THIS WEEK IN ENVIRONMENTAL HEALTH




WEEK # 6 (October 5 - 9, 1998)


AN OVERVIEW OF FOOD MICROBIOLOGY AND FOODBORNE DISEASE

Objectives


At the completion of this lecture, the student should:

1. Discuss why the prevention of foodborne disease has been a problem and why we are seeing ongoing outbreaks.

2. List the foods that have been responsible for outbreaks.

Explain how foods can become contaminated with organisms.

4. Discuss the factors that comprise food ecosystems.

5. List the types of controls for preventing the growth of microorganisms in foods; give one example of a procedure for each type of control and one example of a food for which it is used.

6. Discuss new and/or future approaches to control of foodborne disease.



WHY PREVENTION HAS BEEN AND IS A PROBLEM*


1. No vaccines available for most foodborne pathogens.

2. Consumer education has been insufficient.

3. Food reaches consumer through long chains of industrial production in which many opportunities for contamination exist.

4. We have not understood mechanisms by which contamination and transmission occurred well enough to stop them.

5. We acquire them from animals that appear healthy and thus, detection in these sources is difficult.

6. We have very little information on how the healthy animals which harbor these organisms acquire and transmit them to humans. For example, we must now start investigating the safety of the food that the animals themselves eat and drink.

7. Contaminated foods usually look, smell, and taste normal and the pathogens often survive traditional preparation techniques; e.g. E. coli 0157:H7 can survive gentle heating that a rare hamburger gets.

Each time a new foodborne disease is identified, numerous questions have to be addressed to develop a rational approach to prevention and answering these questions takes time.


STUDY QUESTIONS

1. What is the nature of the pathogen and the disease? What is the animal reservoir?

2. What foods transmit the infection?

3. How does the pathogen get into the food and how does it exist there?

4. How do the animals become infected?

5. How can the disease be prevented?

SUMMARY


New food vehicles of disease carry new problems that are making detection of foodborne disease difficult. For example, because of consumer demand and the global food market, ingredients from many countries may be contained in a single dish, making detection of the incriminating agent especially difficult. Furthermore, these foods have fewer barriers to microbial growth and thus are more easily contaminated.

New pathogens have emerged as a result of increased host susceptibility due to increases in the numbers of immunocompromised agents, aging of population, and malnutrition (ref Morris & Potter).

Changing consumer demands, e.g. demands for foods that are convenient, fresh and all natural with no preservatives, and that promote health (ref Zink).
It now appears that we may need to use a combination of methods to prevent contamination of foods by certain organisms.

*References


1. Collins, JE: Impact of changing consumer lifestyles on the emergence/re-emergence of foodborne pathogens, Emerging Infectious Diseases 3:471-79, 1997.

2. Morris, JG, Jr., and Potter, M: Emergence of new pathogens as a function of changes in host susceptibility, Emerging Infectious Diseases 3:435-41, 1997.

3. Tauxe, RV: Emerging foodborne diseases: an evolving public health challenge, Emerging Infectious Diseases 3:425-34, 1997.

4. Zink, DL: The impact of consumer demands and trends on food processing, Emerging Infectious Diseases 3:467-69, 1997.



CHARACTERISTICS SHARED BY FOODBORNE PATHOGENS


1. Virtually all have an animal reservoir, ie. They are foodborne zoonoses.

2. For the most part, they do not cause illness in infected host animals.

3. Organisms can rapidly spread globally.

4. Many of these organisms are becoming increasingly resistant to antimicrobial agents because of widespread use of antibiotics in animals or in their food.


NEW FOOD VEHICLES
1. Traditional Vehicles

    • Undercooked meat, poultry, seafood
    • Unpasteurized milk

    • 2. Newer Vehicles


    • Eggs that are contaminated internally or externally (this has led to outbreaks in several egg-containing foods such as Caesar salad, lightly cooked eggs in omelettes and French toast, lasagna, meringue pie)
    • Apple cider
    • Venison jerky
    • Produce


    SOURCES OF ORGANISMS IN FOODS*


    Organisms get into food from natural (internal) sources and from external sources to which the food comes in contact from the time it is produced to the time when it is consumed.

    1. Internal Sources

    Most plants essentially sterile except for a few porous vegetables (e.g. radishes and onions) and leafy vegetables (e.g. cabbage and brussel sprouts). Food animals and birds contain a wide variety of normal flora in their digestive, respiratory, and urogenital tracts as well as in the skin, bones, hair, and feathers.


    2. External sources

    • Air (transient organisms from dust) - organisms include Bacillus spp., molds, Staphylococcus, Micrococcus, Sarcina.

    • Soil - because organisms can multiply in soil, their numbers may reach as high as billions per gram.

    • Sewage - a big problem where sewage is used as fertilizer.

    • Water - a great potential source of microorganisms because of its wide variety of uses in the production and processing of foods. For example, it is used for irrigation of corps, washing foods, processing (e.g. pasteurization, canning), cleaning equipment, etc. It is also, of course, consumed by farm animals and used in processed foods.

    • Humans - Numerous people are involved in handling foods between production and consumption. People include those working in the food processing plants, retail stores, restaurants, and home. Microorganisms from improperly cleaned hands and lack of personal hygiene are a primary concern.

    • Food Ingredients - Ingredients and additives are not sterile and thus, must be produced under scrutiny conditions and give antimicrobial treatments prior to addition to foods.

    • Equipment - A wide variety of equipment in food preparation from harvesting to transportation, to processing and finally storage. When larger processing equipment is used continuously for a long period of time and not properly cleaned, microorganisms can multiply and continue to contaminate the food. Household equipment such as cutting boards and knives are a major source of concern.



    *References


    Ray, B: Fundamental food microbiology, CRC Press, Inc., Boca Raton, Florida, 1996.



    PREDOMINANT CONTRIBUTING FACTORS ASSOCIATED WITH
    CONFIRMED FOODBORNE OUTBREAKS
    • Improper refrigeration

    • Poor personal hygiene

    • Inadequate cooking or pasteurization

    • Contaminated equipment

    • Food from unsafe sources (i.e. raw foods that have been fertilized with manure or washed with contaminated water)



      NEW AND/OR FUTURE APPROACHES TO CONTROL OF FOODBORNE DISEASE*

      1. Better surveillance strategies:

      • FoodNet
      • PulseNet

      2. Hazard Analysis and Critical Control Points (HAACP)

      3. Development of strategies to prevent initial infection of animals

      • Examples
      • Developing additives for cattle feed that alter pathogen growth in feed or in animal
      • Immunization of animal reservoirs such as cattle or chickens
      • Chlorinating water sources of chickens

      4. Irradiation

      5. Use of combinations of methods for treating food

      6. Improving methods for diagnosing, managing, and treating severe foodborne illnesses

      7. More prudent use of antibiotics to limit antimicrobial resistance

      8. More research to address several issues

      • Examples:
      • How do pathogens persist in reservoirs?
      • How do pathogens contaminate food and cause disease?



      *References

      1. Tauxe, RV: Emerging foodborne diseases: an evolving public health challenge, Emerging Infectious Diseases 3:425-34, 1997.


      See also:

      1. http://www.fsis.usda.gov/background/bfoodnet.htm (FoodNet - Foodborne Diseases Active Surveillance Network).

      2. http://www.cdc.gov/ncidod/dbmd/pulsenet/pulsenet.htm (PulseNet - The National Molecular Subtyping Network for Foodborne Disease Surveillance).



      • FOODS ARE COMPLEX ECOSYSTEMS

      (Montville, TJ: Principles which influence microbial growth, survival, and death in foods. In Doyle, MP, Beuchat, LR, and Montville, TJ, eds.: Food microbiology: fundamentals and frontiers, ASM Press, Washington, DC, 1997.)

      Ecosystems consist of the environment, the organisms that are contained within and their interactions.


      Factors That Comprise Food Ecosystems*

      1. Presence and specific properties of pathogenic and spoilage microorganisms

      2. Intrinsic and extrinsic factors that influence the growth of these microorganisms

      • Intrinsic Factors - factors inherent to food itself
      • Nutrients
      • Growth factors and inhibitors - naturally occurring or added compounds that may stimulate or retard microbial growth
      • Water activity (Aw)
      • pH

      • Extrinsic Factors - environmental factors

      • Temperature
      • Oxidation-reduction potential



      More Details About Intrinsic Factors


      • Nutrients - Includes carbohydrates, lipids, minerals, and vitamins which supply necessary factors for synthesis of cellular components and for energy requirements for growth of microorganisms.

      • Growth Factors and Inhibitors - Foods may possess naturally occurring or added compounds that stimulate or suppress growth of microorganisms. Natural inhibitors include lysozyme in egg, agglutinin in milk, and eugenol in cloves. Added inhibitors include the numerous preservatives discussed below.

      • Water activity is a measure of the availability of free water for biological functions of microorganisms. These biological functions include transport of nutrients, removal of waste materials, synthesis of cellular materials, and ability to carry out enzymatic reactions. (For specific examples of the Aw of different foods, see Ray reference below.)

      • pH - A measure of the hydrogen ion concentration. Organisms grow under specific pH conditions.



      More Details About Extrinsic Factors

      • Temperature - Determines rate of enzymatic reactions. With every 10C decrease in temperature, rate of an enzyme doubles; with every 10C decrease in temperature, rate of an enzyme reduced to half.

      • Classification of microorganisms according to their temperature of growth (from Ray, below)


      • Group Optimum temperature Range

      • Thermophiles 55C 45-70C
      • Mesophiles 35C 10-45C
      • Psychrophiles 15C -5-20C

        When exposed to temperatures outside these ranges, organisms may grow more slowly or die.

      • Oxidation-reduction potential (Eh) - Oxidation and reduction of substances are the primary means of generating energy for microorganisms and the growth of microorganisms and specific metabolic reactions to generate energy are dependent on oxidation-reduction potential of foods. Because oxidation-reduction potential depends on both the chemical composition of the food and the storage condition (in relation to air), this is both an intrinsic and extrinsic factor.

      • Considerable range of Eh at which different groups of microorganisms can grow (from Ray, below):

      • Aerobes - between +500 and +300 mV
      • Facultative anaerobes - between +300 and -100 mV
      • Anaerobes - between +100 and -250 mV or more



      *References

      1. Montville, TJ: Principles which influence microbial growth, survival, and death in foods. In Doyle, MP, Beuchat, LR, and Montville, TJ, eds.: Food microbiology: fundamentals and frontiers, ASM Press, Washington, DC, 1997.

      2. Ray, B: Fundamental food microbiology, CRC Press, Inc., Boca Raton, Florida, 1996.





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