Persistence of <i>Escherichia coli</i> O157:H7 in Major Leafy Green Producing Soils

Persistence of <i>Escherichia coli</i> O157:H7 in 32 (16 organically managed and 16 conventionally managed) soils from California (CA) and Arizona (AZ) was investigated. Results showed that the longest survival (<i>ttd</i>, time needed to reach detection limit, 100 CFU g^–1 dry soil) of <i>E. coli</i> O157:H7 was observed in the soils from Salinas Valley, CA and in organically managed soils from AZ. Detrended correspondence analysis revealed that the survival profiles in organically managed soils in Yuma, AZ were different from the ones in conventionally managed soils from the same site. Principal component analysis and stepwise regression analysis showed that <i>E. coli</i> O157:H7 survival in soils was negatively correlated with salinity (EC) (<i>P</i> < 0.001), while positively correlated with assimilable organic carbon (AOC) and total nitrogen (TN) (<i>P</i> < 0.01). Pearson correlation analysis revealed that a greater <i>ttd</i> was associated with a larger δ (time needed for first decimal reduction in <i>E. coli</i> population). EC was negatively correlated and TN was positively correlated (<i>P</i> < 0.05) with δ, suggesting that EC and TN likely have a direct impact on <i>ttd</i>. On the other hand, AOC showed a close correlation with <i>p</i> (the shape parameter) that was not directly related to <i>ttd</i>, indicating that AOC might have an indirect effect in the overall survival of <i>E. coli</i> O157:H7 in soils. Our data showed that AOC and EC significantly affected the survival of <i>E. coli</i> O157:H7 in leafy green producing soils and the development of good agricultural practices (manure/composting/irrigation water source management) in the preharvest environment must be followed to minimize foodborne bacterial contamination on fresh produce

Soil properties.

<p>LAT, latitude; LON, longitude; MAT, mean annual temperature; MAP, mean annual precipitation; Org denotes organically managed soil, Conv denotes conventionally managed soil. EC, electrical conductivity salinity; WHC, water holding capacity; T-N, total nitrogen; OC, organic carbon; WSOC, water soluble organic carbon in soil water extract (soil∶water, 1∶1); MBC, microbial biomass carbon.</p

Effect of pH on Luminescence of <i>V. harveyi</i>.

<p>The data represent the average of triplicate soil measurements.</p

Linear regression analysis between assimilable organic carbon (AOC) and water soluble organic carbon (WSOC) in soil.

<p>The data represent the average of triplicate soil measurements.</p

Correlation of soil microbial biomass as determined by microbial biomass carbon (MBC) with soil assimilable organic carbon (AOC) concentrations (5A) and with water soluble organic carbon (WSOC) concentrations (5B).

<p>The data represent the average of triplicate soil measurements.</p

Pearson correlation coefficients between soil properties and MBC, WSOC, and AOC.

<p>MAT, mean annual temperature (°C); MAP, mean annual precipitation (mm); WHC, water holding capacity (%); OC, organic carbon (%); T-N, total nitrogen (%); EC, electrical conductivity salinity (dS m⁻¹); MBC, microbial biomass carbon (mg kg⁻¹); WSOC, water soluble organic carbon (mg kg⁻¹); AOC, assimilable organic carbon (mg kg⁻¹) “+” indicates a positive correlation, “−” indicates a negative correlation,</p>*<p>denotes significant at the 0.05 level,</p>***<p>denotes significant at the 0.001 level.</p

Relative luminescence strength of <i>V. harveyi</i> in response to different glucose concentration and incubation time (3A), and linear range (3B).

<p>Cell suspension (optical density at 600 nm about 1.5) added was 20 µl, incubation time varied from 30 min to 120 min. The data represent the average of triplicate soil measurements.</p

Effect of initial cell concentration and incubation time on sensitivity of the method.

<p>Effect of initial cell concentration and incubation time on sensitivity of the method.</p

Luminescence strength in response to low molecular weight organic carbon sources.

<p>The luminescence of the starved cell in 10 mM MOPS buffer (pH, 7.0) was measured after 30 min of incubation without shaking. The organic carbon source was added to a final carbon concentration of 100 µg l⁻¹. The data represent the average of triplicate soil measurements.</p

Construction of <i>stx</i> and <i>eae</i> mutants (top) and multiplex PCR confirmation of the mutant constructed (bottom).

<p>M represents 100 bp λDNA ladder.</p