Survival of acid-adapted or nonadapted Escherichia coli O157:H7 in apple wounds and surrounding tissue following chemical treatments and storage

Abstract

This study evaluated survival/growth of acid-adapted or nonadapted Escherichia coli O157:H7 inoculated (4 log CFU/wound) in wounds (10 mm deep×6 mm diameter) of apples. Wounds were inoculated with a green fluorescent protein (GFP)-expressing derivative of a rifampicin-resistant strain of E. coli O157:H7 ATCC 43895 and allowed to attach (1 h). Apples were dipped (2 min) in solutions (approximately 25 °C) of water (W), 5% acetic acid (AA), 5% hydrogen peroxide (HP), 0.02% sodium hypochlorite (SH), or not treated (NT), and stored at 25 °C. Survivors were determined in cores (10-mm deep) of the apple wounds (12 mm from center of wound; inner core) and surrounding tissue (18 mm from center of wound; outer core) after homogenizing the samples in Dey–Engley (D/E) neutralizing broth and plating on tryptic soy agar (TSA) and TSA supplemented with 100 μg/ml rifampicin (35 °C, 48 h) after 0, 2 and 5 days. Average bacterial populations at day-0 were 4.0 and 2.0 logs in the inner and outer core, respectively. In the inner core of the untreated apples populations increased to 7.0 logs at day-2, while counts did not exceed 3.0 logs in the outer core during storage of all treatments. Previous acid-adaptation of the cultures did not affect survival of the pathogen. Dipping in W, AA and SH did not reduce initial bacterial populations, while at day-2 of storage inner core counts from W, AA and SH reached 7.1, 5.5 and 6.9 logs, respectively. In contrast, HP reduced initial counts in the inner core by approximately 1.5 logs, but they increased to 7.0 logs by day-2. Populations of all treatments reached 6.6–7.2 logs in the inner core by day-5. Thus, sanitizer treatment did not effectively reduce nor inhibit growth of E. coli O157:H7 contamination in apple wounds and surrounding tissue.