Food Microbiology Lecture Notes

Introduction to Food Microbiology

• Presenter: Dr. Jeanette Robertson

Intended Learning Outcomes

• Students will:
  • Gain an appreciation of the importance of studying microorganisms in food.
  • Acquire theoretical knowledge of the food testing process.
  • Understand the identification of unknown pathogens associated with foodborne infections.

What is Food Microbiology?

• Definitions:
  • Biology: The study of living things.
  • Microbiology: The study of microscopic living things.
  • Food Microbiology: The study of microscopic living organisms in/on food.

Importance of Food Microbiology

• Protects public health by ensuring:
  • Clean, safe, and healthy food for consumers.
  • Prevention of food spoilage.
  • Prevention of foodborne illnesses.
  • Facilitation of food preservation.

Consequences of Microbial Growth in Foods

• Outcomes:
  • Food spoilage.
  • Foodborne illness.
  • Food production enhancement.
• Theme: "The Good, The Bad, and The Ugly" regarding microbial growth in food.

Need for Testing Food for Microbial Growth

• Foods contain diverse populations of microorganisms (bacteria, viruses, yeasts, molds, protozoa).
• Microbial growth can occur during handling, processing, and packaging.
• Foods must meet microbiological guidelines, requiring microbial estimation.
• Rule of Thumb: As microbial counts increase, food quality generally decreases (except for fermented foods).

Reasons for Food Testing

• Routine quality control in food manufacturing.
• Health inspections of food service environments.
• Investigation of foodborne illness outbreaks.
• Microbial contamination can occur at any point in the food supply chain.

Questions Food Microbiologists Address

1. How many microorganisms are present?
   • Enumeration: Counting microorganisms in food samples.
2. What organisms are present?
   • Isolation and Identification: Isolating and identifying organisms in food samples.

Tools for Microbial Count, Isolation, and Identification

• Tools utilized by food microbiologists:
  • Sterile sample pots.
  • Colony counters.
  • Sterile loops, spreaders, swabs, pipettes.
  • Food blenders or stomachers.
  • Sterile Petri dishes and culture media.
  • Sterile scalpels or spatulas.

The Food Testing Process

1. Sample Collection:
   • From food factories or as part of an illness investigation.
   • Methods vary by food type:
     • Liquid foods: Use pipette for non-viscous; weigh for viscous.
     • Solid foods: Weigh sample or collect via sponge/swab.
   • Importance of representative sampling.
2. Sample Preparation:
   • Inoculate onto agar or broth and incubate overnight.
   • Examine colonies and perform identification tests.

Procedures for Food Sample Testing

• Solid Food Preparation:
  • Physical chopping and homogenization.
  • Serial 10-fold dilutions for plating.
  • Incubation at typically 37°C for 24 hours.
  • Expressing counts as CFU (colony-forming units) per mg.

Physiological States of Microbes

• Categories:
  • Viable Cells: Capable of growth.
  • Dead Cells: Inactivated and cannot be cultured.
  • Sub-lethally Injured Cells: May require non-selective recovery media.
  • Viable but Non-Culturable (VBNC): Nutrient limitation state, not recoverable despite viability testing.

Environmental Testing

• Testing not only food but also:
  • Walls, floors, surfaces, belts, containers, air, and storage facilities.
  • Representative samples from surfaces, air, and liquids.

Absence of Food Sample

• In cases where food is eaten and unavailable:
  • Analyze fecal samples from affected individuals as a diagnostic tool.

Clues for Pathogens Causing Illness

• Pathogen-Food Associations:
  • Meat and Poultry: Salmonella, Campylobacter, Listeria, E. coli O157:H7, Clostridium perfringens.
  • Milk and Dairy: Listeria, Campylobacter, Staphylococcus, Salmonella.
  • Seafood: Vibrio, Salmonella, Shigella.
  • Vegetables: Listeria, Shigella, Salmonella, E. coli O157:H7.
  • Eggs: Salmonella enteritidis.

Enterobacteriaceae Family

• Major bacterial group linked to foodborne illnesses.
• Characteristics:
  • Gram-negative rods, generally motile, 1-5 µm in length.
  • Oxidase-negative and facultative anaerobes.
  • Ferment glucose producing acid and/or carbon dioxide.

Identification of Causative Organisms

• Dichotomous Keys:
  • Step-wise identification method based on morphological and biochemical tests.
  • Useful for identifying unknown bacteria through successive choices.

Main Aspects of Identification

• Methods:
  • Growth on agar.
  • Microscopy (Gram stain).
  • Biochemical testing.

Starting with Colony Morphology

• Analyze size, shape, and pigmentation of colonies for preliminary identification.

Useful Media for Enterobacteria

• MacConkey Agar:
  • Selective and differential media for isolation of coliforms and intestinal pathogens.
  • Differentiates lactose fermenters from non-fermenters based on acid production affecting pH.

Attributes of MacConkey Agar

• Nutrition:
  • Pancreatic digest of gelatin and peptones provide growth nutrients.
• Selective Agents:
  • Crystal violet and bile salts inhibit most Gram-positive bacteria.
• Differential:
  • Neutral red indicator reveals pH change due to acid production by fermenting bacteria.

Microbial Identification Techniques

• Microscopy:
  • Reveals cell morphology and Gram reaction.
• Cocci & Bacilli: Identification by shape (e.g., cocci, bacilli) and arrangement.

Limits of Identifying Bacteria

• Gram Stain:
  • Provides preliminary identification but is not definitive.
  • Requires biochemical testing for accurate identification.

Common Biochemical Tests for Enterobacteria

• Include tests such as oxidase, catalase, fermentation, citrate utilization, indole, motility, methyl red (MR), Voges–Proskauer (VP), and triple sugar iron (TSI) agar tests.

Exemplar Dichotomous Key for Enterobacteria

• Framework for biochemical testing identification with examples such as E. coli, Salmonella, and Shigella across multiple tests.