The Role of Predictive The Role of Predictive Microbiology in Microbial Risk Microbiology in Microbial Risk AssessmentAssessment

Robert L. BuchananRobert L. BuchananU.S. DHHS Food and Drug U.S. DHHS Food and Drug AdministrationAdministration

Center for Food Safety and Applied Center for Food Safety and Applied NutritionNutrition

Microbiological Risk Microbiological Risk AssessmentAssessment

Microbial Risk AssessmentMicrobial Risk Assessment

Dramatic changes are occurring Dramatic changes are occurring in the way regulatory issues in the way regulatory issues associated with microbiological associated with microbiological foods safety concerns are foods safety concerns are evaluated as a result of the evaluated as a result of the emergence of quantitative emergence of quantitative microbial risk assessment microbial risk assessment techniques during the past 8 techniques during the past 8 years years

Microbial Risk AssessmentMicrobial Risk Assessment Prior to 1995:Prior to 1995: Generally considered Generally considered

that microbiological food safety that microbiological food safety concerns were too complex to be concerns were too complex to be amenable to formal risk assessment amenable to formal risk assessment techniquestechniques

1994 - 1996:1994 - 1996: First “research” First “research” attempts to conduct microbiological attempts to conduct microbiological risk assessments are publishedrisk assessments are published

1998:1998: First “formal” assessment First “formal” assessment conducted by a U.S. regulatory agency conducted by a U.S. regulatory agency publishedpublished

Barriers to Conducting Barriers to Conducting Microbial Risk AssessmentsMicrobial Risk Assessments

Large variation in virulence of strainsLarge variation in virulence of strains Large variation in susceptibility of hostsLarge variation in susceptibility of hosts Microbial populations change rapidly due Microbial populations change rapidly due

to growth and inactivationto growth and inactivation Large variation in growth and survival Large variation in growth and survival

characteristics of microorganismscharacteristics of microorganisms Microbial contamination sporadicMicrobial contamination sporadic Substantial amount of data on adverse Substantial amount of data on adverse

effects, but dose-response relations effects, but dose-response relations generally not establishedgenerally not established

Examples of Early Examples of Early “Research” Microbial Risk “Research” Microbial Risk AssessmentsAssessments

Drinking Water & Shellfish:Drinking Water & Shellfish: U.S. U.S. EPA; 1988 - 1996 (Rose, Gerba, EPA; 1988 - 1996 (Rose, Gerba, Haas)Haas)

Salmonella Salmonella Enteritidis - Eggs:Enteritidis - Eggs: USDA; USDA; 1996 - 1998 (Buchanan & Whiting; 1996 - 1998 (Buchanan & Whiting; Baker et al.) Baker et al.)

Listeria monocytogenesListeria monocytogenes:: FDA, FDA, USDA, ICMSF; 1994 - 1999 (Peeler & USDA, ICMSF; 1994 - 1999 (Peeler & Bunning; Miller et al.; Buchanan et Bunning; Miller et al.; Buchanan et al.; Whiting et al.)al.; Whiting et al.)

E. coliE. coli O157:H7 in Ground Beef: O157:H7 in Ground Beef: Health Canada, USDA; 1998 - 1999 Health Canada, USDA; 1998 - 1999 (Cassin et al.; Marks et al.) (Cassin et al.; Marks et al.)

U.S. Microbiological Risk U.S. Microbiological Risk AssessmentsAssessments Listeria Listeria

monocytogenesmonocytogenes in in ready-to-eat foodsready-to-eat foods

SalmonellaSalmonella Enteritidis in Eggs Enteritidis in Eggs and Egg Productsand Egg Products

Vibrio Vibrio parahaemolyticusparahaemolyticus in Oystersin Oysters

Listeria Listeria monocytogenesmonocytogenes in Deli Meatsin Deli Meats

EnterohemorrhagEnterohemorrhagic ic Escherichia coliEscherichia coli in Ground Beefin Ground Beef

Fluoroquinolone Fluoroquinolone Resistance in Resistance in CampylobacterCampylobacter

FAO/WHO Microbial Risk FAO/WHO Microbial Risk AssessmentsAssessments

Listeria Listeria monocytogenesmonocytogenes – – ready-to-eat foodsready-to-eat foods

SalmonellaSalmonella in in broilersbroilers

SalmonellaSalmonella Enteritidis in eggsEnteritidis in eggs

CampylobacterCampylobacter in in broilersbroilers

Vibrio Vibrio parahaemolyticusparahaemolyticus in fish and shellfishin fish and shellfish

Enterohemorrhagic Enterohemorrhagic Escherichia coliEscherichia coli in in ground beef and ground beef and produceproduce

What Is Microbial Risk What Is Microbial Risk Assessment?Assessment? Quantitative Quantitative

microbial food safety microbial food safety risk assessment is a risk assessment is a group of modeling group of modeling techniques that allow techniques that allow one to describe the one to describe the relationships between relationships between the presence of a the presence of a hazard in a food and hazard in a food and the likelihood that the likelihood that the hazard will lead the hazard will lead to an adverse public to an adverse public health consequence health consequence

What Is Microbial Risk What Is Microbial Risk Assessment?Assessment?

Risk assessments are a means of Risk assessments are a means of systematically arraying and analyzing systematically arraying and analyzing scientific data that are pertinent to a scientific data that are pertinent to a risk management question:risk management question: StructuredStructured

• to clearly tell what we knowto clearly tell what we know DescriptiveDescriptive

• to characterize how well we know itto characterize how well we know it TransparentTransparent

• to reveal any biasto reveal any bias

What Is Microbial Risk What Is Microbial Risk Assessment?Assessment? The end products of a risk The end products of a risk

assessment are:assessment are: a statement of probability or a statement of probability or

likelihoodlikelihood a statement of uncertainty.a statement of uncertainty.

Microbial Risk Microbial Risk AssessmentAssessment

QualitativeQualitative Probability and uncertainty are Probability and uncertainty are

expressed using descriptor (e.g., low expressed using descriptor (e.g., low risk, high risk)risk, high risk)

Used when eitherUsed when either• Insufficient data to describe risk Insufficient data to describe risk

quantitativelyquantitatively• Insufficient time, resources, or need to do a Insufficient time, resources, or need to do a

full risk assessmentfull risk assessment Many qualitative risk assessment are Many qualitative risk assessment are

actually semi-quantitativeactually semi-quantitative

Microbial Risk Microbial Risk AssessmentAssessment

QuantitativeQuantitative Provides an quantitative estimate Provides an quantitative estimate

of riskof risk• Single point estimateSingle point estimate• Probability profileProbability profile

Done using mathematical modeling Done using mathematical modeling Simple as possible and still answer Simple as possible and still answer

the risk management question the risk management question adequatelyadequately

Microbial Risk Microbial Risk AssessmentAssessment Flexible tool:Flexible tool:

Different typesDifferent types• Risk RankingRisk Ranking – priority setting – priority setting• Product/Pathogen Pathway AnalysisProduct/Pathogen Pathway Analysis – –

intervention strategiesintervention strategies• Risk-RiskRisk-Risk – examination of – examination of

consequencesconsequences• GeographicalGeographical – emergence – emergence

Address all or part of the food chainAddress all or part of the food chain

Microbial Risk Microbial Risk AssessmentAssessment

While specific techniques will vary While specific techniques will vary with the agent be examined, risk with the agent be examined, risk assessments are typically divided assessments are typically divided into four phasesinto four phases Hazard IdentificationHazard Identification Exposure AssessmentExposure Assessment Hazard Characterization (Dose-Hazard Characterization (Dose-

Response Relation + Severity Response Relation + Severity Assessment)Assessment)

Risk Characterization Risk Characterization

Why Dramatic Change?Why Dramatic Change?

InternationallyInternationally WTO SPS and TBT WTO SPS and TBT

AgreementsAgreements Codex Codex

AlimentariusAlimentarius Nationally (U.S.)Nationally (U.S.)

Executive OrderExecutive Order 1994 USDA 1994 USDA

Reorganization Reorganization Act Act

Why Dramatic Change?Why Dramatic Change?

Availability of Availability of personal personal computerscomputers

Availability of Availability of PC-based PC-based software for software for simulation simulation modelingmodeling ““Monte Carlo” Monte Carlo”

TechniquesTechniques

Why Dramatic Change?Why Dramatic Change?

10-year worldwide 10-year worldwide investment in investment in research in research in predictive predictive microbiologymicrobiology Modeling techniquesModeling techniques ModelsModels ExpertiseExpertise TrainingTraining Application software Application software

Predictive MicrobiologyPredictive Microbiology

Predictive MicrobiologyPredictive Microbiology

The use of The use of mathematics to mathematics to describe the describe the behavior of behavior of microorganismsmicroorganisms

1,000,000

Predictive MicrobiologyPredictive Microbiology

Emergence Emergence during the during the 1930’s as a 1930’s as a means of means of describing the describing the inactivation of inactivation of microorganisms microorganisms during thermal during thermal processingprocessing

Thermal Death Time Curve

D = -1/slope

Log #

Time

Predictive MicrobiologyPredictive Microbiology

Expanded greatly during the Expanded greatly during the 1980’s and 1990’s to include 1980’s and 1990’s to include modeling of modeling of GrowthGrowth SurvivalSurvival InactivationInactivation CompetitionCompetition Food Unit OperationsFood Unit Operations

Predictive MicrobiologyPredictive Microbiology

The availability of predictive The availability of predictive microbiology models are critical to microbiology models are critical to quantitative microbiology risk quantitative microbiology risk assessmentassessment

Usually “hidden”Usually “hidden” Mathematical expressions are imbedded Mathematical expressions are imbedded

in the risk assessment modelin the risk assessment model Need to look at the details of the risk Need to look at the details of the risk

assessmentassessment

Understanding Exposure Understanding Exposure AssessmentsAssessments

Risk is Risk is based on based on what goes what goes into the into the consumerconsumers’ mouths’ mouth

Understanding Exposure Understanding Exposure AssessmentsAssessments

Essentially all microbiological Essentially all microbiological data on the frequency and data on the frequency and extent of microbiological extent of microbiological contamination is collected at contamination is collected at sites earlier in food chainsites earlier in food chain

Predictive microbiology to Predictive microbiology to estimate the actual level estimate the actual level ingested by the consumeringested by the consumer

FDA FDA Listeria Listeria monocytogenesmonocytogenes Risk Risk AssessmentAssessment

Most data available on the Most data available on the various foods considered were various foods considered were “at retail”, with some critical “at retail”, with some critical databases at point of databases at point of manufacturermanufacturer

Need to take retail data and Need to take retail data and convert to “time of convert to “time of consumption” dataconsumption” data

Listeria monocytogenesListeria monocytogenes Exposure AssessmentExposure Assessment

Frequency of contamination of Frequency of contamination of foodfood

Number of Lm when contaminatedNumber of Lm when contaminated

Growth before Growth before consumptionconsumption

Frequency of Frequency of consumptionconsumption

Amount food Amount food consumedconsumed

Number of Lm consumed with a Number of Lm consumed with a servingserving

FDA FDA Listeria Listeria monocytogenesmonocytogenes Risk Risk AssessmentAssessment

To estimate exposure needed to To estimate exposure needed to have data and models forhave data and models for Food consumptionFood consumption Food contamination Food contamination Growth, survival and thermal Growth, survival and thermal

inactivation -- refrigeration, inactivation -- refrigeration, storage, and cooking/reheating storage, and cooking/reheating

Growth ModelGrowth Model

SqRT EGR = a ( T - To )SqRT EGR = a ( T - To )

Ratkowsky et al., 1982Ratkowsky et al., 1982

0.0

0.1

0.2

0.3

0.4

-5 0 5 10

Temperature (°C)

Sq

Rt

EG

R

(lo

g/d

)

Dealing with DiversityDealing with Diversity

0

5

10

15

20

25

30

<32 33-35 36-38 39-41 42-44 45-57 48-50 51-53 54-56 57-59 60-63

Home Refrigerator Temperatures (°F)

Semi-Soft Cheese

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 5 10 15 20 25 30

Storage (Days)

Pro

bab

ility

Den

sity

Dealing with Diversity

Dealing with DiversityDealing with Diversity

00.10.20.30.40.50.60.70.80.9

1

0 0.2 0.4 0.6

Growth rate (log/d)

Cu

mu

lati

ve

Fre

qu

en

cy

Multiple Studies with Growth Data Adjusted to 5°CMultiple Studies with Growth Data Adjusted to 5°C

Mathematical ModelingMathematical Modeling

Use Monte Carlo simulations to Use Monte Carlo simulations to increase the accuracy of the increase the accuracy of the predictionspredictions

Considers parameters as Considers parameters as distributions instead of single distributions instead of single valuesvalues

Based on the “Theory of Large Based on the “Theory of Large Numbers”Numbers”

Monte Carlo SimulationMonte Carlo Simulation

What What number number would I get if would I get if I rolled three I rolled three dice?dice?

Monte Carlo SimulationMonte Carlo Simulation

33 33 11 = 7= 7

Monte Carlo SimulationMonte Carlo Simulation

66 66 33 = 15= 15

Monte Carlo SimulationMonte Carlo Simulation

0

5

10

15

20

25

30

0 5 10 15 20

Sum

Fre

quency

Mean = 10.5

FDA FDA Listeria Listeria monocytogenesmonocytogenes Risk Risk AssessmentAssessment

Example 1: Deli (Processed) Example 1: Deli (Processed) meats that support growth meats that support growth Manufacturer to Retail: Growth Manufacturer to Retail: Growth

rate model - temperaturerate model - temperature Retail to ConsumptionRetail to Consumption

• Growth rate model – temperatureGrowth rate model – temperature• Maximum growth model - temperatureMaximum growth model - temperature

FDA FDA Listeria Listeria monocytogenesmonocytogenes Risk Risk AssessmentAssessment

Example 2: Frankfurters Example 2: Frankfurters Manufacturer to Retail: Growth Manufacturer to Retail: Growth

rate model - temperaturerate model - temperature Retail to ConsumptionRetail to Consumption

• Growth rate model – temperatureGrowth rate model – temperature• Maximum growth rate model – Maximum growth rate model –

temperaturetemperature Food preparation: Thermal inaction Food preparation: Thermal inaction

modelsmodels• Percentage eaten without reheatingPercentage eaten without reheating

FDA FDA Listeria Listeria monocytogenesmonocytogenes Risk Risk AssessmentAssessment

Example 3: Example 3: Cultured Dairy Cultured Dairy Products Products Retail to Retail to

ConsumptionConsumption• Survival model – Survival model –

temperature and temperature and pHpH

Impact of Risk Assessment Impact of Risk Assessment on Predictive Microbiologyon Predictive Microbiology The lessons learned about variability and The lessons learned about variability and

uncertainty from risk assessment has greatly uncertainty from risk assessment has greatly expanded our knowledge, techniques, and expanded our knowledge, techniques, and tools that we use in predictive microbiologytools that we use in predictive microbiology

05

1015

Incubation Time (hr)

2.0

2.5

3.0

3.5

Lo

g (

Ba

cte

ria

l C

ou

nt)

SummarySummary Predictive microbiology and microbial Predictive microbiology and microbial

risk assessment are intimately risk assessment are intimately interconnectedinterconnected Cannot do risk assessment with predictive Cannot do risk assessment with predictive

microbiology modelsmicrobiology models Cannot understand predictive microbiology Cannot understand predictive microbiology

models without understanding of variability models without understanding of variability and uncertaintyand uncertainty

An investment in one will aid the advancement An investment in one will aid the advancement of the otherof the other

Part of an overall evolution in food safety Part of an overall evolution in food safety microbiology toward being more quantitativemicrobiology toward being more quantitative