FS 416 Final

What is Pastuer's definition of fermentation?

Life in the absence of oxygen

Know the major categories of antimicrobial preservatives used in food preservation.

Acids Bacteriocins Diacetyl Peroxide

Know the kinds of naturally occurring antimicrobial agents available for food preservation.

Acetic Acid Lactic acid Citric acid

What is vinegar?

Acetic acid

How is vinegar made?

Made by microorganisms Acetobacter and Gluconobacter Gluconobacter oxidans Strictly aerobic Grow as biofilms Produce Ethanol (wine, hard cider, etc.) ... Acetic acid Preservation of Acetic acid

What are the steps in beer making

Malting: Mashing: Wort Boiling with hops and cooling Fermentation(4-14C for 8-10 days) Ales: Saccharomyces cerevisiae (top) Lagers: Saccharomyces carlbergensis(bottom) Post fermentation treatment

What are the three types of fermentation pathways?

The Entner-Doudoroff pathway (ED) Embden-Meyerhof Parnas (best) Heterofermative pathway

What is specific example of the use of Nisin?

used for milk, cheese, dairy products, canned food, maynonnaise, salad dressing, and baby food GRAs, and inhibits botulinum spores Sanitizes spores to heat and reduce thermal treatment (not approved in the US)

What is a specific example of the use of Pediocin?

used to inhibit Listeria, and increase shelf life of salads and salad dressing o More effect than nisin in meats o Improve proformance in fatty foods

What are starter cultures?

• Accidental fermentation • Back-slopping • Defined starter cultures Culture that is added to start fermentation

What is the sequela of Salmonellosis?

• Aseptic reactive arthritis • Reiter's syndrome • Ankylosing spondylitis

Be able to define bacteriocins.

• Bacteriocins are proteinaceous or peptidic toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s). They are similar to yeast and paramecium killing factors, and are structurally, functionally, and ecologically diverse.

What are the three different methods of starting a meat fermentation?

. 1. The native bacteria o II. Back-slopping Lactobacilli Coagulase-negative staphylococci Micrococci n Yeasts o III. Defined starter cultures Pediococcus acidilactici ¨ Homofermentative ¨ Can tolerate 6% salt ¨ Not proteolytic ¨ Not lipolytic ¨ Can grow at 43-50oC ¨ Used at 10^7 CFU/g

What is the microbial succession in sauerkraut?

1. Day 1-3, aerobic bacteria die off, facultative anaerobes grow 2. Day 3-7 heterfermentators LAB -lactic and acetic acid grow (lactobacillus pantarum)(leuconostoc mesenteroides) (lactobacillus brevis) 3. Homofermentors LAB grow (months) (lactobacillus brevis) 4. Storage up to year Lactobacillus brevis and heterofermenators metabolize pentose

What are the step in sauerkraut fermentation?

1. shred sound healthy cabbage 2. Place it into a vassel which can contain up tp 1000 tons 3. add salt to an equilibrium concentration of 2% 4. tightly pack and make an anaerobic environment May be stored in vessel up to a year

LAB genomes have how many protein encoding genes?

2000-3000

What is malic acid?

A by-product of the TCA cycle, occurs naturally in grapes, and decreses as they ripen

What does sequela mean? What is its significance in the context of food microbiology?

A sequela is a pathological condition resulting from a disease, injury, therapy, or other trauma. Typically, a sequela is a chronic condition that is a complication which follows a more acute condition.

What do we know about nisin?

Applications of Nisin: Milk, cheese, other dairy products, canned foods, mayonnaise, salad dressings, and baby foods GRAS for inhibition of botulinum spores Sensitizes spores to heat to reduce thermal treatments - not approved in the US Often used with other inhibitors Modified atmosphere packaging of fish & meat Increases shelf life & delays toxin production by type E Cb Modified atmosphere packaging of pork Prevents L. monocytogenes growth Pasteurization of liquid whole eggs Doubles shelf life

What is the food fermentation definition?

Bio Processes that change food properties while bacteria generates energy in the absence of oxygen o Acid production o Flavor o Nutrient bio-availability o Gas production o Texture

What is the Bio-processing definition of fermentation?

Biological process whether or not oxygen is present Example vinegar, MSG, antibiotics

2. What is MicroGARD? How is it made? How is it used?

Cultured milk product-essentially a starter culture that can used with a variety of products-cheese, yogurt, ice cream, salad dressing, and sour cream made by Danisco, Inc Made:by fermenting milk with Propionibacterium shermanii to produce acetic acid, proteins and a bacteriocin

Be able to compare and contrast the use of native yeast or defined yeast inoculum in wine making.

Defined yeast inoculum: Consistent product and faster process Saccharomyces cerevisiae or bayanus 106-107 cfu/ml Native yeasts: Less predictable product Saccharomyces, Kloeckera, Hansenia, Candida, Pichia, Hansenula Grapes; Immature grapes: ~10 to 1,000 cfu/g Ripe grapes: 104-106 cfu/ml Equipment

2. Have a good understanding of the legal definition of probiotics.

Dietary ingredients taken by mouth to supplement the diet, which are not labeled as food, are not the sole item of a meal, and are not taken to prevent, cure, or treat a disease

What are the three major categories of fermented foods of non-western societies?

Fermentation based on yeast and molds: Miso, soy sauce, tempeh, sake, oncom Fermentations based on LAB Kim chi Fermentaions based on other bacteria Kefir, Natto

What are the factors that affect "controlled acidification?"

Food's initial pH Foods's buffering capacity Pathogen of greatest concern Nature of fermentable carbohydrate Microbial growth rates (LAB & pathogens) at refrigeration & abuse temperatures

What does microbial genomics deal with?

Genetic fluidity in lactic acid bacteria LAB genomic Bacteria plasmids

What is the common definition of fermentation?

Gentle bubbling or "boiling" action (production of CO2)

What are the basic steps in white wine making?

Grape juice drained from skin immediately after crushing Juice is clarified Sent to fermentation tank

Know the major classes of bacteriocins. Know the basics behind this categorization.

Group III - Large (>30kDA) heat labile Group IV - Contain lipid or carbohydrate Group II Small heat stable proteins with normal amino acids Leader sequence [Gly-Gly-Xaa] Three sub-classes (IIa, IIb, IIc) Pediocin-Like (Class IIa) Tyr-Gly-Asp-Gly-Val-Xaa-Cys Pediocin PA-1, pediocin AcH, sakasin A, sakasin P, leuocin A, bavaricin MN, and curvacin A Have activity against Listeria monocytogenes Group I - "Lantibiotics" - contain unusual amino acids Lanthionine, dehydroalanine, dehydrobutyrine, methyl lanthionine Nisin - Most well characterized lactic acid bacteria bacteriocin Commercially available - Nisaplin Nisin A - His 27 Nisin Z - Asp 27

What are the ingredient in beer making>?

Hops (for flavor and antimicrobial activity) Yeast (to produce alcohol and carbon dioxide) Water Malt (to provide fermentable carbohydrate Adjuncts (less expensive cereals, grains, corn) to augment or replace malt

What is the significance of the presence of antibiotic-resistant food borne pathogens, such as select strains of Salmonella, in our food supply?

If they are antibiotic resistant and found in our food and if an individual contacts the illness they will not be able to get treated with antibiotics

What are the different means of using bacteriocins in foods?

Improve quality of fermented foods: Inhibit unwanted bacteria Promotes starter culture Use starter cultures that produce bacteriocins to kill the native bacteria of the food

2. Be able to discuss the prospective research areas of interest on probiotics.

Investigate benefits of probiotics in the context of: a. Diet b. Well-characterized probiotic bacteria c. Individual human intestinal systems d. Principles of microbial ecology e. Develop new probiotic strains

What are the basis flavor compounds produced in fermented dairy products?

Lactic acid is primary product: acid taste Diacetyl: Butter flavor Acealdehyde: Yogurt/ green apple flavor Proteolysis of casien: bitterness

What can LAB plasmids encode for?

Lactose metabolism Proteinase activity Oligopeptide transport Bacteriophage resistance Exopolysaccharide production Citric acid utilization

What are the major components of the human GI tract?

Large intestine Small intestine:nuetral pH Stomach:acidic

Be able to compare old and modern fermentors used for making vinegar.

Modern fermentors: Submerged culture reactor Pump oxygen through at rates equal to the volume of fermentor Old fermentors: Leave wine in wooden barrels exposed air, inoculate it with Gluconobacter, and wait "Trickling" fermentor Large wooden box with slatted sides to allow oxygenation Filled with wood shavings Gluconobacter grows as biofilms Rotating sprayer sprays the alcoholic liquid on the top of chips

Have a good understanding of the microbiology of fermented sausages.

Natural microbiota of meat is Gram (-) aerobic putrefying spoilage bacteria LAB lower pH and prevents spoilage n Dry and semi dry sausage is most common type of fermented meats USDA FSIS requirements: ¨ Nitrite and other curing agents ¨ pH <5.0 ¨ Moisture to protein ratio <3.1:1.0 Nitrite and other curing agents Inhibits Clostridium botulinum Contributes to the cured meat taste Converts myoglobin to nitrosomyoglobin (the pink color of cured meat) Other important ingredients Fermentable sugars n Glucono-delta-lactone (GDL) ¨ GDL ... Gluconic acid ... lactic and acetic acids (accelerated acidification) The starter cultures

How does serotyping work?

O: somatic found on the external surface of the bacterial outer membrane H: Flagellar anitgens associated with peritricious flagella, Heat liable, Can produce one or two diffent sets of flagellar antigens Vi: Capsular: Found in Typhi, ParatyphiC, and Dublin

What is the major role of microbial fermentation in cocoa?

Provides flavor The seeds and mucilage are removed from the pods Put into heaps or containers Fermented for 2-8 days

What do we know about polylysin?

Polymer of 25-30 lysine residues No secondary or tertiary structure Produced by Streptomyces albulus Effective at 100 ug/mL Activity against bacteriophages, molds, yeasts and bacteria

What are reservoirs for Salmonella

Poultry Eggs Meat Fruits and vegetables Contaminated shellfish

What is the generic scheme for making dairy products?

Prepare milk at farm Receipt and storage plant Clarification Formulation Heat treatment Homogenization Inoculation and incubation Cooling, formulation, and packing

What is the Biochemistry definition of fermentation?

Process in which bacteria use to make energy from CHO in absence of oxygen

Be able to compare and contrast white wine and red wine making.

Red wines: Skins and juice fermented Fermented 7 day at 20-30C Aged Different grapes White wines: Just purified juice is fermented 1-2 weeks at 10-18C Not aged Different grapes

What are the basic steps in manufacturing fermented sausage?

Reduce particle size of high quality meat by grinding add curing salts, 3% glucose, spices, and starter culture 10^7 Blend ingredients Vacuum stuff into to casings incubate Heat to inactivate the starter culture and pathogens Age(dry) the sausages

What are the important ingredients used in vegetable fermentation and what are their roles?

Salt: flavor, fluid release, help create anaerobic conditions, microbial selection Spices and seasoning: tumeric color Antioxidants: asorbic acid, citric acid, sulfur dioxide Antimicrobial: sorbic acid

What are the basis steps in vegetable fermentation?

Select sound, undamaged, uniformly sized, ripe vegetables Pretreat, by peeling, blanching, or cooking Place whole, pierced, shredded, or slice vegetables in fermentation vessel, also add starter culture if necessary Completely cover with brine, seal vessels, and ensure anaerobic environment Let fermentation take course, time depends on temp, product, and bacteria present Distribute final fermented product fresh, packaged, pasteurized or unpackaged

What is the significance of malolactic acid fermentation in wine making?

Significance of malolactic acid: decreases the acidity, and creates mellower flavor

What are the substrates and products of yeast growth in wine fermentations?

Substrates: glucose and fructose Products: Ethanol Carbon dioxide Glycerol - smooths taste and imparts viscosity Flavor compounds: Higher alcohol esters & aldehydes Succinic acid & acetic acid - undesirable

What factors influence grape flavor?

Sugar to acid ratio Climate Soil Vine age Other factors

What is the wisconsin process?

Tanaka et al (1980) - "Wisconsin Process" for bacon manufacture Nitrites in cured meats carcinogenic Bacon spiked with 103 botulinum spores Bacon w/o LAB +120 ppm nitrite - 58% became toxic Bacon w/ LAB + sucrose + 40-80 ppm nitrite - <2% became toxic USDA approved the "Wisconsin Process" for bacon manufacture in 1986

Be able to define "biopreservation."

The use of Lactic Acid Bacteria (LAB), their metabolic products, or both to improve or ensure the safety and quality of foods that are not typically seen as fermented foods."

What are general characteristics of bacteriocins?

They do NOT kill the bacteria that produce them o Have antimicrobial activity against "closely related" species o Gram (+) o May have activity against Gram (-) o Chelating agents Hydrostatic pressure o Injury

Be able to describe the symptoms of Salmonellosis.

Typhoid and paratyphoid strains: o Incubation period: 7- 28 days o Symptoms:diarrhea, prolonged and spiking fever, abdominal pain, headache, prostration • Non-typhoid strains o Incubation period: 8-72 hours o Symptoms: non-bloody diarrheal stools and abdominal pain

Significant differences exist between the past and present rankings of select foodborne pathogens, including Salmonella. Be able to discuss at least three possible reasons for the difference.

a. Changing demographics b. Changing food preferences c. Food available Preservation methods available

Salmonella species cause food borne illness with low mortality rates. Do we need to be concerned about the organism and the illness that it causes? Why or why not?

a. Yes we do because even if the acute illness is not severe the sequela can be and have long term affects.

Know the basic steps of red wine making.

grape juice and skins go directly to fermentor Skins float and the juice is pumped over the skins to extract anthocyanins and phnolics Liquid pressed through the skins into a fermentation tank Skins removed

What is the major role of microbial fermentation in coffee processing?

main role is to liberate the beans Adds complexity and depth Fermentations in coffee: Coffee cherries that contain the beans are surrounded by fleshy mucilage Remove mucilage and liberate beans Mechanically Enzymes Dehydration Fermentation Mechanical depulping Beans + residual mucilage + water Native yeast, molds, LAB, and gram-negative bacteria

What are probiotics based on FAO/WHO definition?

o "Live microorganisms which when administered in adequate amounts confer a beneficial physiological effect on the host"

What does SAccharomyces make when grown aerobically versus anaerobically?

o Anaerobically: makes carbon dioxide, alcohol , a little ATP o Aerobic: CO2 and ATP

What differentiates sour dough bread from other breads?

o Has another organism in it usually Lactobacilluius sanfranciscensis o Improved flavor, texture

Know the preservation factors in alcoholic beverages.

o High acidity/Low pH(wine) o Ethanol (all alcoholic beverages) o Sulfites (wines)

Be able to discuss the main properties associated with probiotics?

o Human origin o Documented health effects o Safe o Good technological properties o Resistance to pancreatic enzymes, acid and bile o Adhesion to intestinal mucosa

A food fermented with a bacteriocin producing culture can be used as an ingredient in another food. When do you need to include the ingredient as a preservative on the food label? When do you need NOT to include the ingredient as a preservative?

o If it used as an ingredient and extends shelf life than do not list as a preservative o If it used as an ingredient fort he the sole purpose of extending shelf life then list as a perserative

Who is Metchnikoff? What are some of his accomplishments as a scientist

o Metchnikoff was a Russian zoologist best known for his pioneering research in immunology. o In particular, he is credited with the discovery of phagocytes. o He won a nobel prize for discovering phagocytosis

Know the ingredients and the basic steps of bread making.

o Mix flour, sugar, fat, salt, add yeast, and add water o Knead dough o Ferment (proofing; 28-32oC for several hours) o Remix (punching) o Portion dough into loaves o Ferment o Bake (200C, ~30 minutes)

What are the major modes of action by bacteriocins?

o Pore formers o Rapid efflux of pre-accumulated ions, aa, and ATP o Degrade RNA o Inhibit protein synthesis

What are the benefits of starter cultures?

o Rapid acid production o Decreased rate of contamination o Production of flavor compounds o More consistent products o Greater process predictability

What is Saccharomyces?

o Saaharomyces is a sugar fungus Saccharo: sugar Myces: fungi Found on grapes and fruits

What are the virulence factors involved in the virulence of Salmonella species?

o Siderophores Retrieves essential iron from host tissues o Competes with host transferrin, lactoferrin, and ferritin ligands o Diarrheagenic enterotoxin o Thermolabile cytotoxic protein Localized in the bacterial outer membrane & released in hostile environments Inhibits protein synthesis and lyses host cells o Capsular polysaccharide Vi antigen o The length of LPS Defines the rough (short-LPS) & smooth (LPS) phenotypes Prevents attack by the host immune system o Porins Outer membrane proteins that function as channels in regulating the influx of nutrients, antibiotics, etc. DETE • : plasmids

What is the significance of Saccharomyces?

o Significance: helps make alcohol beverages alcoholic, and give them a bubbly taste o Feeds off sugar in bread and helps it rise, and breakdown sugar

What are the common substrates used in fermented foods of non-western societies?

o Soybean o Rice o Wheat o Peanut

3. Be able to discuss the relevant benefits associated with probiotics?

o Species specific interactions with the host o Proposed health effects are 'true' o No health risk to consumer o Strain stability o Production at large scale o Oxygen tolerance o Survival of passage through the intestinal tract o Immune modulation o Pathogen exclusion o Enhanced healing of damaged mucosa o Prolonged transient colonization

What are the intrinsic and extrinsic factors that influence yeast growth in wine fermentations?

pH Oxygen concentration Sugar concentration measured in Brix Temperature Additional ingredients Other indigenous microbial flora

Why would one need to use an enrichment method while detecting Salmonella?

• Because there is a zero tolerance in RTE food, because even 1 sublethally injured cfu is a health hazard • So you want to enrich it so even one cell can be detected • Enriched by: pre enrichment, selective enrichment, than immunoassay

Have a good understanding of the technology and microbiology of pickle fermentation.

• Bulk tanks: 8- 10,000 gallons • Brine: 6% NaCl, .1-.4% CaCl2 • Cucumbers- 1% glucose and fructose • Undergo homolactic fermentation by Lactobacillus plantarum o Final pH 3.7 < pH 4.6 for high acid foods o Stored 1 year in tanks • Floaters or bloaters o Causes: co2 produced by cucumbers respiration o Control- reduce co2 from the brine by purging with N2

What does "climax community" mean? What is the significance of the term in the context of probiotic bacteria?

• Climax Community - Ecosystem in which progression of species has stopped, all niches are filled, and a characteristic group of organisms is maintained o The metabolic activity is strongly influenced by host physiology and everything that affects host physiology

What are examples of fermented vegetables products?

• Cucumbers • Sauerkraut • Niche Products o Peppers o kimchi

Be able to define "controlled acidification."

• Customized Application - product specific o Other metabolites may enhance inhibition: Diacetyl Hydrogen peroxide Bacteriocins • Organic acids inhibit microbial growth o Examples: Acetic acid, Lactic acid, Citric acid o Often added to food as a chemical "preservative" o LAB can produce acid in the food

Be able to recognize and provide examples of desirable selection criteria for probiotics.

• Developing future strains: o Quality of probiotic strains o Testing the stability of strain characteristics o Probiotics for specific target groups o Non-viable probiotics o Alternative applications • Testing the stability of strain characteristics: o Adherence properties o Viability • Quality of probiotics: o Retain the strain characteristics o Retain the purity of preparations o Careful identification of the strains used o Placement of strains in an international type culture collection o Avoidance of old or non-existing nomenclature o Hygienic preparation of products

What are the biochemical traits that help us differentiate Salmonella Species from other members of the family Enterobacteriaceae?

• Facultatively anaerobic • Gramnegative • Rodshaped • Motile with peritrichous flagella, but Nonflagellated variants and nonmotilestrains with dysfunctional flagella exist • Chemoorganotrophic with both respiratory and fermentative pathways • Acid and gas production from carbohydrates • Oxidase negative • Catalase negative • Grow on citrate as sole carbon source • Produce hydrogen sulfide, Decarboxylate, lysin& ornithin • Do not hydrolyze urea

Be able to recognize and provide examples of health benefits for probiotic bacteria.

• Healthier gut microbiome • Increased digestion

What is the difference between homofermentative and heterofermentative?

• Homofermentative bacteria are used to make lactic acid, whereas heterofermentative bacteria contribute flavor

Define and describe LAB

• Lactic acid bacteria o Composed primarily of Lactococcus, Lactobacillus, Leuconostoc, Pediococcus, and Streptococcus o Functionally defined group organisms Used to prepare and preserve foods Distantly related o Convert sugar to lactic acid Homofermentative vs heterofermentative o Separation between catabolic and biosynthetic pathways o Plasmid can encode for Lactose metabolism Proteinase activity Oligopeptide transport Bacteriophage resistant Citric acid utilization • Key role in production of fermented foods and beverages o Impact on food health and agriculture

What is the legal status of bacteriocin use in the US?

• Lactic acid bacteria are GRAS for production of fermented foods o GRAS - allows product to be used for an application without additional approval o GRAS status of LAB for production does not make them or their metabolic products GRAS for other applications • Nisin is the ONLY bacteriocin with GRAS status (1988) • Polylysine is GRAS for use in cooked rice and sushi rice (2003) • Labeling & Marketing o Bacteriocins that are not GRAS are regulated as food additives o Require premarket approval by the FDA o A food fermented by bacteriocin-producing starter cultures can be used as an ingredient in a second food product o If used as an ingredient and it also extends shelf life - do not need to list it as a preservative o If used as an ingredient for the purpose of extending shelf life - need to list it as a preservative

1. Be able to discuss various types of fermentation based on LAB and their significance.

• Lactic acid bacteria converts sugar/lactose to lactic acid with heterofermentations and homofermentation processes. There's a separation between the catabolic and biosynthetic pathways. o Catabolic pathways: EMP fermentations: 6 carbon sugar to a 3 carbon sugar to pyruvate to lactic acid or acetic acid or aldehydes Heterofermentative: six carbon sugar to five carbon sugar to aledehydes or 3 three carbon to pyruvic acid to lactic acid Entner-doudoroff: six carbon sugar to three carbon to pyrvic acid to lactic acid Heterolactic n <85% Lactic Acid • CO2, EtOH, Formic acid, Acetic Acid

3. What is Nisin?

• Nisin is a polycyclic antibacterial peptide produced by the bacterium Lactococcus lactis that is used as a food preservative.

1. Be able to compare/contrast sauerkraut and pickle fermentations

• Pickle fermentations: o Bulk tanks: 8-10,000 gallons o Brine: 6% NaCl, 0.1-0.4% CaCl2 o Cucumbers contain 1% each of glucose and fructose o Undergo homolactic fermentation by Lactobacillus plantarum ¨ Final pH ~3.7 < pH 4.6 for high acid foods ¨ 5 log reduction in pathogens ¨ Stored 1 year in tanks n "Floaters" or "Bloaters" ¨ o Causes: C02 produced by cucumber respiration, Leuconostoc mesenteroides - heterofermenter ¨Control:, Reduce C02 from the brine by purging with N2 • Sauerkraut fermentations: o Bulk Tanks - 100 Tons cabbage o Preparation ¨Shredding ¨Dry Salting - 2% Salt ¨Packed tight - removes O2 ¨Temperature - 18oC o No starter culture

What is polylysine?

• Polylysine is an antimicrobial peptide

What are the different uses of fermentation?

• Preserve vegetables and fruit • Deepen characteristic sensory properties- flavor, aroma, and texture • Destroy naturally occurring toxins in raw materials • Enrich product with desired microbial metabolites (lactic acid or amino acids) • To create new products • Increase dietary value

What is the chicken a la King example?

• Saleh and Ordall (1955) - "Chicken a la King" o L. bulgaricus or L. lactis + Fermentable sugar o Spiked with C. botulinum + incubation at 30oC o 16/16 of samples w/o LAB became toxic o 3/16 of samples w/ LAB became toxic • Inhibition of Clostridium botulinum growth under temperature abuse o C. botulinum unable to grow at < pH 4.6 o LAB + fermentable sugar (glucose or lactose) are added to food

What types of genes are located on a generic bacteriocin operon? What are the roles of these genes?

• Structural gene • Immunity • Maturation • Processing • Export • Control of bacteriocin synthesis • Encodes a pre-peptide • Protects cells from being killed by the bacteriocin • Cleavage of leader sequence (e.g., Gly-Gly) • Modification and excretion of the prepeptide • Turn bacteriocin synthesis on and off

3. Why is the human GI tract considered to be a "microbiome?"

• The average human contains 400+ species of bacteria (microbiome) o 30-40 of these species constitute 99% of bacteria in healthy humans • Humans contain 1013 to 1014 human cells and 1013 to 1014 bacterial cells • Humans are born sterile inside. o Colonized by bifidobacteria when breast-fed. o Colonized by lactobacilli when fed with cow's milk. o Colonized by strict anaerobes with the introduction of solid food. • Once established, the composition and distribution of human microbial component are very hard to change. • Indigenous bacteria • Colonize the host by adhering to the intestine o Transient bacteria • Foodborne bacteria just passing through Dietary factors, even starvation, make surprisingly little difference in the distribution of bacteria in the GI tract! o Colonization influenced by many factors o Gastric acidity o Bile salt concentration o Peristalsis o Digestive enzymes o The immune response