Food Microbiology

08/11/11

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)

Growth of Microorganisms in Food

• Warm• Oxygen-rich

Food SpoilageGenerally not harmfulMay 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. 2003. Appl. Environ. Microbiol. 69(4):2321-2329

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: entericaSerovars: 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

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 PreservationPreventing 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

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

Food Preservation (con’t)