Comparison of temperature and humidity from 2001, 2012, and 2023 in the survival of Escherichia coli O157:H7 in romaine lettuce (Latuca sativa) from California and Texas

Outbreaks linked to E. coli O157:H7 related to leafy and romaine lettuce vary from year to year; factors such as temperature and humidity may contribute to this irregularity. The planet’s temperature has been rising since the beginning of the 21st century, affecting the quality of food production and the survival of pathogens. Outbreaks in leafy greens, particularly romaine lettuce, have been linked to E. coli O157:H7 in almost all cases. This pathogen can survive in soil, roots, and leaves, allowing lettuce to reach the market with traces of E. coli O157:H7.The first chapter of this thesis presents a literature review, providing an overview of the weather conditions and rising temperature levels affecting the planet, with a focus on their impact on food production. It also discusses lettuce production in the United States and the market demand for romaine lettuce. Finally, it addresses outbreaks related to romaine lettuce and survival studies of E. coli O157:H7 in romaine lettuce. The second chapter focused on the survival of E. coli O157:H7 under varying temperature and humidity conditions from three different years in Salinas Valley, California: 2001 (no outbreaks), 2012 (7 outbreaks), and 2023 (a recent year with one outbreak). Growth chambers were used to copy the weather, and overhead irrigation was simulated. The Year 2023 had significantly higher temperatures than 2001 and 2012 (P < 0.05). There was no significant difference in humidity between the years (P > 0.05). Regarding bacterial counts, it was necessary to transform them to Log CFU/ml to adjust the normality of the data. Results showed that the day of sampling and the year had a strong interaction with E. coli O157:H7 counts (P < 0.05). The third chapter was similar to chapter two; the new location for the weather conditions was Lubbock, Texas. The Quaker Ave Research Farm, located at Texas Tech University, provided natural soil for the experiment. The temperature and humidity recorded from the website did not show significant differences between 2001, 2012, and 2023 (P > 0.05). Instead of 28 days, microbial counts were evaluated for 23 days after inoculation and showed significant differences between days (P < 0.05). The results showed that 2012 had a higher survival rate of E. coli O157:H7 in leaves than 2001 (P < 0.05), but 2023 shared similarities with both years.