Characterizing relationships between bacterial indicators of foodborne pathogens on Mexican produce in the pre-harvest and post-harvest environment Open Access

Usher, Rachel (Spring 2020)

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In recent decades, the United States has experienced a rise in the number of foodborne disease outbreaks impacting millions of Americans every year. The majority of these new outbreaks originate from contaminated produce. Testing for foodborne pathogens such as pathogenic E. coli and Salmonella in agricultural samples is expensive and time consuming, so indicator bacteria of fecal contamination are commonly used to detect poor sanitation on agricultural products.

There are many potential sources of produce contamination in the agricultural environment including irrigation water, soil, farming equipment, and farmworker hands. Currently, there is little understanding of the fitness and relationships between indicator bacteria and pathogenic enteric bacteria on these surfaces and substrates in the agricultural environment. This study aims to characterize the relations between different bacterial indicators to see if these relationships are modified by sample type at different processing stages from Mexican farms distributing different types of produce.

Between 2011 and 2012, samples were collected from 11 farms producing three types of produce (cantaloupe, tomato, jalapeños), on different associated sample types (produce, soil, irrigation water, hand-rinse), before and after harvest. Each sample was analyzed for the presence and concentration of three bacterial indicators: E. coli, coliforms, and Enterococcus. In an analysis of the principal component 1 scores we found significant differences in the mean score values between produce samples in the pre-harvest (p < 0.005) and post-harvest (p < 0.001) environment in addition to significant differences between hand-rinse scores (p < 0.05). In all of these comparisons, the cantaloupe associated scores were significantly larger than tomato and jalapeño. We also found significant differences in the indicator bacteria relationships in the discriminant analysis among cantaloupe samples compared to the other produce types. Overall, relationships between bacterial indicators vary significantly by associated sample and produce type indicating physical characteristics of produce, such as the rough skin of a cantaloupe, may create a more ideal habitat for colonization by indicator bacteria than the waxy produce surfaces of jalapeño and tomato. This study will contribute much needed initial research characterizing the relative abundance and association of indicator bacteria within the agricultural environment.

Table of Contents

Literature Review 8

Data and Methods 20

Study Sites 20

Study Design 21

Statistical Analyses 22

Results 24

Descriptive Statistics 24

Examining the quantity and concentration of indicators 24

Assessing relationships between indicators 25

Contribution of bacterial indicators to the contamination profile 30

Contamination modified by sample type and associated produce type 31

Relationship between bacterial indicators 33

Strengths and limitations 35

Implications 37

References 38

Figures and Tables 48

Table 1. Summary statistics 48

Table 2. Principal component results 49

Figure 1. Significantly different produce and hand-rinse cantaloupe PC1 scores 50

Figure 2. Discriminant analysis plots by produce type and time period 52

Table 3. Discriminant analysis score summaries 54

Appendix A: IRB Approval 56

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