Relationships Among Microbial Indicators of Fecal Contamination on Produce Farms in Mexico Open Access

Abstract

Background: Produce is the leading single ingredient cause of foodborne outbreaks and illnesses in the US, and detecting and preventing contamination before it reaches consumers is essential. Microbial indicators, which are related biologically and ecologically to foodborne pathogens, are used to test for potential routes of pathogen contamination along the fecal-oral pathway. Goal: This study examines the relationships between the presence and concentrations of four microbial indicators of fecal contamination in agricultural samples and how they differ based on sample type, produce type, and stage in the production process. Methods: Hand-rinse, soil, water, and produce, samples collected from 11 tomato, jalapeño, and cantaloupe farms in Mexico were tested for the presence and concentration of pathogens and four microbial indicators of fecal contamination (fecal coliforms, Enterococcus, generic E. coli, and somatic coliphage). Linear and logistic regression, as well as chi-squared and Spearman's correlation were used to assess the relationships between the microbial indicators on each sample type, controlling for potential effect modifiers (produce type, time of sample collection, and stage in the production process). Results: Significant relationships between concentrations of Enterococcus and fecal coliforms were observed across all sample types Several other significant relationships between microbial indicators were observed with no apparent trends by either sample type or indicator-pair. Conclusions: These results provide evidence to support the idea that, while some indicators may be related in certain settings, unknown factors influence the presence of indicators in the agricultural environment and using multiple indicators may be the best way to test for the presence of fecal contamination in produce farms. Implications: These results should be considered as safety guidelines and standards for produce are being developed, since the choice of indicator can have implications on whether fecal contamination is being identified, and whether the potential presence of various types of pathogens is being accurately predicted.

Table of Contents

Literature Review.. 3
Microbial Indicators. 9
Indicators and Pathogen Relationships11
Indicator Relationships. 13
Regulations. 15
Introduction. 19
Materials and Methods. 25
Study area. 25
Sample collection. 25
Microbial indicator analyses27
Statistical Analyses. 29
Logistic Regression. 29
Linear Regression. 31
Results. 32
Descriptive Statistics. 32
Indicator concentrations. 32
Indicator prevalence. 34
Discussion. 36
Key Results. 36
Limitations. 39
Strengths. 40
Implications. 41
Conclusions. 43
Tables. 47
Table 1. 47
Table 2. 48
Table 3. 49
Table 4. 50
Table 5. 51
Table 6. 52
Appendix A.. 53
Appendix B. 54
Appendix C. 55

About this Master's Thesis

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School	Rollins School of Public Health
Department	Epidemiology
Subfield / Discipline	Global Epidemiology - MPH & MSPH
Degree	MPH
Submission	Master's Thesis
Language	English
Research Field	Environmental Sciences Health Sciences, Epidemiology
Keyword	Produce Contamination Foodborne Illness Microbial Indicators
Committee Chair / Thesis Advisor	Leon, Juan, Emory University
Committee Members	Bartz, Faith, Emory University
Partnering Agencies	Emory University schools, faculty or affiliated programs University, college or educational institution (other than Emory)

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