Study: E. coli in Lettuce Growing Environment Have Stress Adaptation Traits Enabling Farm-to-Fork Persistence
In a new study published in BMC Microbiology, Escherichia coli isolated from irrigation water, soil, and lettuce demonstrated a combination of biofilm formation, acid tolerance, and antibiotic resistance traits that may help the bacteria persist throughout the farm-to-fork continuum. The researchers, from Lebanese University and American University of Beirut, said the findings underscore the importance of preventing contamination pre-harvest, rather than relying solely on post-harvest control measures.
The researchers analyzed 18 environmental E. coli isolates collected from irrigation water, soil, and lettuce grown in the Litani River Basin region of Lebanon. The isolates underwent phenotypic analysis for antibiotic susceptibility, biofilm formation traits, and tolerance to acetic acid, as well as whole genome sequencing (WGS) to identify virulence-associated genes, antimicrobial resistance determinants, and plasmids.
Although most isolates remained susceptible to the antibiotics tested, two isolates (11.1 percent) were classified as multidrug resistant. In contrast, moderate-to-strong biofilm formation traits were common and observed in 83.3 percent of isolates. Several isolates demonstrated reduced susceptibility to acetic acid concentrations comparable to those used in household produce washing.
The researchers also identified significant positive correlations among acid tolerance, biofilm formation, and antibiotic resistance, suggesting these stress adaptation traits frequently occurred together. However, while antibiotic resistance phenotypes closely matched the presence of corresponding resistance genes, neither biofilm formation nor acid tolerance could be linked to individual genetic determinants, indicating these characteristics are likely controlled by multiple regulatory and environmental factors.
Genome analyses showed widespread distribution of genes associated with adhesion, iron acquisition, biofilm formation, and plasmid carriage, suggesting that environmental E. coli possess a range of genes that support persistence in agricultural environments. However, the study found no evidence suggesting that individual biofilm-associated genes or plasmids could explain the observed stress-adaptation phenotypes.
The authors emphasized that the isolates generally did not exhibit high-risk clinical resistance profiles, such as extended-spectrum β-lactamase production, but argued that the co-occurrence of biofilm formation, acid tolerance, and antimicrobial resistance genes could still enhance bacterial survival and complicate microbial control efforts in produce production systems.
The researchers concluded that environmental selection pressures may favor the development of multiple complementary stress-adaptation mechanisms rather than isolated resistance traits. They said these findings support integrated, preventive food safety strategies that reduce contamination in irrigation water, soil, and other environmental reservoirs before produce reaches consumers.
Looking for quick answers on food safety topics?
Try Ask FSM, our new smart AI search tool.
Ask FSM →
