Evaluation of the microbial safety of commercially produced tomatoes in South Africa and the development of a novel enrichment broth for the identification of Escherichia coli O157:H7

Abstract

Since the discovery of Escherichia coli O157:H7 in the early 1980s and the increased association of foodborne outbreaks linked to fresh produce, a growing number of research articles relating to this subject have been published. Although leaps in our understanding of foodborne pathogen association with fresh produce have been made, our knowledge regarding this subject remains limited, with many research findings being contradictory. The microbial safety of commercially produced tomatoes in South Africa was investigated during 2012- 2014. Although no tomatoes tested were contaminated with either Salmonella enterica or Escherichia coli O157:H7, the presence of Salmonella enterica in the packinghouse of the first farm investigated in 2012 does indicate a potential risk associated with fresh tomatoes. Rapid, sensitive, and accurate detection of foodborne pathogens is vital to ensure produce safety. Conventional culturing techniques should be improved to reduce the analysis time, increase the efficiency of recovery, improve the resurrection and growth of injured cells, and to exclude other microorganisms present in the samples. The part of this dissertation therefore aimed to investigate the application of EnBase® technology for the enhanced recovery of sublethally injured E. coli O157:H7 from the environment and particularly surface water. Limiting fermentable carbohydrate availability through enzymatic glucose release would hypothetically prevent metabolic overload during enrichment cultivation, allowing enhanced resuscitation and growth of target organisms. This was found to be the case when E. coli O157:H7 was grown axenically under pristine conditions. EnPresso® B and variations thereof performed sub-optimally compared to buffered peptone water when used for the enrichment of E. coli O157:H7-spiked surface water in the presence of naturally occurring coliforms. The utilization of EnBase® technology for the selective and differential isolation and enumeration of sub-lethally injured E. coli O157:H7 from spiked surface water on solid media were not effective due to interference of the buffering capacity of EnPresso® B with the pH indicator neutral red. Understanding the biological and ecological factors enabling survival and persistence of human enteric bacteria in fresh produce, as well as better detection and monitoring techniques, are crucial in developing pre- and postharvest strategies to ensure safe produce in the future.