1. Food Microbiology
08/11/11

2. Growth of Microorganisms in Food
Ideal environment for microbial growth:
Moist
Nutrient-rich
pH neutral
Water activity (aw): designate amount of water available in foods Pure water aw = 1.0 Most bacteria aw > 0.90
Most fungi aw > 0.80
Low pH: inhibits most bacterial growth &
production of some toxins
(Exceptions: lactic acid bacteria and fungi)
Warm
Oxygen-rich
Fresh meats and milk have high water content
Supports microbial growth
Breads, nuts and dried foods have low water availability
Defined populations can grow in these specific environments
Water activity (aw) used to designate amount of water available in foods
Pure water has aw of 1.0
Most bacteria require aw of above 0.90
Most fungi require aw of above 0.80
Important in determining which organisms can survive and thrive on specific foods
Many microorganisms inhibited by acid conditions
Exception include lactic acid bacteria
Lactic acid bacteria used in fermentation process of food production
Also prime cause of spoilage of unpasteurized milk and other foods
Fungi able to survive at relatively low pH
Most acid foods spoil from fungal contamination as opposed to bacteria
pH can determine bacteria’s ability to produce toxin
Toxin production of many organisms is inhibited by acid pH
Storage temperature
Atmosphere

3. Food Spoilage Generally not harmful May be presence of
Food-borne bacterial pathogen
Fungal toxin
Common spoilage bacteria
Pseudomonas (meat, fish, eggs)
Psychrophilic organisms
Endospore forming organisms (Clostridium & Bacillus species)
Common spoilage fungi (lower aw & acidic environments)
Rhizopus Penicillium Aspergillus
Pepe, O. et al Appl. Environ. Microbiol. 69(4):
Bacteria: souring, slime
Bacillus (bread)-roping
Fungi: contaminate maple syrup, bread (low water and acidic environments)

4. Foodborne Illness “food poisoning”
Infection : colonization by a pathogen
Most food-borne pathogens: obligate anaerobes
Salmonella
Campylobacter
Escherichia coil O157:H7
Intoxication: ingestion of microbial toxin
Staphylococcus aureus
Clostridium botulinum
Foodborne infection requires consumption of living organisms
Symptoms do not appear for at least one day after ingestion
Major symptom usually diarrhea
Thorough cooking of food immediately before consumption will kill organisms
Prevent infection
Foodborne illness commonly caused by
Salmonella
Campylobacter
Escherichia coil O157:H7
Food intoxication
Illness resulting from consumption of an exotoxin produced by organisms growing in food product
When food is ingested it is the toxin responsible for illness not organism
Common causes of foodborne intoxication are
Estimated millions of cases of food poisoning occur each year
Preventable

5. Common Foodborne Illnesses

6. Salmonellosis (Infection)
1.4 million cases (100s of deaths) Gastroenteritis/typhoid fever
Organism: Salmonella enterica
Subspecies: enterica
Serovars: Enteritidis, Typhi, Typhimurium
Virulence factors:
Type III secretion system
Cell invasion
Toxin that inhibits the immune system
Associated with poultry products
Inadequate cooking
Cross-contamination common
Sensitive to stomach acid-
must ingest high numbers
Why associated with poultry? Difference between Typhi (typhoid fever) and typhimurium (mouse typhi- can cause disease in humans just originally found in mice_? motile, gram negative
uptake by intestinal epithelium)
Serovar (serological variant)
Enteric fever: fever, headache, constipation, abdominal pain, …internal bleeding, shock, death
Typhus: Rickettsia bacteria

7. Staphylococcus aureus (Intoxication)
S. aureus: not good at competing with other organisms
Superantigen enterotoxin:
Heat stable
Mechanism of toxin:
Poorly understood
Affect sub-epithelial macrophages (inflammation)
Change in local vascular permeability
Look this up- projectile vomitting
S. Aureus not g
Staphylococcus aureus
Produces toxin that causes nausea and vomiting
Thrives in moist, rich foods in which other organisms have been killed or inhibited
Survives well in unrefrigerated foods with high salt content
Source of S. aureus generally human carrier
Organism is inoculated into food during preparation
Food left at room temperature allows organism to grow and produce toxin
Toxin is heat stable and not inactivated by cooking
ood at competeing but fine once other org arent around (following cooking)

8. Food Preservation
Preventing growth and metabolic activities of organisms that cause spoilage and foodborne illness
Canning
Steam under pressure
Destroys spoilage and
pathogenic organisms
(including endospores)
High acid environment
Pasteurization
High temperatures for short periods
Reduces number of spoilage organisms
High acid environment: limits growth of sporeformers and toxin production

9. Food Preservation (con’t)
Cooking
Only destroys non-spore forming organisms
If undercooked, organisms can survive
Refrigeration
Slows growth rate
Freezing
Stops microbial growth
Drying
Inhibits microbial growth by decreasing available moisture
Does not alter taste of food significantly
Freezing: Water unavailable due to ice formation
organisms remaining can grow when food is thawed