## 3 Results
### Acid and Chlorine Tolerance of STEC O157:H7 on Inoculated Lettuce During Cold Storage  
The median log reduction due to acid treatment for all the strains increased from 0.18 (95% CI = \(-\)0.48, 0.97) on day 0 to 0.32 (95% CI = \(-\)0.35, 1.37) at 5 d of cold storage. The log reduction values due to acid treatment according to strain and harvest temperature were as shown in Figure 10. There were weak to no correlations between cold storage days and log reduction due to acid treatment, for lettuce harvested at either 9 \({}^{\circ}\)C (r = 0.28) or 17 \({}^{\circ}\)C (r = 0.02). The interaction of harvest temperature and strain significantly (\(p\) < 0.001, ANCOVA, \(\eta^{2}\) = 0.08 [95% CI = 0.03, 1.00]) affected the log reduction due to acid treatment during cold storage. The Salinas 2019 outbreak strain had significantly higher log reduction at 9 \({}^{\circ}\)C (0.81) than at 17 \({}^{\circ}\)C (0.07) [\(\eta^{2}\) < 0.01, 95% CI = 0.00, 1.00], see Figure S7. Regression tree analysis optimally explained the variance in the increase in log reduction in 6 splits (CP = 0.017, xerror = 0.950). The Salinas 2019 outbreak strain on lettuce harvested at 9 \({}^{\circ}\)C had the highest increase (1.1, probability = 0.07) under cold storage (\(\geq\)2 days); see Figure 11. The Central Coast 2018 outbreak strain had the lowest log reduction due to acid treatment during cold storage (0.11, probability = 0.43). Cold storage day explained the highest variance (49.3%) of change in log reduction due to acid treatment (Figure S5). Figure 8: Effects of harvest temperature, strain and cold storage period on the formation of persister cells in lettuce. The computed effect size (R\({}^{2}\)) for the model was 0.152. Persister fraction is presented as a proportion (0, 1) and the analysis was conducted using the beta regression function. Significant (\({}^{\circ}\)) values < 0 indicate decreasing persister formation with increasing values, whereas significant values > 0 indicate increasing persister formations with increasing values. Strain was transformed into a dummy variable with the Central Coast 2018 outbreak strain as a baseline category. Figure 10: Boxplots of the changes in acid (**A**) and chlorine (**B**) tolerance of the STEC O157:H7 cells in leafy greens stored under cold temperature over 5 days. Acid and chlorine tolerance were measured as difference in log reduction due to acid and chlorine treatment, respectively, from 0 days of cold storage. Solid boxplots represent 25 th and 75 th percentile of the data. Outliers are shown by solid dots. Each boxplot represents one strain with at least three biological replicates and two technical replicates. Figure 9: Regression tree analysis of the combined effects of harvest temperature, strain and cold storage period on the formation of VBNC cells over the cold storage period. The criteria used in splitting the trees are indicated for each decision node and the relative probability of each decision indicated as a %. The median log reduction due to chlorine treatment increased from 0.93 (95% CI = \(-\)0.34, 1.91) on day 0 to 1.01 (95% CI = \(-\)0.00, 1.80) at 5 days of cold storage. Partial correlations accounting for the chlorine concentrations showed weak relationship between cold storage days and log reduction due to chlorine treatment at 9 \({}^{\circ}\)C (0.08) and 17 \({}^{\circ}\)C (0.19). ANCOVA analysis with chlorine concentration as a covariate showed that harvest temperature and strain did not significantly (\(p\) > 0.05) affect the log reduction due to chlorine treatment (Figure S8). The effects of harvest temperature, strain and cold storage with chlorine concentration as a covariate were assessed using regression trees (Figure 12). Produce that was inoculated with outbreak strains Salinas 2019 and Yuma 2018 and stored for >2 days in cold storage after chlorine washing (>20 ppm) had the highest log reduction (1.8, probability = 0.07). Produce that was stored for \(\geq\)2 days in the cold had higher log reduction than that stored in the cold for <2 days.