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Keywords:

  • E. coli O157:H7;
  • microbial diversity;
  • Salmonella ;
  • soil

Abstract

Aims

Three soils that varied in their physicochemical characteristics and microbial diversity were inoculated with Escherichia coli O157:H7 and Salmonella to determine the relative impact of abiotic and biotic factors on the pathogens' survival when the soil was held at 25°C.

Methods and Results

Three soils that were classified as having low, medium and high microbial diversity were divided into two batches for adjustment to 20% of water-holding capacity and to 40% of water-holding capacity. Soils were inoculated with both green fluorescent-labelled E. coli O157:H7 and red fluorescent-labelled Salmonella (5 log CFU g−1 dry weight) and held at 25°C. Pathogens inoculated into an acidic soil died off within 9 weeks, whereas they were still detected in the other two soils by enrichment culture after 18 weeks. Moisture did not affect inactivation of E. coli O157:H7, but did affect Salmonella inactivation in soil having the greatest organic load and microbial diversity. Using multiple linear regression analysis, 98·7% of the variability in the inactivation rate for E. coli O157:H7 was explained by a model that included the variables of initial pH and electrical conductivity. Salmonella's inactivation rate was predicted by a model that included pH and initial cell numbers of copiotrophic and oligotrophic bacteria.

Conclusion

This study provided evidence of specific properties that impact inactivation of E. coli O157:H7 and Salmonella in soils at 25°C.

Significance and Impact of the Study

Identification of factors influential in the die-off of enteric pathogens will assist in developing guidelines for safe intervals between field contamination events and planting or harvesting of fresh-cut produce crops.