Lecture 23 (Ch 16: Food microbiology and disease)

What are the differences and the reason why bacterial and eukaryotic microbes are used differently as direct microbial foods?

bacteria are very small, have extremely high nucleic acid content, and contain endotoxins that humans cannot safely digest.

What are the reasons we ferment food?

• Food preservation: inhibit growth of many spoilage organisms and foodborne pathogens 

• Improve digestibility: break down macromolecules → tenderize meat and vegetable products

• Add nutrients and flavors (e.g., vitamins and flavorful metabolites)

(lactic) acid fermentation

process in which microbes break down sugars without oxygen and produce organic acids—such as lactic acid, acetic acid, or other acids—that preserve food and give it a sour flavor.

starter culture 

specific microorganisms that are added to food to begin and control fermentation; strains carefully selected to produce desirable flavors and textures as well as protection against contamination from other microbes

unripened vs ripened cheeses

• unripened cheese only relies on bacterial fermentation to acidify and coagulate proteins (curd)

• ripened cheese undergo further processing to expel water and further fermentation to enrich flavor

alcoholic fermentation

yeasts ferment sugars to produce ethanol and carbon dioxide; used to make a variety of alcoholic beverages, vinegar, and bread

food spoilage

refers to microbial changes that render a product obviously unfit or unpalatable for consumption, based on microbial metabolism

• Acid fermentation = sour taste

• Oxidation of fats = rancidity 

• Decomposition of proteins = putrefaction

• Alkaline fermentation = bitter taste

Contrast lactic acid and ethanolic fermentation and what they are used for in food production

• lactic acid fermentation: sugar → lactic acid. used for yogurt, cheese, pickled foods 

• ethanolic fermentation: sugar → ethanol + CO2. done by yeasts. used for beer, wine, bread

intrinsic risk factors for food spoilage 

• Water availability: More water = faster spoilage (meat, milk). Low water = slower spoilage (bread, nuts)

• pH: fungi can grow at lower pH (and water availability) than most bacteria

• Nutrients: more nutrients = more bacterial growth

• Biological barriers: rinds, shells (postpone not eliminate)

• Antimicrobial chemicals: spices, herbs

extrinsic risk factors for food spoilage

• Storage temperature: warm temps = fast growth; cold slows growth; psychrophiles still grow in fridge

• Atmosphere: O2 speeds spoilage, but some grow even without O2 (e.g. improperly canned foods)

food poisoning

refers to presence of toxins produced by microbes present in food; symptoms arise within hours, cooking may not destroy toxins

food infection

refers to presence of pathogens on ingested food that proliferate and cause disease in the body – symptoms may take a day or more to appear, cooking of food before eating will prevent

What distinguishes a pathogen from a spoilage organism?

• A pathogen causes disease in humans or other hosts, while a spoilage organism causes deterioration of food or materials but does not necessarily cause disease.

• Bacterial/virulence factors that contribute to disease are often encoded together in pathogenicity islands in its genome

Select an appropriate means of food preservation from a provided list and be able to describe why they work

• salt/sugar: creates a hypertonic environment, causing water to leave

• dehydration (osmolarity): removes water

• radiation: damages DNA, kills microbes

• organic acids: lowers pH and makes environment too acidic for microbes

• low pH: most microbes prefer neutral pH

exotoxin

proteins produced and secreted by various types of bacteria; kill host cells and unlock their nutrients

endotoxin

a part of lipopolysaccharide (LPS) of Gr- bacteria; released only when the bacteria die and the cell wall breaks apart

Why is botulism of particular concern in canned goods?

Clostridium botulinum is a Gram-positive, spore-forming anaerobe commonly found in soil, and its spores can survive normal processing; if the canning process isn’t hot enough, the spores can germinate in the oxygen-free environment, produce a powerful neurotoxin, and cause potentially fatal illness—even though the disease is rare.

Name the most common causative agents of diarrheal diseases.

• Most diarrheal disease is caused by viruses

  • Rota- and noroviruses are the main culprits.

• Among the bacteria, Salmonella enterica serovar Typhimurium and Campylobacter are the most frequent causes of self-limiting diarrheal disease, other outbreaks include specific strains of E. coli.

enterotoxin

a type of exotoxin that targets the cells lining the GI tract, often causing symptoms such as diarrhea and vomiting. can cause gastrointestinal disease without ever producing infection (i.e., growth inside of the host).