Food Microbiology 08/11/11

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

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. 2003. Appl. Environ. Microbiol. 69(4):2321-2329 Bacteria: souring, slime Bacillus (bread)-roping Fungi: contaminate maple syrup, bread (low water and acidic environments)

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

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 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

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)

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

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