In Summary
- Human pathogens on the surface of crops (and other plants) decline with time, the rate of decline depends on air temperature, humidity, hours of sunshine.
- Strong sunlight can rapidly destroy pathogens on crops. Reduction to non-detectable levels has been observed after two weeks.
- Experiments have found that injured leaves release nutrients which can help sustain pathogens. Pathogens which have managed to enter the plant (called internalisation) can also survive longer.
- It’s important to emphasise that under certain conditions, pathogens can persist for extended periods, the longest reported is 177 days.
The interval between the last application of water and harvest
The numbers of human pathogens on plant phylloplanes (the above soil surfaces of plants) decline with time. A number of factors such as air temperature, humidity, hours of sunshine; control how rapidly the pathogen numbers decline. However, a number of scientists have attempted to measure the decline under a variety of environmental conditions and a number of common themes have emerged.
Hutchison and co-workers (2008) monitored the decline of E. coli O157, Campylobacter and Salmonella applied in irrigation water to the leaves of lettuce and spinach growing in fields under commercial conditions in the UK. Under conditions of strong sunlight, the applied bacteria typically became too low to count within two weeks. For all three of the typical spring and summer growing conditions monitored by the study, there was an initial rapid decline in bacterial numbers during the first week after application.
A similar rapid decline within five days has also been reported by Barker-Field and colleagues (2009). This Australian study observed that there was a 2-log (100 times) reduction in numbers of E. coli applied to uninjured iceberg lettuce using deliberately contaminated irrigation water. In addition to the rapid decline in bacterial numbers, the study is particularly notable because it showed that the nutrients which leak from injured lettuce leaves help sustain bacteria and prolongs their survival. Thus, particular care with water quality should be taken when irrigating injured or damaged crops.
However, although there is good evidence that an initial rapid decline in the numbers of bacteria applied to leaves and plant surfaces occurs; the persistence of small numbers of bacteria can stretch for as long as several months. The longest reported interval is 177 days in Georgia, USA over a mostly winter period when sunlight was weak and contained low levels of sterilising ultra-violet light. A summary of related, relevant research is provided in Table 1 below. The key message from these studies is that human pathogens will perish faster if they are introduced into environments where there are already-established microbial communities and also, that under certain circumstances, bacteria can become internalised into the plant tissues, which can also extend their life. Internalisation tends to involve single numbers of pathogenic bacteria and has not yet been shown to make any meaningful contribution to human illness. Bacterial cells on the surfaces of plants diminish quickly from exposure to harsh environmental factors such as drying and exposure to UV light, which promote bacterial death. Those internal to the plant persist for longer because they are at least partly shielded from such lethal environmental stresses.
There is some research into understanding whether all bacteria die when their measured numbers decline on leaves. Some research scientists believe that some of the reductions observed in bacterial populations on plant surfaces may not caused by bacterial death, but by the bacteria switching to a survival mode called ‘viable but non culturable’ (VBNC). VBNC cells can’t be cultured using traditional microbiological lab methods, so won’t be part of a count on a lab report. Some VBNC cells may however resuscitate and become fully pathogenic if they are exposed to favourable environmental conditions such as the inside of a mammalian gut.
Table 1: A summary of bacterial survival on plant surfaces (modified from an original table collated by Delaquis et al 2007)
| Investigator | Produce type | Bacteria used | Experimental details | Experimental outcomes |
|---|---|---|---|---|
| Solomon et al. (2002) | Butterhead lettuce, cv. Tom Thumb | ATCC 43895 | Greenhouse setting, 30-day-old plants, contaminated by spray | Recovery from blended leaf tissue samples for up to 30 days following inoculation |
| Solomon et al. (2002) | Green ice lettuce, unspecified cultivar | ATCC 43895 | Greenhouse setting, contaminated dairy manure, irrigation water, plants grown from seedlings | Transfer to external leaf surfaces and internalization demonstrated by cultural procedures and microscopy |
| Wachtel and Charkowski (2002) | 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. (2004) | Lettuce, parsley, unspecified cultivar | Non-toxigenic B6-914 | Field setting, contaminated dairy, poultry manure composts, irrigation water, plants grown from seedlings | Detection on tissues from both plants species by a rinse method and culturing for up to 177 days |
| Franz et al. (2005) | Lettuce, cv. Tamburo | Non-toxigenic B6-914 | Laboratory setting, hydroponic system, contaminated potting soil, grown from seed or seedlings | Internalization indicated by recovery from surface-sterilized, ground tissue samples |
| Cooley et al. (2006) | Lettuce, unspecified cultivar | E. coli O157:H7 Odwalla | Laboratory setting, contaminated seeds or seedlings contaminated with cell suspensions, co- contaminated with two epiphytic bacteria | Survival and growth on seedlings and mature plants inhibited by co-contamination with E. asburiae, supported by with W. paucula on roots and leaves of soil-grown plants |
| Macarisin et al. (2013) | Spinach, Emilia, Lazio, Space and Waitiki | E. coli O157:H7 EDL933 | Phytotron in containment level two laboratory | At least 14 days (when the experiment was terminated). Leaf roughness is positively associated with numbers of E. coli that can attach to leaves. |
In terms of assessing water use risks, despite the initial rapid decline; it is apparent that human pathogens such as E. coli O157 can potentially survive for extended periods on plant surfaces. In addition, there is a growing body of evidence that such pathogens can be internalised into the plant where their survival is extended. In many cases, the reported survival time exceeds the length of time routinely allowed for growing by commercial growers. Thus, although the assessment system takes full account of rapid initial declines in bacterial numbers, leaving one week between water application and harvest is not a critical control method and it is still important to ensure that the water applied to a crop is free from pathogens that can cause human illness.
