Purpose: The goal of this study was to investigate whether different washing methods and storage times changed the indicator bacterial counts as well as the epiphytic bacterial communities on lettuce.
Methods: Four treatments were evaluated: unwashed, washed in tap water, in 100 ppm bleach solution or 30 ppm peroxyacetic acid solution (Tsunami® 100), immediately after washing or following 5 days of low temperature storage at 4°C. Indicator microorganisms (E.coli, coliforms, aerobic bacteria, yeast and mold) were quantified by culture methods, and bacterial community profiles were compared using Automated Ribosomal Intergenic Spacer Analysis (ARISA), to identify differences among treatments. ANOVA and ANOSIM were used to assess significant differences in the data.
Results: Culture methods showed that washing significantly reduced microbial indicators initially (APC, yeast, and molds; P < 0.05), but the difference between unwashed and washed samples diminished after storage. Bacterial community analysis revealed a significant difference among treatments using operational taxonomic unit relative abundance data in three experiments: global R = 0.70; global R = 0.30, global R = 0.49 (P < 0.001 in all cases). Significant differences among treatments persisted after 5 days of storage in all experiments; global R = 0.56, P < 0.001; global R = 0.22, P < 0.05; global R = 0.97, P = 0.001). Therefore, although reductions in indicator bacterial counts obtained after washing diminished after storage, differences in bacterial community profiles persisted. Different washing treatments had long-lasting effects on the epiphytic bacterial communities.
Significance: Impacts of washing on lettuce microbiome could result in different food safety risks through interactions between foodborne pathogens and other species of epiphytic bacteria.