25: Food Microbiology

Food Microbiology

I. Food and Beverages

A. Source of Nutrients
B. Sources of Contamination

  1. Soil:

    • Plants grow in soil.

    • Animals come into contact with soil.

    • Soil organisms in food are usually not pathogens, but can cause spoilage.

  2. Food handling:

    • Harvest and slaughter practices.

    • Food handlers themselves.

    • Unsanitary working conditions.

  3. Improper food storage:

    • Improper refrigeration is a major cause of food-related illness.

  4. Globalization:

    • Import from countries where standards may be lower.

Support of Microbial Growth

A. Categories

  1. Based on perishability:

    • Perishable: Easily supports growth; ex: fresh starberry

    • Semi-perishable: Spoil less quickly; ex: raisins

    • Non-perishable: Edible for a long time; ex: canned food

  2. Factors influencing spoilage:

    • Extrinsic factors; ex: How the food is stored.

    • Intrinsic factors (Directly related to the food itself)

B. Water Activity (awa_w)

  • Definition: Amount of H2OH_2O available and accessible to microorganisms.

  • Range: 0 to 1

  • Spoilage fungi: aw≥0.8a_w ≥ 0.8

  • Spoilage bacteria: aw≥0.9a_w ≥ 0.9

  • Examples:

    • Fresh meat and fish: aw=0.99a_w = 0.99

    • Bread: aw=0.95a_w = 0.95

    • Dried fruit: aw=0.6a_w = 0.6

    • Instant coffee: aw=0.2a_w = 0.2

C. Controlling Food Spoilage

  1. Reduce awa_w:

    • Drying: Using the sun or an oven.

    • Freeze-drying: Freezing then drying under a vacuum.

  2. Control of temperature:

    • Refrigeration: 4°C (except for psychrotolerant organisms).

    • Cooking: Using high heat.

  3. Pasteurization:

    • Controlled heating at temperatures well below boiling.

    • Flash pasteurization (High-Temperature Short-Term - HTST): Quick heating to 72°C for approximately 15 seconds, then rapid cooling.

    • Ultrahigh Temperature (UHT) sterilization: Heated to >135°C for 2-5 seconds, then cooled; no refrigeration needed; room temperature storage for approximately 3 months.

II. Foodborne Diseases

A. Introduction

  1. Gastroenteritis:

    • Inflammation of the stomach and intestinal lining.

    • Caused by a variety of microorganisms.

    • Results from contamination of food and water.

  2. Bacteria:

    • Enterotoxins: Act on the small intestine lining.

    • Cause massive fluid secretion into the intestinal lumen, leading to vomiting and diarrhea.

  3. Worldwide diarrheal diseases:

    • Leading cause of childhood death.

    • Second only to respiratory diseases as a cause of adult death.

  4. In the US:

    • 48 million cases of food-related diseases annually.

    • 128,000 hospitalizations.

    • 3,000 deaths.

  5. Two primary types of food-related diseases:

    • Foodborne infection.

    • Foodborne intoxication.

B. Foodborne Infection

  • Ingestion of pathogen.

  • Followed by growth in host, tissue invasion, and release of toxins.

  1. Cholera:

    • Causative agent: Vibrio cholerae

    • Transmission: Fecal-oral

    • Mechanism:

      • Secretes choleragen (CT), an AB exotoxin.

      • Binds to epithelial cells of the small intestine.

      • Plasma membrane highly permeable to water; intestinal lining damaged, leading to rice-water stools.

      • B pentamer: Mucolytic enzymes that penetrate intestinal mucus. Fimbriae attach to cell surface.

    • Choleragen (CT) mechanism of action:

      1. B subunits bind to receptor.

      2. A subunit enters & dissociates.

      3. A1A_1 ADP-ribosylates G proteins.

      4. G protein activation leads to adenylate cyclase activation.

      5. Increases cAMP production.

      6. Activates CFTR chloride channels.

      7. Efflux of Cl−Cl^-.

      8. Secretion of H2OH_2O & ions into lumen.

      9. Rapid fluid loss from intestine.

    • Symptoms:

      • Profuse diarrhea: Lose 10-15 L fluid.

      • Electrolyte imbalance and H2OH_2O loss.

      • Increased concentrations of blood proteins.

      • Circulatory shock and collapse (loss of BP, diminished blood circulation, & inadequate blood flow to tissues).

      • Death.

    • Treatment: Fluid and electrolyte replacement.

  2. Salmonellosis:

    • Mainly S. Typhimurium and S. Enteritidis.

    • Sources: Meats, poultry, fish, eggs, dairy products.

    • US statistics: approximately 45,000 cases/year reported, but may be 2-3 million.

    • Mechanism: Invade intestinal lining and damage tissue, leading to diarrhea and abdominal pain.

    • Gram-negative bacteria release endotoxin, causing fever.

  3. Escherichia coli:

    • Transmission: Contaminated food & water

    • Rapidly acting, dehydrating condition.

    • Traveler’s diarrhea.

    • Pathogen usually absent from normal environment.

    • Different categories (strains) cause disease in different ways:

      • Enterotoxigenic E. Coli (ETEC): Bind to intestinal epithelial cells & produce enterotoxin leading to secretion of electrolytes & water into lumen -> diarrhea.

      • Enteroinvasive E. Coli (EIEC): Invade cells & produce enterotoxin -> diarrhea.

      • Enteropathogenic E.coli (EPEC): Bind to cells, cause lesions & damage microvilli -> diarrhea.

      • Enterohemorrhagic (EHEC): Toxins kill vascular endothelial cells -> diarrhea; responsible for many E.coli outbreaks in US.

C. Foodborne Intoxication

  1. Overview:

    • Pathogen secretes toxin, which contaminates food and is then ingested.

    • Presence of living organisms is not required.

    • Produces symptoms shortly after food consumption because growth of organism is not required.

  2. Food-borne botulism (example):

    • Causative agent: Clostridium botulinum (obligate anaerobe, endospore-forming) found in soil & aquatic sediments.

    • Source of infection: Home-canned food not heated sufficiently to kill endospores.

    • Mechanism:

      1. Endospores germinate and produce toxin during vegetative growth.

      2. Food is eaten without adequate cooking, so toxin remains active, and disease results.

      3. Botulinum toxin is a neurotoxin that binds to synapses of motor neurons.

      4. Cleaves synaptobrevin (synaptic vesicle membrane protein) preventing exocytosis & release of acetylcholine

      5. No contraction in response to motor neuron activity

      6. Flaccid paralysis results

    • Symptoms:

      • Occur 18 to 24 hours of toxin ingestion.

      • Blurred vision, difficulty swallowing & speaking, muscle weakness, nausea, vomiting

      • Without treatment, 1/3 of patients die within a few days due to respiratory or cardiac failure.

      • < 200 cases/yr in US

    • Infant botulism:

      • Occurs in infants < 1 year old.

      • Ingest endospores naturally found in honey or house dust.

      • Endospores germinate in the intestine.

      • C.botulinum multiplies and produces toxin.

      • Death may result from respiratory failure.