Food microbiology

• Food Microbiology


  The study of microscopic living things in/on food
• Why the study of Food Microbiology is Important

  • Provide clean, safe, healthy food to consumer
  • Prevent food spoilage
  • Prevent Foodborne Illness
  • Food Preservation
• Food must be tested for microbial growth
• Enumeration


  Microbiologists count the numbers of microorganisms present in a food sample
• Isolation and Identification


  Food microbiologists isolate and identify the organisms present in the food sample
• The Food Testing Process

  1. Sample taken from food factory, supermarket (routine quality control testing) or as part of foodborne illness investigation

  2. Sample prepared for culture: weighed, homogenised (stomacher or blender) and diluted (1:10)

  3. Sample inoculated onto agar and/ into broth medium, incubated overnight. Colonies examined and enumerated

  4. Identified by a series of tests: colonial morphology, Gram stain, biochemical tests

  5. Results recorded and test report prepared
• Different physiological states microbes found in/on food may be present in

  • Viable cells
  • Dead cells
  • Sub-lethally injured cells
  • Viable but non-culturable cells (VBNC)
• The food environment is also tested
• Sometimes food suspected of being involved in a foodborne infection is no longer available
• Food-Pathogen Pairings


  • Meat and Poultry: Salmonella, Campylobacter, Listeria, E. coli O157:H7, Clostridium perfringens
  • Milk and dairy products: Listeria, Campylobacter, Staphylococcus, Salmonella
  • Seafood (fish and shellfish): Vibrio, Salmonella, Shigella
  • Vegetables (esp. salad vegetables): Listeria, Shigella, Salmonella, E. coli O157:H7, other types of E. coli
  • Eggs: Salmonella enteritidis
• Enterobacteriaceae


  • Large family of bacteria, recognised as a major bacterial cause of foodborne illness
  • Examples: Salmonella, Escherichia coli, Klebsiella, Shigella, Enterobacter and Citrobacter
  • Gram negative rods, generally motile, around 1-5 µm in length
  • Oxidase-negative
  • Facultative anaerobes
  • Ferment glucose to acid and/or carbon dioxide
• Dichotomous keys


  • Method used to characterize and identify living organisms, including bacteria
  • Step wise process to work through based upon successive choices between two alternatives
• Main aspects of identification

  • Growth on agar
  • Microscopy (Gram stain)
  • Biochemical testing
• MacConkey agar


  • Selective and differential media
  • Used for the isolation of coliforms and intestinal pathogens
  • Used in the differentiation of lactose fermenting from lactose non-fermenting gram-negative bacteria
  • Fermentation of carbohydrates by bacteria results in production of acid and gas
• Microscopy reveals cell shape and Gram reaction, but is not discriminatory enough to definitively identify bacteria
• Common biochemical tests for enterobacteria

  • Oxidase test
  • Catalase test
  • Carbohydrate fermentation tests
  • Citrate utilization Test
  • Indole Test
  • Motility Test
  • Methyl Red (MR) Test
  • Voges–Proskauer (VP) Test
  • Triple Sugar Iron (TSI) Agar Test (H2S production)
  • Urease Test
• Food spoilage


  Food is considered spoiled if sensory changes make it unacceptable to the consumer
• Spoiled food is not necessarily unsafe
• Causes of food spoilage

  • Can be both microbial and non-microbial
  • Microbial spoilage results from decomposition of the food and the formation of metabolites
• Sources of microbes on food

  • Everything food comes into contact with is a potential source of contamination
• Factors that affect the growth of microorganisms in food

  • Intrinsic Factors
  • Extrinsic Factors
  • Processing Factors
  • Implicit Factors
• Intrinsic Factors


  • Factors related to the food itself; inherent part of a food, so hard to manipulate/change
• Extrinsic Factors


  • Factors related to the food environment
• Processing Factors


  • Related to food processing
• Implicit Factors


  • Related to interactions with other microbes in the food
• Carbohydrate-rich foods that are high in moisture content (water activity) e.g. some fruits and vegetables initially show spoilage by fungi
• Denser carbohydrates e.g. grains and corn are also susceptible to mold growth, particularly if stored in moist conditions
• Meat and dairy products – contain proteins and fat that support bacterial growth
• Biological structures


  • Extrinsic Factors
  • Processing Factors
  • Implicit Factors
  • Food Composition
• Factors related to the food environment

  • Temperature of storage
  • Relative humidity
  • Gases in the environment
• Processing Factors are related to food processing
• Implicit Factors are related to interactions with other microbes in the food
• Food Composition


  • Carbohydrate-rich foods that are high in moisture content (water activity) e.g. some fruits and vegetables initially show spoilage by fungi
  • Molds produce enzymes that weaken skin of fruit
  • Once mold degrades carbohydrates in the skin, bacteria can colonize the food
  • Bacteria produce hydrolytic enzymes e.g. pectinase
  • Example: Pectobacterium carotovorum which causes soft rots
• Denser carbohydrates e.g. grains and corn are also susceptible to mold growth


  • Particularly if stored in moist conditions
  • Examples: Penicillium spp
• Meat and dairy products

  • Contain proteins and fat that support bacterial growth
  • Putrefaction is common—resulting from the anaerobic breakdown of proteins
  • Yields foul-smelling amine compounds
• Unpasteurised milk spoilage


  1. Milk starts out as an opaque liquid

  2. Lactococcus lactis naturally present in the milk over-grows and produces acid

  3. Additional acid tolerant organisms grow e.g. Lactobacillus spp

  4. Yeasts and molds then predominate and degrade the lactic acid and the acidity of the milk decreases

  5. This allows protein digesting bacteria to become active

  6. Their activity causes a bitter taste and putrid odour

  7. Finally the milk becomes clear due to coagulation of protein and fats
• Substrate


  • Pectin
  • Proteins
  • Carbohydrates
  • Lipids
• Food Example


  • Fruits
  • Meat
  • Starchy foods
  • Butter
• Enzymes involved


  • Pectinases
  • Proteases
  • Hydrolysing Enzymes
  • Lipolytic enzymes
• Chemical Reactions or Processes

  • Pectinolysis
  • Hydrolysis and deamination
  • Hydrolysis and fermentation
  • Hydrolysis and fatty acid degradation