Purpose: This research studied the extent of Escherichia coli K12 internalization through the tomato stem scar by post-harvest inoculation without aid of temperature or pressure difference to determine the depth to which internalized organisms may travel within channels in tomato stems.
Methods: Stem-intact vine tomatoes were washed and surface-sanitized using 70% ethanol; samples (n=6) were air dried for 1 h. Tomato stems were inoculated with 100 μl 7.0±0.1 log CFU/ml rifampicin-resistant E. coli K12. After 1 h attachment period, stems were aseptically removed. Samples were then held at 25°C for 24 h to allow for microbial internalization through stem scars. Afterward, stem scars were excised into 3 pieces with 0.6 cm height each (top, middle, and bottom) relative to the intact stem. Samples were placed in stomacher bags and pummeled with 25 ml 0.1% peptone diluent for 1 min; samples were serially diluted and spread on tryptic soy agar containing 0.1g/liter rifampicin (TSAR). TSAR plates were incubated at 35°C for 24 h and colonies enumerated. Logarithmically-transformed data were analyzed using Student’s t-test to identify differences in E. coli numbers by stem scar depth. Uninoculated stem scar samples were visualized using scanning electron microscopy (SEM) to determine physical characteristics of stem scars.
Results: E. coli K12 (2.8 ± 1.6 log CFU/cm3) were recovered from the top stem scar samples but not the middle and the bottom pieces. The SEM images of internal stem scar samples showed vascular bundle structures with approximately 100 μm length and 15 μm diameter.
Significance: These results suggest deposit of aqueous microbial suspension onto tomato stem could result in internalization through vascular bundles structures, however internalized microbes are unable to travel to cells deep within the stem.