## 3 Results
### Transitions in Physiological States of STEC O157:H7 on Romaine Lettuce During Cold Storage  
#### 3.3.1 Changes in Culturable Cells During Cold Storage  
This study assessed the changes in STEC O157:H7 culturable cells on Romaine lettuce during cold storage by computing the log reduction over the period, calculated with day 0 as the baseline. Strains from the Central Coast outbreak and Yuma outbreak consistently showed higher median log reduction values over the cold storage period for Romaine lettuce harvested at 9 and 17 \({}^{\circ}\)C compared to day 0 (Figure 5A). Correlation analysis of the log reduction in culturable cells over the period of storage showed a weak relationship  
Figure 4: Boxplots representing (from top) change in log reduction of culturable cells due to (**A**) exposure to pH 2.5 for 2 h, and (**B**) exposure to chlorine for 30 s. X- axis shows harvest temperature. Y- axis shows change in reduction in log CFU/g. Solid boxplots represents 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. between reduction and harvest temperature at 9 (r = \(-\)0.1) and 17 \({}^{\circ}\)C (r = \(-\)0.1). Analysis of covariance (ANCOVA) with days as a covariate was used to assess the effect of the interaction between harvest temperature and strain on the culturable cells during cold storage. The interaction of harvest temperature and strain did not significantly (\(p\) > 0.05) affect the levels of the culturable cells on stored lettuce. However, the main effects of strain (\(\eta^{2}\) = 0.14, 95% CI = 0.09, 1.00) and harvest temperatures (\(\eta^{2}\) = 0.01, 95% CI= 0.00, 1.00) significantly (\(p\) < 0.05, ANCOVA) influenced the change in culturable cells. The Central Coast outbreak strain had significantly (\(p\) < 0.05, ANCOVA) higher log reduction values compared to other strains over the period of cold storage for lettuce harvested at both temperatures (Figure S4). For the other strains, harvest temperature had no significant (\(p\) > 0.05, ANCOVA) effect on the log reduction of culturable cells during cold storage. Figure 5: Boxplots representing culturable cells (**A**), injured cell percentage (**B**), persister cell percentage (**C**), and VBNC cells (**D**) during 5 days of cold storage. Culturable cells and VBNC cells are shown as reduction (log CFU/g), and persister cells, and injured cells are shown as percentages (%) on \(Y\)-axis. Solid boxplots represent the 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. To assess the effect of harvest temperature, cold storage days, strains and their interactions on log reduction of culturable cells, a regression tree analysis was used. Optimal pruning of the tree was performed using a defined complexity parameter (CP), known to determine how deep a tree will grow, and cross-validation error (xerror): a CP that minimized the xerror was selected to optimally account for explained variance. The tree had three splits optimally explaining the variance of culturable cells during cold storage (CP = 0.01, xerror = 0.856). While regression trees have the disadvantage of variation in the splits, bootstrapping (\(n\) = 100) was used to minimize variation in establishing the most important factors explaining variance in the culturable cells. Number of cold storage days accounted for 56.1% of the variance and was the most important factor explaining the reduction in culturable cells (Figure S5).