Cultivar was more influential than bacterial strain and other experimental factors in recovery of Escherichia coli O157:H7 populations from inoculated live Romaine lettuce plants

Authors: DING, QIAO - University Of Maryland; Gu, Ganyu; Nou, Xiangwu; MICALLEF, SHIRLEY - University Of Maryland

Submitted to: Microbiology Spectrum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2024
Publication Date: 2/16/2024
Citation: Ding, Q., Gu, G., Nou, X., Micallef, S.A. 2024. Cultivar was more influential than bacterial strain and other experimental factors in recovery of Escherichia coli O157:H7 populations from inoculated live Romaine lettuce plants. Microbiology Spectrum. https://doi.org/10.1128/spectrum.03767-23.
DOI: https://doi.org/10.1128/spectrum.03767-23

Interpretive Summary: E. coli O157:H7 has caused repeated outbreaks associated with consumption of fresh-cut romaine lettuce in recent years. Scientists at University of Maryland, in collaboration with ARS scientists, examined multiple factors affecting E. coli O157:H7 recovery from inoculated romaine lettuce to gain insights on its survivals. The study determined that lettuce cultivar had the strongest impact on the recovery. The findings in this study are useful for optimizing detection procedures for E. coli O157:H7.

Technical Abstract: The contamination of Romaine lettuce with Escherichia coli O157:H7, linked to multiple foodborne disease outbreaks, is frequently tracked back to pre-harvest production stages. Methods used to inoculate, retrieve and enumerate cells on live plants to evaluate lettuce-bacterial interactions vary, confounding comparability of different studies. Various experimental variables and sample processing methods for recovering and quantifying E. coli O157:H7 from live Romaine lettuce were assessed. The factors evaluated were cultivar, bacterial strain, incubation time, inoculated leaf side and sample processing method. The recovery level from the popularly grown green Romaine cultivar 'Rio Bravo' was higher than that from the red variety 'Outredgeous'. In fact, cultivar was found to exert the most significant effect on the recovery level of E. coli O157:H7 from Romaine lettuce, an influence that held up even when considered in interactions with all other factors. Other significant variables modulating E. coli O157:H7 levels detected on lettuce were incubation time, strain and leaf side inoculated. The sample processing method, on the other hand, did not affect bacterial recovery. The overall recovery of E. coli O157:H7 from lettuce incubated for 24 hours post-inoculation was greater than that at 48 hours post-inoculation, with the recovery rate being affected by lettuce cultivar, bacterial strains and leaf side inoculated. Higher counts obtained for strain EDL933 compared to the outbreak strain 2705C emphasized the need to use relevant strains for the system being studied. Furthermore, bacterial counts retrieved from plants inoculated on the abaxial side were higher than those from adaxial side-inoculated plants, although this difference was affected by the bacterial strain and the post-inoculation incubation period. While the results of this study emphasize the importance of selecting relevant cultivar-strain pairs, findings also suggest that the inoculation and incubation procedures should be standardized. In addition, the strong effect of cultivar exerted on the E. coli O157:H7-lettuce association supports a need to start collecting cultivar information during foodborne illness outbreak investigations to facilitate pathogen-plant interaction studies in identifying plant traits that may increase food safety risk.