Food Microbiology
AN INTRODUCTION
FOURTH EDITION
FOURTH EDITION
Food Microbiology
AN INTRODUCTION
Karl R. Matthews, Kalmia E. Kniel, and Thomas J. Montville
Karl Matthews
Department of Food Science, Rutgers University, New Brunswick, New Jersey
Kalmia E. Kniel
Department of Animal and Food Sciences, University of Delaware, Newark, Delaware
Thomas J. Montville
Department of Food Science, Rutgers University, New Brunswick, New Jersey
Washington, DC
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Library of Congress Cataloging-in-Publication Data
Names: Matthews, Karl R., author. | Kniel, Kalmia E., author. | Montville, Thomas J., author.
Title: Food microbiology : an introduction / Karl R. Matthews, Department of Food Science, Rutgers University, New Brunswick, New Jersey, Kalmia E. Kniel, Department of Animal and Food Sciences, University of Delaware, Newark, Thomas J. Montville, Department of Food Science, Rutgers University, New Brunswick, New Jersey.
Description: 4th edition. | Washington, DC : ASM Press, [2017] | Revision of: Food microbiology / Thomas J. Montville, Karl R. Matthews, Kalmia E. Kniel. 3rd ed. 2012. | Includes bibliographical references and index.
Identifiers: LCCN 2017003891 (print) | LCCN 2017005242 (ebook) | ISBN 9781555819385 (hardcover) | ISBN 9781683673125 (ebook)
Subjects: LCSH: Food—Microbiology.
Classification: LCC QR115 .M625 2017 (print) | LCC QR115 (ebook) | DDC 664.001/579—dc23
LC record available at https://lccn.loc.gov/2017003891
doi:10.1128/9781555819392
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
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Cover image credits: Tiger Images/Shutterstock (Hamburger), Phuengphol/Shutterstock (Cabbage), Maksym Yasinskyy/Shutterstock (Blue Cheese), CDC PHIL: #6708, CDC, 1976 (Yersinia enterocolitica), CDC PHIL: #18134, National Institute of Allergy and Infectious Diseases (NIAID) 2011 (Salmonella).
Contents
Preface
About the Authors
SECTION |
1 | BASICS OF FOOD MICROBIOLOGY |
1
The Trajectory of Food Microbiology
Introduction
Who’s on First?
Food Microbiology, Past and Present
To the Future and Beyond
Summary
Suggested reading
Questions for critical thought
2
Microbial Growth, Survival, and Death in Foods
Introduction
Food Ecosystems, Homeostasis, and Hurdle Technology
Foods as Ecosystems
Classical Microbiology and Its Limitations
Limitations of Detection and Enumeration Methods
Plate Counts
Selective, or Differential, Media
Most-Probable-Number Methods
Enrichment Techniques
Physiological States of Bacteria
Introduction
Injury
Viable but Nonculturable
Quorum Sensing and Signal Transduction
Introduction
Quorum Sensing
Signal Transduction
Biofilms
Factors That Influence Microbial Growth
Intrinsic Factors
Extrinsic Factors
Homeostasis and Hurdle Technology
Growth Kinetics
Microbial Physiology and Metabolism
Carbon Flow and Substrate-Level Phosphorylation
The TCA Cycle Links Glycolysis to Aerobic Respiration
Conclusion
Summary
Suggested reading
Questions for critical thought
3
Spores and Their Significance
Introduction
Spores in the Food Industry
Low-Acid Canned Foods
Bacteriology of Sporeformers of Public Health Significance
Heat Resistance of C. botulinum Spores
Spoilage of Acid and Low-Acid Canned and Vacuum-Packaged Foods by Sporeformers
Spore Biology
Structure
Macromolecules
Small Molecules
Dormancy
Resistance
Freezing and Desiccation Resistance
Pressure Resistance
γ-Radiation Resistance
UV Radiation Resistance
Chemical Resistance
Heat Resistance
The Cycle of Sporulation and Germination
Sporulation
Activation
Germination
Outgrowth
Summary
Suggested reading
Questions for critical thought
4
Detection and Enumeration of Microbes in Food
Introduction
Sample Collection and Processing
Analysis
Metabolism-Based Methods
Surface Testing
Summary
Suggested reading
Questions for critical thought
5
Rapid and Automated Microbial Methods
Introduction
Sample Processing
Requirements and Validation of Rapid Methods
Rapid Methods Based on Traditional Methods
Immunologically Based Methods
Molecular Methods
A Potpourri of Rapid Methods
Summary
Suggested reading
Questions for critical thought
6
Indicator Microorganisms and Microbiological Criteria
Introduction
The Purpose of Microbiological Criteria
The Need to Establish Microbiological Criteria
Definitions
Who Establishes Microbiological Criteria?
Sampling Plans
Types of Sampling Plans
Establishing Limits
Indicators of Microbiological Quality
Indicator Microorganisms
Metabolic Products
Indicators of Foodborne Pathogens and Toxins
Indicator Organisms
Fecal Coliforms and E. coli
Metabolic Products
Application and Specific Proposals for Microbiological Criteria for Food and Food Ingredients
Current Status
Summary
Suggested reading
Questions for critical thought
SECTION |
2 | FOODBORNE PATHOGENIC BACTERIA |
7
Regulatory Issues
Introduction
U.S. Agencies Involved in Food Regulation
The USDA
The FDA
The CDC
Surveillance
When an Outbreak Occurs
Agroterrorism
What’s Next?
Global Perspective
“It Takes a Village” and Maybe More
Summary
Suggested reading
Questions for critical thought
8
Bacillus cereus
Outbreak
Introduction
Characteristics of the Organism
Environmental Sources
Foodborne Outbreaks
Characteristics of Disease
Dose
Virulence Factors and Mechanisms of Pathogenicity
The Emetic Toxin
Enterotoxins
B. cereus as a Medical Pathogen
The Spore
Summary
Suggested reading
Questions for critical thought
9
Campylobacter Species
Outbreak
Introduction
Characteristics of the Organism
Environmental Susceptibility
Reservoirs and Foodborne Outbreaks
Characteristics of Disease
C. jejuni and C. coli
Other Campylobacter Species
Epidemiological Subtyping Systems Useful for Investigating Foodborne Illnesses
Infective Dose and Susceptible Populations
Virulence Factors and Mechanisms of Pathogenicity
Cell Association and Invasion
Flagella and Motility
Toxins
Other Factors
Autoimmune Diseases
Immunity
Summary
Suggested reading
Questions for critical thought
10
Clostridium botulinum
Introduction
Botulism
Characteristics of the Disease
Toxic Doses
Characteristics of C. botulinum
Classification
Tolerance to Preservation Methods
Sources of C. botulinum
Occurrence of C. botulinum in the Environment
Occurrence of C. botulinum in Foods
Virulence Factors and Mechanisms of Pathogenicity
Structure of the Neurotoxins
Genetic Regulation of the Neurotoxins
Mode of Action of the Neurotoxins
Summary
Suggested reading
Questions for critical thought
11
Clostridium perfringens
Introduction
A Spore’s-Eye View of Clostridium perfringens Toxicoinfections
The Foodborne Illness
Cruel and Unusual Punishment
Incidence
Vehicles for C. perfringens Foodborne Illness
Factors Contributing to C. perfringens Type A Foodborne Illness
Preventing C. perfringens Type A Foodborne Illness
Identification of C. perfringens Type A Foodborne Illness Outbreaks
Characteristics of C. perfringens Type A Foodborne Illness
Infectious Dose for C. perfringens Type A Foodborne Illness
The Organism
Overview
Classification: Toxin Typing of C. perfringens
Control of C. perfringens
Reservoirs for C. perfringens Type A
Virulence Factors Contributing to C. perfringens Type A Foodborne Illness
Heat Resistance
C. perfringens Enterotoxin
Summary
Suggested reading
Questions for critical thought
12
Enterohemorrhagic Escherichia coli
Outbreak
Introduction
Categories of E. coli
Characteristics of E. coli O157:H7 and Non-O157 EHEC
Acid Tolerance
Antibiotic Resistance
Inactivation by Heat and Irradiation
Reservoirs of E. coli O157:H7
Detection of E. coli O157:H7 and Other EHEC Strains on Farms
Factors Associated with Bovine Carriage of E. coli O157:H7
Cattle Model for Infection by E. coli O157:H7
Domestic Animals and Wildlife
Humans
Disease Outbreaks
Geographic Distribution
Seasonality of E. coli O157:H7 Infection
Age of Patients
Transmission of E. coli O157:H7
Examples of Foodborne and Waterborne Outbreaks
Characteristics of Disease
Infectious Dose
Mechanisms of Pathogenicity
Attaching and Effacing
The Locus of Enterocyte Effacement
The 60-MDa Plasmid (pO157)
Conclusion
Summary
Suggested reading
Questions for critical thought
13
Listeria monocytogenes
Outbreak
Introduction
Characteristics of the Organism
Classification: The Genus Listeria
Susceptibility to Physical and Chemical Agents
Listeriosis and Specific Foods
Ready-to-Eat Foods
Milk Products
Cheeses
Meat and Poultry Products
Seafoods
Other Foods
Environmental Sources of L. monocytogenes
Food-Processing Plants
Prevalence and the Regulatory Status of L. monocytogenes
Human Carriers
Foodborne Outbreaks
Characteristics of Disease
Infectious Dose
Virulence Factors and Mechanisms of Pathogenicity
Pathogenicity of L. monocytogenes
Specific Genes Mediate Pathogenicity
Summary
Suggested reading
Questions for critical thought
More questions than answers
14
Salmonella Species
Outbreak
Introduction
Characteristics of the Organism
Biochemical Identification
Taxonomy and Nomenclature
Serological Identification
Physiology
Reservoirs
Characteristics of Disease
Symptoms and Treatment
Preventative Measures
Antibiotic Resistance
Infectious Dose
Pathogenicity and Virulence Factors
Specific and Nonspecific Human Responses
Attachment and Invasion
Growth and Survival within Host Cells
Virulence Plasmids
Other Virulence Factors
Summary
Suggested reading
Questions for critical thought
15
Shigella Species
Outbreak
Introduction
Classification and Biochemical Characteristics
Shigella in Foods
Survival and Growth in Foods
Characteristics of Disease
Foodborne Outbreaks
Virulence Factors
Genetic Regulation
Conclusions
Summary
Suggested reading
Questions for critical thought
16
Staphylococcus aureus
Outbreak
Characteristics of the Organism
Historical Aspects and General Considerations
Sources of Staphylococcal Food Contamination
Resistance to Adverse Environmental Conditions
Foodborne Outbreaks
Incidence of Staphylococcal Food Poisoning
A Typical Large Staphylococcal Food Poisoning Outbreak
Characteristics of Disease
Toxic Dose
Toxin Dose Required
Microbiology, Toxins, and Pathogenicity
Nomenclature, Characteristics, and Distribution of Enterotoxin-Producing Staphylococci
Introduction to and Nomenclature of the Staphylococcal Enterotoxins: Current Classification Scheme Based on Antigenicity
Staphylococcal Regulation of Staphylococcal Enterotoxin Expression
Summary
Suggested reading
Questions for critical thought
17
Vibrio Species
Outbreak
Introduction
Characteristics of the Organism
Epidemiology
Characteristics of Disease
Susceptibility to Physical and Chemical Treatments
V. cholerae
Isolation and Identification
Reservoirs
Foodborne Outbreaks
Characteristics of Disease
Virulence Mechanisms
V. mimicus
Foodborne Outbreaks
Characteristics of Disease
Virulence Factors
V. parahaemolyticus
Classification
Reservoirs
Foodborne Outbreaks
Characteristics of Disease
Virulence Mechanisms
V. vulnificus
Classification
Susceptibility to Control Methods
Reservoirs
Foodborne Outbreaks
Characteristics of Disease
Virulence Mechanisms
V. fluvialis, V. furnissii, V. hollisae, and V. alginolyticus
Summary
Suggested reading
Questions for critical thought
18
Yersinia enterocolitica
Outbreak
Introduction
Characteristics of the Organism
Classification
Susceptibility and Tolerance
Characteristics of Infection
Reservoirs
Foodborne Outbreaks
Mechanisms of Pathogenicity
Pathological Changes
Virulence Determinants
Chromosomal Determinants of Virulence
Other Virulence Determinants
Pathogenesis of Yersinia-Induced Autoimmunity
Summary
Suggested reading
Questions for critical thought
SECTION |
3 | OTHER MICROBES IMPORTANT IN FOOD |
19
Lactic Acid Bacteria and Their Fermentation Products
Introduction
The Biochemical Foundation of Food Fermentation
Catabolic Pathways
Genetics of Lactic Acid Bacteria
Dairy Fermentations
Starter Cultures
Production of Aroma Compounds
Vegetable Fermentations
Ingredients and Additives Used during Fermentations
Sauerkraut Fermentation
Pickle Fermentation
Meat Fermentations
Summary
Suggested reading
Questions for critical thought
20
Yeast-Based and Other Fermentations
Introduction
Fermentations That Use Yeast
Bread
Beer
Wine
Vinegar Fermentation
Cocoa and Coffee Fermentations
Cocoa
Coffee
Fermented Foods of Non-Western Societies
Summary
Suggested reading
Questions for critical thought
21
Spoilage Organisms
Introduction
Meat, Poultry, and Seafood Products
Origin of the Microbiota in Meat
Origin of the Microbiota in Poultry
Origins of Microbiota in Finfish
Origins of Microbiota in Shellfish
Bacterial Attachment to Food and Food Contact Surfaces
Microbial Progression during Storage
Muscle Tissue as a Growth Medium
Factors Influencing Spoilage
Control of Spoilage of Muscle Foods
Milk and Dairy Products
Milk and Dairy Products as Growth Media
Psychrotrophic Spoilage
Spoilage by Fermentative Nonsporeformers
Spore-Forming Bacteria
Yeasts and Molds
Spoilage of Produce and Grains
Types of Spoilage
Mechanisms of Spoilage
Influence of Physiological State
Microbiological Spoilage of Vegetables
Microbiological Spoilage of Fruits
Microbiological Spoilage of Grains and Grain Products
Summary
Suggested reading
Questions for critical thought
22
Molds
Introduction
Isolation, Enumeration, and Identification
Aspergillus Species
A. flavus and A. parasiticus
Other Toxigenic Aspergilli
Penicillium Species
Significant Penicillium Mycotoxins
Fusaria and Toxigenic Molds Other than Aspergilli and Penicillia
Toxigenic Fusarium Species
Detection and Quantitation of Fusarium Toxins
Other Toxic Molds
Summary
Suggested reading
Questions for critical thought
23
Parasites
Outbreak
Introduction
Protozoa
Cryptosporidium spp.
Cyclospora cayetanensis
Toxoplasma gondii
Giardia intestinalis
Other Protozoa of Interest
Helminths
Roundworms (Nematodes)
Tapeworms (Cestodes)
Flukes (Trematodes)
Detection
Preventative Measures
Summary
Suggested reading
Questions for critical thought
24
Viruses and Prions
Introduction
Viruses
Elementary Virology
Viruses as Agents of Foodborne Illness
Bacteriophages in the Dairy Industry
Beneficial Uses of Viruses
Prions
A Short History of the Prion
Prion Biology
Summary
Suggested reading
Questions for critical thought
SECTION |
4 | CONTROL OF MICROORGANISMS IN FOOD |
25
Chemical Antimicrobials
Introduction
Factors That Affect Antimicrobial Activity
Organic Acids
Parabenzoic Acids
Nitrites
Phosphates
Sodium Chloride
Water Activity
Disinfectants
Sulfites
Chlorine
Quaternary Ammonium Compounds
Peroxides
Ozone
Naturally Occurring Antimicrobials
Lysozyme
Lactoferrin and Other Iron-Binding Proteins
Avidin
Spices and Their Essential Oils
Onions and Garlic
Isothiocyanates
Phenolic Compounds
Phage Therapy
Summary
Suggested reading
Questions for critical thought
26
Biologically Based Preservation and Probiotic Bacteria
Introduction
Biopreservation by Controlled Acidification
Bacteriocins
General Characteristics
Bacteriocin Applications in Foods
Probiotic Bacteria
The Human GI Tract Is a Microbial Ecosystem
Summary
Suggested reading
Questions for critical thought
27
Physical Methods of Food Preservation
Introduction
Physical Dehydration Processes
Drying
Freeze-Drying
Cool Storage
Controlled-Atmosphere Storage
Modified-Atmosphere Packaging
Freezing and Frozen Storage
Preservation by Heat Treatments
Technological Fundamentals
Thermobacteriology
Calculating Heat Processes for Foods
Heat Resistance?
Summary
Suggested reading
Questions for critical thought
28
Nonthermal Processing
Introduction
Acceptance
High-Pressure Processing
Ozone
Ultraviolet Light
Pulsed Electric Fields and Pulsed Light
Oscillating Magnetic Fields
Ultrasound
Conclusions
Summary
Suggested reading
Questions for critical thought
29
Sanitation and Related Practices
Introduction
Food Safety Objectives
Good Manufacturing Practices
General Provisions (Subpart A)
Buildings and Facilities (Subpart B)
Equipment (Subpart C)
Production and Process Controls (Subpart E)
DALs (Subpart G)
Sanitation
SSOPs
HACCP
Food Safety Modernization Act
Conclusions
Summary
Suggested reading
Questions for critical thought
Critical Thinking Skills
Useful Websites for Food Safety Information
Glossary
Answers to Crossword Puzzles
Answers to Selected Questions for Critical Thought
Index
Preface
THE GLOBAL population is projected to increase to more than 9 billion by 2050. The study of food microbiology is essential to ensuring that there will be sufficient high-quality, safe food to feed these growing numbers. This book provides a taste of the complexity and challenge of this goal. The Fourth Edition has been thoroughly updated and revised to reflect the global nature of the field.
Today’s students have a wealth of information available at their fingertips by virtue of access to the Internet. Utilizing and recognizing the quality of that information, however, calls for critical thinking, innovative approaches, and healthy skepticism. We have incorporated a greater breadth of material that underscores the global audience served by the textbook. New homework problems have been added based on positive feedback from instructors, and these problems serve as a vital learning tool.
Capturing the interest and attention of our target audience, undergraduates, is integral to opening the door to the field of food microbiology and from there to countless career opportunities. We have included authors’ notes, special box material written by students and international experts, chapter summaries, and a glossary. The book is user-friendly for instructors too; answers to selected critical thought questions are provided.
This book is divided into four sections. Instructors and students should be aware that the Fourth Edition contains a substantial amount of material not found in previous editions. Each chapter is self-contained, facilitating the instructor’s flexibility to present the material in different order.
The first section of the book covers the foundational material, describing how bacteria grow in food, how the food affects their growth, the control of microbial growth, spores, detection, and microbiological criteria. The material has application for food microbiology laboratory courses, too.
Instructors may choose to use the other three sections in virtually any order. The foodborne pathogenic bacteria are presented in alphabetical order in section II. A greater emphasis has been given to foodborne illness outbreaks occurring outside of the United States, and the chapter on Regulatory Issues discusses international regulations.
Section III contains chapters on beneficial microbes, spoilage organisms, and pathogens that are not bacteria. Lactic acid bacteria and yeast fermentations are covered separately. Molds are covered both as spoilage organisms and as potential toxin producers. Since viruses likely cause more than half of all foodborne illnesses, treatment of viruses has been expanded to include explanations of lytic and temperate phages, the importance of bacteriophage infection prevention in the dairy industry, and utilization of phages for pathogen control. Prions are not bacteria, molds, or viruses; in fact, they are not microbes at all. However, they are a major biological concern to the public and food safety experts, and they are covered in the same chapter as viruses. Underscoring the global breadth of this edition, we have expanded the chapter on parasites, which are important sources of disease in many parts of the world. Section IV covers the chemical, biological, and physical methods of controlling foodborne microbes and closes by examining industrial and regulatory strategies for ensuring food safety.
The authors are grateful to the many students who provided perspective on the level and depth of coverage and contributed photographs that enhanced this edition. Special thanks go to Yangjin Jung, Hyein Jang, and Jingwen Gao for development of tables and figures for this edition.
We thank the team at ASM Press for facilitating completion of the book. The guidance of Larry Klein was essential to keeping us on track to complete this edition. Ellie Tupper’s enthusiasm and eye for detail ensured that we would “get it right.”
We hope that Food Microbiology: An Introduction, Fourth Edition, makes the subject come alive and encourages you to explore careers in food microbiology. The food microbiome is complex and offers a wonderful playground for food microbiologists. Enjoy your adventure.
KARL R. MATTHEWS
KALMIA E. KNIEL
THOMAS J. MONTVILLE
About the Authors
KARL R. MATTHEWS is Professor of Microbial Food Safety at Rutgers University. He received a Ph.D. from the University of Kentucky in 1988. Dr. Matthews has earned an international reputation for his work on the interaction of foodborne pathogens with fresh fruits and vegetables. This includes demonstrating the internal localization of bacteria during growth of leafy greens. He further showed that the internalization process is a passive event by demonstrating the internalization of fluorescent polystyrene beads. Dr. Matthews has also been active in research on antimicrobial resistance of foodborne bacteria and development of novel antimicrobial agents.
Author’s Statement
My interest in microbiology was sparked one summer when I was working on a dairy farm. I regularly drank raw milk, but one time after doing so I became extremely ill (I won’t go into the messy details). I became intrigued by microorganisms associated with milk and the disease bovine mastitis. These beginnings led me to an exciting career in food microbiology, where every day seems to bring a new challenge to be addressed. I have traveled throughout the world (Bhutan, China, Costa Rica, France, Iran, Ireland, Oman, Turkey, Peru, Philippines, and many, many more countries) conducting workshops and addressing issues associated with food safety. During those trips I have had the opportunity to enjoy the culture and cuisine of those countries and made many lasting friends.
KALMIA E. KNIEL is Professor of Microbial Food Safety in the Department of Animal and Food Sciences at the University of Delaware. She received her Ph.D. from Virginia Tech in Food Science and Technology in 2002. Her doctoral work focused on protozoan parasites. After that, she was a postdoctoral microbiologist at the USDA Agricultural Research Service’s Animal Parasitic Diseases Laboratory. She is now nationally recognized as a leading expert in transmission of viruses, protozoa, and bacteria in the preharvest environment. Dr. Kniel has been active in researching the mechanisms behind the survival and inactivation of norovirus, hepatitis A virus, and other enteric viruses prevalent in our water and foods. She is an active advocate for teaching food safety at all levels and has been involved with elementary and secondary education. At the University of Delaware, she teaches courses on foodborne outbreak investigations and the basics of food science and food safety from farm to fork.
Author’s Statement
I received my first microscope when I was 10, and I was hooked. My children now use it to look at plant cells and pond life, and what a kick it is seeing them light up looking into those little lenses. Serving as a teaching assistant for a pathogenic bacteriology laboratory course changed my life. I relished working with the students, challenging them with fecal unknowns and mock sputum samples. The best part was seeing their reaction as they identified an unknown bacterium and observed growth on their petri plates. It’s that level of excitement from students of all ages that I love. Working with students in my laboratory and in the classroom is an honor. We all share a great curiosity for science. I believe that food microbiology is the greatest science, as it includes basic scientific inquiries with an applied twist. I have been fortunate to work with a myriad of amazing people, and with them at my side I look forward to the challenges of every day.
THOMAS J. MONTVILLE is Distinguished Professor Emeritus at Rutgers University, where he received his B.S. in 1975. Dr. Montville received his Ph.D. from the Massachusetts Institute of Technology (MIT) and then worked at the U.S. Department of Agriculture (USDA) as a research microbiologist before returning to Rutgers as a professor. He has published over 100 research papers on Clostridium botulinum, Listeria monocytogenes, antimicrobial peptides, and Bacillus anthracis. Dr. Montville is a fellow of the American Academy of Microbiology and a fellow of the Institute of Food Technologists.
Author’s Statement
My desire to know how things work drew me to science. When I was a child, my relatives saved their broken appliances so that I could take them apart and see how they worked. My attraction to science was fed by microscopes; the ability to see bacteria sucked me into the field of microbiology. Rods, cocci, spores, motile, tumbling, germinating before my very eyes! Careers in science, and life in general, can take strange turns. My undergraduate goal was just to get a good job where I didn’t have to work the night shift. But my professors badgered me to attend graduate school and I ultimately returned to Rutgers as a professor, where my laboratory station from my undergraduate Applied Microbiology course became part of my research laboratory.
I
BASICS OF FOOD MICROBIOLOGY
1 The Trajectory of Food Microbiology
2 Microbial Growth, Survival, and Death in Foods
3 Spores and Their Significance
4 Detection and Enumeration of Microbes in Food
5 Rapid and Automated Microbial Methods
6 Indicator Microorganisms and Microbiological Criteria
1 |
The Trajectory of Food Microbiology |
Learning Objectives
The information in this chapter will help the student:
Introduction
Who’s on First?
Food Microbiology, Past and Present
To the Future and Beyond
Summary
Suggested reading
Questions for critical thought
A former president of the American Society for Microbiology (Box 1.1) defined microbiology as an artificial subdiscipline of biology based on size. This suggests that basic biological principles hold true, and are often discovered, in the field of microbiology. Food microbiology is a further subdivision of microbiology. It studies microbes that grow in food and how food environments influence microbes. In some ways, food microbiology has changed radically in the last 20 years. The number of recognized foodborne pathogens has doubled. “Safety through end product testing” has given way to “safety by design” provided by Hazard Analysis and Critical Control Points (HACCP). In the United States, the Food Safety Modernization Act introduced Hazard Analysis Risk-Based Preventative Controls. Genetic and immunological probes have replaced biochemical tests and reduced testing time from days to minutes. In other ways, food microbiology is still near the beginning. Louis Pasteur would find his pipettes in a modern laboratory. Julius Richard Petri would find his plates (albeit plastic rather than glass). Hans Christian Gram would find all the reagents required for his stain. Food microbiologists still study only the microbes that we can see under the microscope and grow on agar media in petri dishes. Experts suggest that only 1% of all the bacteria in the biosphere can be detected by cultural methods.
Box 1.1
Preparing for the future
Membership in professional societies is a great way to advance professionally, even as a student (who benefits from reduced membership fees and is eligible for a variety of scholarships). Professional societies provide continuing-education and employment services to their members, provide expertise to those making laws and public policies, have annual meetings for presentation of the latest science, and publish books and journals. The three main societies for food microbiologists are listed below. Applications for membership can be obtained from their websites.
The American Society for Microbiology (ASM) (http://www.asm.org) is the largest life science society in the world. It has 27 divisions covering all facets of microbiology from microbial pathogenesis to immunology to antimicrobial agents to food microbiology, and it publishes 12 scholarly journals. ASM Press is the publisher of this book.
The Institute of Food Technologists (IFT) (http://www.ift.org) devotes itself to all areas of food science by discipline (food microbiology, food chemistry, and food engineering), as well as by commodity (cereals, fruits and vegetables, and seafood) and by processing (refrigerated foods). The IFT is a nonprofit scientific society with over 17,000 members working in food science, food technology, and related professions in industry, academia, and government. The IFT publishes four journals, sponsors a variety of short courses, and contributes to public policy and opinion at national, state, and local levels.
The International Association for Food Protection (IAFP) (http://www.foodprotection.org) is the only professional society devoted exclusively to food safety microbiology. IAFP is dedicated to the education and service of its members, as well as industry personnel. Members keep informed of the latest scientific, technical, and practical developments in food safety and sanitation. IAFP publishes two scientific journals, Food Protection Trends and Journal of Food Protection.
This chapter’s discussion of microbes per se sets the stage for a historical review of food microbiology. The bulk of the discussion concerns bacteria. Most of this book deals with bacteria. Viruses and prions are covered later in a single chapter because so little is known about them. This chapter ends with some thoughts about future developments in the field.
Let there be no doubt about it: the microbes were here first. It is a microbial world (Fig. 1.1). If the earth came into being at 12:01 a.m. of a 24-h day, microbes would arrive at dawn and remain the only living things until well after dusk. Around 9 p.m., larger animals would emerge, and a few seconds before midnight, humans would appear. The microbes were here first, they cohabit the planet with us, and they will be here after humans are gone. Life is not sterile. Microbes can never be (nor should they be) conquered, once and for all. The food microbiologist can only create foods that microbes do not “like,” manipulate the growth of microbes that are in food, kill them, or exclude them by physical barriers.
Figure 1.1
It’s a microbial world.
Bacteria live in airless bogs, thermal vents, boiling geysers, us, and foods. We are lucky that they are here for microbes form the foundation of the biosphere. We could not exist without microbes, but they would do just fine without us. Photosynthetic bacteria fix carbon into usable forms and make much of our oxygen. Rhizobium bacteria fix air’s elemental nitrogen into ammonia that can be used for a variety of life processes. Degradative enzymes allow ruminants to digest cellulose. Microbes recycle the dead into basic components that can be used again and again. Microbes in our intestines aid in digestion, produce vitamins, and prevent colonization by pathogens. For the most part, microbes are our friends.
From the dawn of civilization until about 10,000 years ago, humans were hunter-gatherers. Humans were lucky to have enough. There was neither surplus nor a settled place to store it. Preservation was not an issue. With the shift to agricultural societies, storage, spoilage, and preservation became important challenges. The first preservation methods were undoubtedly accidental. Sun-dried, salted, or frozen foods did not spoil. In the classic “turning lemons into lemonade” style, early humans learned that “spoiled” milk could be acceptable or even desirable if viewed as “fermented.” Fermenting food became an organized activity around 4000 B.C.E. (Table 1.1). Breweries and bakeries sprang up long before the idea of yeast was conceived.
Table 1.1 Significant events in the history of food microbiologya
Humans remained ignorant of microbes for thousands of years. In 1665, Robert Hooke published Micrographia, the first illustrated book on microscopy that detailed the structure of Mucor, a microscopic fungus. In 1676, Antonie van Leeuwenhoek (Fig. 1.2) used a crude microscope (Fig. 1.3) of Hooke’s design to see small living things in pond water. Microbiology was born!
Figure 1.2
Antonie van Leeuwenhoek.
Figure 1.3
Replica of van Leeuwenhoek microscope.
Nonetheless, it took another 200 years to prove that microbes exist and cause fermentative processes. In the mid-1700s, Lazzaro Spallanzani showed that boiled meat placed in a sealed container did not spoil. Advocates of spontaneous generation, however, argued that air is needed for life and that the air was sealed out. It took another 100 years for Louis Pasteur’s elegant “swan-necked flask” experiment (see Box 27.1) to replicate Spallanzani’s experiment in a way that allowed access to air but not to microbes. Napoleon, needing to feed his troops as they traveled across Europe, offered a prize to anyone who could preserve food. Nicolas Appert (Fig. 1.4) won this prize when he discovered that foods would not spoil if they were heated in sealed containers, i.e., were canned. Thus, canning was invented without any knowledge of microbiology. Indeed, it was not until the 1900s that mathematical bases for canning processes were developed.
Figure 1.4
Stamp honoring Nicolas Appert.