of other stresses. Under the influence of this factor, bacterial cells respond very quickly to unfavorable changes in their environment. Often these responses are phenotypic and remain in place only during stress (37).
Low pH Tolerance
Brown et al. (37) demonstrated "acid habituation" (38), a phenotypic response to an environmental insult, for five strains of Escherichia coli. These strains showed a wide range of intrinsic acid tolerance, which for each strain was enhanced by exposure to nonlethal acidity (pH 5) before exposure to a lethal acid challenge (pH 3). Neutralization of the growth medium partially reversed tolerance to acid stress, underlining that acid habituation is a phenotypic response.
Furthermore, acid tolerance was correlated with changes in the fatty acid composition of the cell membrane. During acid habituation, monounsaturated fatty acids (16:1w7c and 18:1w7c) in the phospholipids of E. coli were either converted to their cyclopropane derivatives (cy17:0 and cy19:0) or replaced by saturated fatty acids. The degree of acid tolerance of the five strains of E. coli was highly correlated with the membrane cyclopropane fatty acid content, which may enhance the survival of cells exposed to low pH.
Low Water Activity Tolerance
Bacterial cells, when confronted by lowered water activity, regulate the internal environment by rapidly accumulating compatible solutes such as glycine betaine or carnitine (39). The solutes, which may be scavenged by the cell, exert their influence at very low concentrations; the effect is demonstrated both in limits and rate growth. These compounds appear also to provide cryotolerance as well as osmotolerance
We have argued that a thorough understanding of microbial ecology and physiology offers the best opportunity to control microbial populations in food and reverse the upward trend in the incidence of foodborne disease. Many food preservation strategies have their origin in empirical observations practiced for thousands of years. The systematic collection and collation of data on microbial behavior in foods spawned the discipline of food microbiology, within which predictive microbiology (quantitative microbial ecology) has accelerated our understanding of the microbial ecology of foodborne bacteria. Studies in microbial physiology will further enhance our knowledge and offer new possibilities for food preservation. The most disturbing aspect of the current crisis is that simple application of existing knowledge would lead to a marked reduction in the incidence of foodborne disease. Education of food handlers and consumers in basic hygiene and the consequences of temperature abuse is urgently needed as is a greater depth of understanding for those in technical positions in the food industry or those with regulatory responsibilities. Furthermore, an appreciation of the need for shared responsibility for food safety within all sectors of the continuum from farm to table, including the consumer, has to be developed. The U.S. Food Safety Initiative draft document emphasizes this point, as does the structure of the Australian Meat Research Corporation's Microbial Food Safety Key Program (53).
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