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Chapter 2

Microbial Growth, Survival, and Death Surv val, and Death in Foods

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Learning Objectives1. Distinguish among different methods of

culturing foodborne microbesg2. Recognize intrinsic and extrinsic factors used

to control microbial growth3. Water activity concept4. Affect of water activity on microbial growth

and ecologygy5. Qualitatively predict the size of microbial

populations and their rate of growth6. Relate biochemistry to energy production and

metabolic products of foodborne bacteria2

Food Microbiology• Regulated by numerous government

agenciesagencies• Requires understanding of microbes as

well as complex food systems

Campylobacter is the second-most reported cause of food-borne illness. 3

Agencies Involved in Food SafetyUSDA – US Department of Agriculture

• regulates meat and poultry via Food Safety and Inspection Service (FSIS)

• pressures food processors to initiate ”voluntary” recallpressures food processors to initiate voluntary recall

FDA – Food and Drug Administration• regulates all foods except meat, poultry and alcohol• issues mandatory recalls and quarantines foods• regulates drugs and medical devices

CDC – Center for Disease Control and Prevention• provides epidemiological expertise for outbreaks of foodborne

illnesses• oversees PulseNet, a DNA fingerprinting clearinghouse that

links geographically dispersed cases to a common outbreak

EPA – Environmental Protection Agency• insecticides, fungicides, herbicides 4

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Hurdle TechnologyInteraction of intrinsic and

extrinsic factors d i hi h determines which microbes grow in an environment

Hurdle technology - uses multiple p

environmental factors to inhibit microbial growth

- important preservation strategy 5

Food as EcosystemsThe food environment consists of:

intrinsic factors - inherent to the foodintrinsic factors - inherent to the food• pH• water activity• nutrientsextrinsic factors - external to the food• temperature• gaseous environment• other bacteria

Both can be manipulated to preserve food6

Microbial Ecology

• the study of the interactions between th h i l h i l d t t l the chemical, physical, and structural aspects of a niche and the composition of its specific microbial population

• ”interactions” emphasize the dynamic complexity of food ecosytems complex ty of food ecosytems

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Heterogeneity of Food• Heterogeneous on a scale of

micrometersmicrometers• Gradients of pH, oxygen, and nutrients• Example - food poisoning outbreak

associated with aerobic foods caused by ”strict anaerobe” Clostridium botulinum

• Oxygen is driven out by cooking and diffuses in slowly so that most of the product remains anaerobic

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Plate Count Methods for Enumeration of Bacteria

• Classical estimation method for counting bacterial cell numberscell numbers

• Assumes every cell forms one colony and every colony originates from one cell

colony forming unit - CFU• Limitations:Limitations: not all bacteria grow under the same conditions not all bacteria grow as single cells samples require dilution or enrichment

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Plate Counts

• Used to quantify bacterial populations- >250 CFU / ml for liquids- >2,500 CFU / ml for solids

• Food sample is homogenized 1:10 in buffer followed by serial dilutions

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Serial Dilution

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Spread Plate vs Pour Plate

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Plate Count

6.5 x 105 Countable plate25–250 colonies13

Simple Right?• Used to be referred to as the total plate count• This is not accurate Why?This is not accurate, Why?

- 35 degrees C and aerobic growth were standard but do not detect anaerobes or bacteria growing at 15 degrees C

- important pathogens may go undetected• Variations of the standard aerobic plate count

would increase specificity for certain types of bacteria

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Selective and Differential Media

• Selective media- includes a component that favors the

growth of the desired microbe - may contain an ingredient that inhibits

the growth of competing microbes• Differential media• Differential media

- a component is added to the media that enables colonies of the target microbe to look different to other colonies

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Mannitol Salt Agar• Staphylococci• Selective agent - 7.5% salt

inhibits growth of most gmicrobes

• Mannitol fermentation differentiates between pathogenic and non-pathogenic staphylococci- S aureus ferments - S. aureus ferments

mannitol turning phenol red to yellow

- S. epidermidis cannot ferment mannitol and appears red 16

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Thiosulfate-Citrate-Bile salts-Sucrose• Vibrio• Selective agents – sodium

citrate, sodium thiosulfate, oxgall which provide an g palkaline pH of 8.6 and slows growth of other microbes

• Sucrose fermentation differentiates between vibrios - V. cholerae ferments

sucrose turning pH indicator brom thymol blue yellow

- V. parahaemolyticuscannot ferment sucrose and appears blue-green 17

Xylose Desoxycholate (XLD) Agar• Enteric species like

Salmonella and Shigella • Selective agents – sodium

desoxycholate inhibits gram-y gpositive bacteria

• Three reactions differentiate Salmonellafrom other enteric bacteria - Xylose fermentation

stimulates Salmonella black centers - Lysine decarboxylation

produces alkaline conditions which stimulate

- Hydrogen sulfide production

Salmonella - black centers are due to hydrogen sulfide productionOther enteric bacteria remain acid yellow or red without a black center 18

Most Probable Number Methods• Used to estimate MPN of organisms in a sample when

low numbers of bacteria are expected (<30 CFU / ml)p ( )- Selective agents are added to a broth medium - One milliliter of sample is transferred to the

broth- At least three 10-fold serial dilutions are

inoculated- Turbid growth is recorded- MPN is derived from a statistical chart

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After incubation, a 3-digit number is produced based on the number of positive tubes per set. The number corresponds to a MPN/100 ml of water on a statistical table. 20

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Enrichment Techniques• Zero tolerance or 0 cells per 25 grams of sample is

required for food safety against Salmonella, Listeria, and Escherichia coli O157:H7

• Must be able to find one or more cells among millons of other bacteria

• How?- Pre-enrichment using non-selective media

Use selective media to isolate the pathogen- Use selective media to isolate the pathogen- Confirm identity via biochemical tests and/or

genetic authentification• Preenrichment allows the bacteria to repair,

recover and regain resistance to selective agents. 22

Traditional flowchart for isolation of Salmonella

species from foodsspecies from foods

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Physiological States of Bacteria• Outside of the lab, bacterial communities are

highly dynamic• Each community may involve the following

physiological states1. Injured and unable to grow on selective

media2 Viable but non-culturable2. Viable but non-culturable3. Biofilm – structured communities4. Coordination of activities as a population via

cell-cell communication24

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Injury• Defined – inability of cells exposed to sublethal

stress to grow on selective media, while g ,retaining culturability on non-selective media

• Sub-lethal levels of heat, radiation, acid, or sanitizers may injure rather than kill cells

• Injured cells are less resistant to selective h d l agents or have increased nutritional

requirements• Recovery of injured cells may be needed before

growth on selective media 25

Bacterial Injury following Mild Stress

Non-selective mediumlethality

Selective mediuminjury

Injured bacteria are not detected on the selective media26

Cell Injury is a Threat to Food Safety

1. If injured cells are classified as dead during heat resistance determination during heat resistance determination, then the heat process will be ineffective

2. Injured cells that escape detection may repair before food is eaten and cause illness

3 Selective agents may be common food 3. Selective agents may be common food ingredients like salt or organic acid that prevent repair and lead to underestimation of the microbial levels

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How are Bacteria Injured?• Heating, freezing, and detergents

- Injury: damage membranes and leak Injury: damage membranes and leak cytoplamic components

- Repair: membrane integrity is reestablished

• Osmoprotectants prevent injury

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• Oxygen toxicity – damages nucleic acid• Recovery:

- add detoxifying agents such as catalase to the mediumGrow anaerobically- Grow anaerobically

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Repair of Injured Bacterial Cells• Requires RNA and protein synthesis • Lag phase is prolonged• Environmental factors influence the extent and

rate of repair• Example

- Listeria monocytogenes injured at 550C, 20 mins

- Starts repair immediately at 370C and is complete in 9h

- At 40C, repair is delayed for 8-10 days and is complete in 16-19 days

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Viable but Non-Culturable (VBNC)

• Defined - VBNC bacteria cannot be cultured on any ymedium but still cause disease

• Vegetative cells differentiate into a dormant cell as a survival strategy for non-gysporulating bacteria

• VBNC cells are found in marine, soil, and gastrointestinal environments

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Viable but Non-Culturable (VBNC) • Most often induced by nutrient

limitation• Also induced by salt concentration,

exposure to hypochlorite, and shifts in temperature

• How the VBNC state occurs is mostly unknown

• Estimated that 99% of the bacteria in the biosphere may be VBNC

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Bacterial Cellular Communication• Two different gene regulation

mechanisms1. Quorum sensing – population size2. Signal transduction - environment

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Quorum SensingCommunication via small chemical molecules to coordinate

activities as a population instead of as single cells

Low Cell Density Hi h C ll D nsitLow Cell Density High Cell Density

Light

luxR luxI C D A B E G

LuxRLuxR

LuxIAcyl-ACPSAM

luxR luxI C D A B E G

LuxI

LuxR

unstableinactive

Luciferase

autoinduction 34

Two-Component Signal Transductionresponse to environmental stress / stimuli

Signal does not diffuse across the membraneKinase transmits signal across the membrane via a conformational change

Modulates gene expression in respone to the environment

Phosphorylated response regulator activates gene

expression35

Role of Quorum sensing in Food Microbiology

• Speculated that QS has a role in food ilspoilage

• Not much data to support this• Signal molecules are found in food

products such as milk, chicken soup, bean sproutsp

• Components of food may mimic or alter QS signaling systems

• Probiotics may inhibit QS signaling36

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BiofilmsDefined - a community of

microbes associated with a surface and typically encased surface and typically encased in an extracellular polymeric substance matrix or EPS

biofilm on a piece of lettuce37

Campylobacter cells attached to chicken juice

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Biofilm: a Multi-Step Process- motility- random contacts

- appendages- EPS matrix

- nutrient dependent growth- cell-cell binding interactions

1-Planktonic bacteria 2-Surface attachment 3-Microcolony

water filled voids, heterogeneity, stabilized by the EPS matrix, quorum sensing coordinates activities

4-Macrocolony 5-Detachment

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Cells in a biofilm are more resistant to heat, chemicals, and sanitizers than are

planktonic cells

First recognized in 1684 by Antonie First recognized in 1684 by Antonie van Leeuwenhoek- Dental caries as the biofilm- Vinegar as the antimicrobial- Planktonic cells showed no motilityy- After vinegar wash, plaque

bacteria still motile

Dental plaque40

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Factors that Govern Antimicrobial Resistance of a Biofilm

• Thickness, age, densitysi l i s l ti shi b t bi ss d • simple inverse relationship between biomass and susceptibility cannot be made

• Presence of abiotic particles• reduce diffusive transport• increase sorptive capacity

• Concentration of antimicrobial agentConcentration of antimicrobial agent• higher doses than for planktonic

• Species composition• co-culturing species improve resistance

• Genetic makeup of the microbe-role remains unclear41

5 Protective Mechanisms

No/slow growth zones VBNC cellsDefense mechanisms42

• Persister cells: subpopulation of microbes with genetic characteristics that allow their survival when exposed to an antibiotic without undergoing genetic change; type of dormant cellcell

• involves a metabolic shutdown not a genetic mutation

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Multicellular Characteristics of Biofilms Contribute to Bacterial Survivability within a Biofilm

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Biofilms: Special Challenges for the Food Industry

• Foodborne pathogens form biofilms on food surfaces and food contact equipmentfood surfaces and food contact equipment

• Only proper cleaning ensures that cells in the biofilm are reached by sanitizers such as trisodium phosphate

• Other methods of control:Ultrasound- Ultrasound

- Superhigh magnetic fields- Proteolytic and glycolytic enzymes- Equipment with highly polished surfaces

impede biofilm formation 45

Intrinsic Factors that Influence Microbial Growth

Characterisitics inherent to the food itself are called intrinsic factorsare called intrinsic factors

• pH• Water activity (aw)• Oxidation-reduction potential• Natural compounds that inhibit

bacterial growth• preservatives

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Acid-Base Balance: the Concept of pH• The amount of H+ in a solution

is expressed as pH (potential of hydrogen)

• Increasing [H+], increases acidityacidity

• Increasing [OH] increases alkalinity

• Too high or too low pH, enzymes change in shape cannot promote chemical reactions in a cell reactions in a cell

• Chemical reactions in a cell are sensitive to pH

• Most biochemical processes in a cell involve OH and H+ ions

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• 1 unit change in pH is a

pH of Common Foods

g pchange factor of 10 in the concentration of proton ions

• pH is defined by pH = log10[H+]pH = log10[H ]

• Food with a pH of 6 is 10 times more acidic than one with a pH of 7

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pH and Foodborne Pathogens• Lower the pH

- the more energy the cell needs to maintain a neutral intracellular pH

- the less energy producing toxins the less energy producing toxins • Foods with lower pH require less processing• Foodborne pathogens

- grow optimally at neutral pH- do not grow below pH 4.4 or above pH 8- persist or slowly diep y

• Spoilage microbes can grow in acid products

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pH Regulates Gene Expression

• Exernal pH regulates- Proton transport- Amino acid degradation- Stress response- Virulence

• Neutral pH is maintained to prevent proteins from denaturing

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Bacteria Sense pH

• Accumulation of anions or t i th t lprotons in the cytoplasm

• Change in transmembrane proton gradient

• Altered protein conformation due to protonation and deprotonation of amino acids deprotonation of amino acids

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Maintaining pHHomeostatic response

• Hydroxyl ions cannot pass through the membrane but protons do, wihch affects the pH of the cytoplasm

• Gram-negative enteric bacteria maintain a pH of 7.8- With a shift in acidity, K+/H+ antiporter pumps in

K+ neutralizing the pH- With a shift to alkaline, the Na+/H+ antiporter

acidifies the cytoplasm

• Proton antiporters are goodfor transient shifts in pH

• Prolonged exposures requiremetabolic changes

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Maintaining pHAcid Tolerance Response

Amino acid-dependent decarboxylase/antiporter systems• Consumption of protons during

decarboxylation• Decarboxylase catalyzes the decarboxylation

of an organic moleculeDeiminase and deaminase systemsDeiminase and deaminase systems

• produce ammonia which combines with intracellular protons to yield the ammonium ion (NH4

+) thereby raising pH

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Proton Pump

decarboxylase/antiporter systems

Deiminase and deaminase systems 54

Maintaining pHSynthesis of Acid Shock Proteins

• Regulatory genes induced by pH

SensorSensor

Protein chaperones 55

Water Activity

• Major factor in controlling microbial th d h i l ti i f dgrowth and chemical reactions in food

• Inhibition is due to availability of free water NOT the amount of water

• Free water supports the growth of microbes

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• Water binds the surface of solutes as they dissolve changing the property of the solution

• Free water decreases as solutes are dissolved

• Microbes must compete with the solutes for free molecules

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Water Activity Defined• Water activity (aw) is a measure of free water or the

energy status of water in a system (food)

W d f d • Water activity is defined as

• Movement of water vapor from food depends on temperature, humidity, and food composition

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Water in Food• All foods contain some water

F t • Free water - Held inside the cell- Maintains properties of free water- May be removed by pressure

• Bound waterBound water- Part of the cellular molecular structure- Reduced motility- Does not maintain properties of free water

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• A low water activity inhibits microbial growth

Water Activity as a Preservative

growth• Salt, sugar, or drying lower the water

activity of foods- used as preservative of food to decrease

the water activity and to preserve the f dfood

- sugar and salt bind water- drying removes water

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Typical aw of various foodsFoods aw

Fresh raw fruits vegetables meat and fish >0 98Fresh, raw fruits, vegetables, meat, and fish >0.98Cooked meat, bread 0.95-0.98Cured meat products, cheeses 0.91-0.95Sausages, syrups 0.87-0.91Rice, beans, peas 0.80-0.87Jams marmalades 0 75 0 80Jams, marmalades 0.75-0.80Candies 0.65-0.75Dried fruits 0.60-0.65Dehydrated vermicelli, spices, mild powder 0.20-0.60

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Microbes have different awrequirements for growth

• Decreasing aw- increases lag phaseg p- decreases growth rate- decreases the number of cells at stationary

phase• Grouped by their minimal aw requirement• Reflects the mechanisms used by each bacterium

to protect against osmotic stressp g• Compatible solutes are synthesized

- Accumulate intracellularly- Equilibrate the intracellular water activity- Do not interfere with cellular metabolism

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Minimal aw required for growth of foodborne microbes at 25oC

Group of Minimal aw requiredGroup of microorganisms

Minimal aw required

Most bacteria 0.91-0.88Most yeasts 0.88Regular molds 0.80H l hili b t i 0 75Halophilic bacteria 0.75Xerotolerant molds 0.71Xerophilic molds and osmophilic yeasts

0.62-0.60

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Extrinsic Factors that Influence Microbial Growth

Extrinsic factors are external to the food• Temperature

- Growth rate slower at 4oC- Physiology differs at 4oC- Genes expressed differ at 4oC

• Gas composition

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Requirements for Growth in the Cold• Maintain membrane fluidity

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Requirements for Growth in the Cold

• Physiology changes• Must overcome a block in

translationtranslation• Accumulate cold shock

proteins

• Accumulate compatible solutes

- PeptidesPeptides- Amino acids - Betaine- Sugars- Taurine- carnitine

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helicase

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Temperature Regulates Virulence Factors of Pathogens

• Yersinia entercolitica – expression of 16 proteins from 7 operons on the virulence plasmid is high at p p37oC, weak at 22oC and undetectable at 4oC

• Shigella - virulence gene expression is high at 37oC not expressed at 30oC

• Listeria monocytogenes – lyse blood cells if grown at 37oC not at 25 or 4oCat 37 C not at 25 or 4 C

• Vibrio cholerae – cholera toxin expression is regulated by temperature

• Escherichia coli – virulence genes are regulated by temperature 68

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Relative growth rates of bacteria at different temperatures

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Temperature Control is a Key Factor for Food Safety

• ”Keep hot foods hot and ld f d ld” lid cold foods cold” – valid

statement• ”40-140” rule

- Food should be held below 40oF and above 140oF 140 F

• These are the temperature ranges for microbial growth

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Time and Temperature Abuse• Leading cause of

foodborne illnessf

• Occurs when the temperature of food is allowed to fall in the danger zone for too long a timeg

• Effect of time and temperature on vegetative bacteria in moist foods

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The effect of the amount of food on its cooling rate in a refrigerator and its chance of spoilage.

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Food Hazard vs Food Risk• Hazard is the potential to cause harm or be a

source of damageg• Risk is the probability that the hazard will lead

to injury• Food microbiology is risk management

- Reduces the level of hazard to an acceptable probability of risk

G d b f i h d d ibl • Ground beef is a hazard due to possible presence of pathogens. - Cooked well-done risk of illness is eliminated- Cooked rare has a higher risk of illness

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Effect of time-temperature exposures on vegetative bacteria in moist foods

TempTemp

C TempF

C

Food Safety Magazine; The “Danger Zone” ReevaluatedBy Frank L. Bryan, Ph. D., M.P.H.

Hours

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Homeostasis and Hurdle Technology• Demand for fresh food has decreased the use of

intrinsic factors such as salt and sugar as a sole means of inhibiting microbes

• Instead, multiple-hurdle technologies are used that deoptimizes several factors

• Prefer inhibitory factors that work by different h imechanisms

• Hurdle effects can be synergistic or additive• Example: bacteriocins used together with other

antimicrobials or high-pressure, pulse electric fields 75

Bacteria should not be able to jump over all hurdles

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• Homeostasis is the property of a system in which variables are regulated so that internal conditions remain stable and relatively constantand relatively constant

• Growth is inhibited if cells channel energy needed for growth into maintaining homeostasis

• If the energy needed exceeds the cell’s • If the energy needed exceeds the cell s capacity to generate it, then the cell dies

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Growth Kinetics• All phases of the microbial growth

curve are affected by time and temperature

• Bacteria divide by binary fission and growth curves are plotted as cells or CFU per milliliter

• Yeast replicate by budding so optical p y pdensity is plotted

• Fungi replicate by branching / hyphal elongation and dry weight of the cell is plotted against time

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Phases of GrowthLag Phase: little or no cell division; 1 hr - days; intense metabolic activity

Log Phase: logarithmic growth; generation time is a constant minimum; peak activity

Stationary Phase: growth rate slows; death balance growth; population

bilistabilizes

Death Phase: dead cells exceed new cells formed, continues until only a fraction of cells remain or die out entirely 79

• Microbial inhibitors canExtend the la phase- Extend the lag phase

- Decrease the size of the stationary phase population

- Increase the death rate• Growth rate decreases 2-fold with every 10oC

d i decrease in temperature• Time and temperature have a greater effect on

the microbial load than does the initial microbial load

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Microbial Physiology and MetabolismMetabolism – all chemical reactions and

physical workings of a cell

Metabolism is an energy-balancing act i h i l i i h l since chemical reactions either release

or require energy

Two types of chemical reactions:anabolism – biosynthesis; process that

forms larger macromolecules from smaller molecules; requires energy input; dehydration synthesis input; dehydration synthesis (reactions release water)

catabolism – degradative; breaks the bonds of larger molecules forming smaller molecules; releases energy; hydrolytic reactions (water is used to break hydrogen bonds) 81

• Nutrient molecules, like allmolecules, have energyassociated with the electronsthat form bonds betweentheir atoms

• Various reactions in catabolicpathways concentrate theenergy into bonds of ATP,which serves as a carrier ofenergy

• ATP energy can be releasedyeasily and quickly due tounstable bonds

• ATP provides energy foranabolic reactions

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Generation of ATP

• Foodborne bacteria produce energy byoxidizing reduced compounds

• Energy released during oxidation-reduction reactions is trapped within thecell in the form of ATP

• Specifically, an inorganic phosphategroup is added to ADP with the input ofenergy or phosphorylation

• When the third phosphate is removed(dephosphorylation) energy is released.

• Organisms use 3 mechanisms ofphosphorylation to generate ATP fromADP1 – substrate level phosphorylation2 – oxidative phosphorylation3 - photophosphorylation

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Subtrate Level PhosphorylationDirect transfer of a high-energy

phosphate group from one p p g pcompound (substrate) to ADP

1,3-diphosphoglyceric acid + ADP ATP + 3-phosphoglyceric acid

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Oxidative Phosphorylation

• Synthesis of ATP coupled with electron transportEl t t t h i• Electron transport chain -energy released from the transfer of electrons (oxidation) of one compound to an electron carrier, which then passes the electrons to th n pa th ctr n t a series of electron carriers (reduction) to molecules of oxygen or other oxidized molecules

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Chemiosmosis: a mechanism that uses a proton gradient across the cytoplasmic membrane to generate ATP

• Electrons from NADH pass Electrons from NADH pass down the electron transport chain while protons are pumped across the membrane

• Proton gradient is established• Protons in higher g

concentration on one side of the membrane diffuse through ATP synthase

• Releases energy to synthesize ATP 86

Glucose - most common carbohydrate energy source used by cells.

Catabolism: getting materials and energy

Most common pathway to break down glucose is glycolysis

Carbohydrate metabolism is the breakdown of carbohydrate molecules yto produce energy.

Three major pathways1. Aerobic respiration2. Anaerobic respiration3. Fermentation 87

Respiration: series of reactions that convert glucose to CO2 and allows the cell to recover significant amounts of energy

Fermentation: when facultative and aerotolerantb l h l l i h anaerobes use only the glycolysis scheme to

incompletely oxidize glucoseAerobic respiration: When oxygen is used as the

final electron acceptor at the end of the respiration scheme to produce H2O.

Anaerobic respiration: Does not use molecular h fi l l b oxygen as the final electron acceptor, but uses

nitrogen or compounds of nitrogen or sulfur or compounds of sulfur, or other inorganic substances as the final electron acceptor.

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Summary of Aerobic Respiration

Total possible ATP yield is 404 from glycolysisg y y2 from the Krebs cycle34 from electron

transport

Two ATPs are expended in glycolysis, so a maximum yield of 38 ATPs for yield of 38 ATPs for prokaryotes

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Fermentation DefinedA process that:1. releases energy from sugars or

other organic molecules2 does not require oxygen2. does not require oxygen3. does not use the Krebs cycle or

an electron transport chain4. uses an organic molecule as the

final electron acceptor5 produces small amounts of ATP 5. produces small amounts of ATP

because much of the energy remains in the chemical bonds of the organic end-product, such as lactic acid or ethanol

6. substrate-level phosphorylation 90

FermentationElectrons are transferred (along with protons) from reduced coenzymes (NADH, NADHP) to pyruvic acid or its derivatives.

Final electron acceptors are reduced to the end-products

An essential function of the second stage of fermentation is to ensure a steady supply of NAD+to ensure a steady supply of NADand NADP+ so that glycolysis can continue.

ATP is generated only during glycolysis 91

End Products of Various Fermentations

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Overview of Respiration

and and Fermentation

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Summary

• Food microbiology is a highly specialized bfi ld f i bi lsubfield of microbiology

• The amount of energy a microbe makes depends on the metabolic pathways it can use

• Plate count determines how many • Plate count determines how many organisms can grow on a given medium under the incubation conditions used

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Summary

• Selective media allow the low numbers f ifi th t b t d of specific pathogens to be enumerated

when they are in a larger population of other bacteria

• Enrichment allows very low numbers of specific pathogens to be detected in spec f c pathogens to be detected n the presence of large numbers of other bacteria

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Summary

• Most probably number method allows very low numbers of bacteria to be estimated low numbers of bacteria to be estimated from statistical tables

• Injured cells may escape detection by cultural methods but can still cause illness when consumed

• Viable but non-culturable cells are just Viable but non culturable cells are just that

• Intrinsic factors such as pH and water activity

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Summary

• Intrinsic factors such as pH and water ti it d t i i f t h activity and extrinsic factors such as

temperature and time can be manipulated to control microbial growth

• Amount of available water determines which organisms can grow in a food and which organisms can grow in a food and influences lag time, growth rate, and final cell density

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Summary

• A water activity of 0.85 is the lowest that allows growth of pathogens making it a allows growth of pathogens making it a cardinal value in food microbiology

• Hurdle technology challenges bacteria by several mechanisms to inhibit microbial growth

• Microbial growth curve consists of lag log • Microbial growth curve consists of lag, log, stationary, and death phases of growth. Each phase is important in food safety and can be manipulated.

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