Download Lecture 14 Food Microbiology

Food Microbiology
08/11/11
Growth of Microorganisms in Food
Ideal environment for microbial growth:
•Moist
•Nutrient-rich
•pH neutral
• Warm
• Oxygen-rich
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)
Food Spoilage
Generally not harmful
May be presence of
• Food-borne bacterial pathogen
• Fungal toxin
Pepe, O. et al. 2003. Appl. Environ.
Microbiol. 69(4):2321-2329
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
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
Common Foodborne Illnesses
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 acidmust ingest high numbers
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
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
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