The phylloplane is a relatively hostile environment for most microorganisms. Factors such as drying, ultraviolet (UV) radiation, and nutrient scarcity contribute to a rapid decline in pathogen numbers after water application. Hutchison and co-workers investigated the decline of Escherichia coli O157, Campylobacter, and Salmonella following irrigation of lettuce and spinach in UK fields (1). They reported a consistent rapid initial drop in bacterial counts within the first week, particularly under high sunlight conditions, with levels typically falling below detection within two weeks. Barker-Reid and colleagues in Australia found a similar pattern, observing a 2-log reduction in E. coli on uninjured iceberg lettuce within five days2. However, they also demonstrated that injured leaves leaking nutrients supported longer bacterial survival, underscoring the importance of good crop condition.
A study examining zoonotic pathogen (a pathogen that can spread from animals to humans) survival on spinach and lettuce irrigated with water contaminated with E. coli O157, Salmonella Typhimurium, Campylobacter jejuni, and Listeria monocytogenes showed that higher levels of initial contamination were associated with longer survival (3). Across multiple growing seasons and replicates, pathogens typically became undetectable within one month, with sunlight intensity again strongly influencing the rate of decline.
Despite the evidence for rapid initial die-off, longer-term persistence has also been documented. One field study reported survival of non-toxigenic E. coli for up to 177 days under low-light winter conditions in Georgia, USA (4). A summary of related, relevant research is provided in Table 1 below.
Table 1: A Summary of Bacterial Survival on Plant Surfaces (Modified from Delaquis et al. (5))
| Investigator | Produce Type | Bacteria used | Experimental Details | Experimental Outcomes |
|---|
| Solomon et al. (6) | Butterhead lettuce, cv. Tom Thumb | ATCC 43895 | Greenhouse setting, 30-day-old plants, contaminanted by spray | Recovery from blended leaf tissue samples for up to 30 days following inoculation |
| Solomon et al. (6) | Green ice lettuce, unspecified cultivar | ATCC 43895 | Greenhouse setting, contaminated dairy manure, irrigation water, plants grown from seedlings | Transfer to external leaf surfaces and internalisation demonstrated by cultural procedures and microscopy |
| Wachtel and Charkowski (7) | Lettuce, cv. Prizehead | Four E. coli O157:H7 | Laboratory setting hydroponic system, soil, contamination through irrigation water, plants grown from seedlings | Strong association with the root system shown by cultural techniques and fluorescence microscopy |
| Islam et al. (4) | Lettuce, parsley, unspecified cultivar | Non-toxigenic B6-914 | Field setting contaminated dairy-poultry manure compost, irrigation water, plants grown from seedlings | Detection on tissues from both plant species by a rinse method and culturing for up to 177 days |
| Franz et al. (8) | Lettuce, cv. Tamburo | Non-toxigenic B6-914 | Laboratory setting, hydroponic systems, contaminated potting soil, grown from seed or seedlings | Internalisation indicated by recovery from the surface-sterilised, ground tissue samples |
| Cooley et al. (9) | Lettuce, unspecified cultivar | E. coli O157:H7 Odwalla | Laboratory setting contaminated seeds or seedlings contaminated with cell suspensions, co-contaminated with two epiphytic bacteria | |
| Macarisin et al. (10) | Spinach, Emilia, Lazion, Space, and Waitiki | E. coli O157:H7 EDL933 | Phytotron in contaminant level two laboratory | At least 14 days (when the experiment was terminated). Lead roughness is positively associated with numbers of E. coli that can attach to leaves. |
The key finding from the studies in the above table is that pathogens introduced to an environment with an established microbial community are less likely to persist, though internalisation into plant tissues may extend survival. Such internalised bacteria are protected from surface stresses and can persist longer, though typically at low levels. Internalisation has been demonstrated in lettuce and spinach, with some bacteria associating with root systems or being detected inside surface-sterilised tissues (11).
Pathogen persistence is also influenced by structural and behavioural microbial adaptations. Biofilm formation provides a protective layer that helps shield bacteria from environmental stresses, including UV radiation. Between 30% and 80% of leaf surface bacteria are estimated to exist within biofilms, particularly in nutrient-rich areas like leaf veins. These structures have been shown to develop within two days on cotyledons, hypocotyls, and roots of various sprouts (12-14). Some foodborne pathogens, including Shiga toxin-producing E. coli (STEC), have shown increased survival in the rhizosphere compared to leaves, likely due to reduced exposure to damaging sunlight (15).
Bacteria are dynamic and adapt quickly to environmental pressures. Studies have shown that bacterial populations on plant surfaces become more UV-resistant as the growing season progresses. Adaptation to plant surfaces may also influence virulence, although further work is needed to understand this relationship fully (16, 17). Further, apparent die-off may not always reflect cell death. Bacteria may instead enter a viable but non-culturable (VBNC) state, rendering them undetectable by conventional microbiological methods. While not confirmed to contribute to human illness, the potential for resuscitation under favourable conditions, such as those in the mammalian gut, remains a concern.
