The interval between the last application of water and harvest

The reason that the risk assessment programme asks about the time interval between last watering and harvest is because numbers of human pathogens on plant phylloplanes (the surface of the plant) decline with time.  A number of factors such as air temperature, humidity, hours of sunshine; control how rapidly the decline in pathogen numbers occurs.  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 monitored the decline of E. coli O157, Campylobacter and Salmonella applied in irrigation water to the leaves of lettuce and spinach growing in fields 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 5 days has also been reported by Barker-Field and colleagues.  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 helps sustain bacteria and prolongs their survival.  Thus particular care with water quality should be taken when irrigating injured 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 winter period when sunlight was weak and contained low levels of sterilising ultra violet light.  A summary of related, relevant research is provided in the Table 1 below.  The key messages 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 extends 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.

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.

Butterhead lettuce, cv. Tom Thumb

ATCC 43895

Greenhouse setting, 30-day-old plants, inoculated by spray

Recovery from blended leaf tissue samples for up to 30 days following inoculation

Solomon et al.

Green ice lettuce, unspecified cultivar

ATCC 43895

Greenhouse setting, inoculated dairy manure, irrigation water, plants grown from seedlings

Transfer to external leaf surfaces and internalization demonstrated by cultural procedures and microscopy

Wachtel and Charkowski

Lettuce, cv. Prizehead

Four E. coli O157:H7

Laboratory setting, hydroponic system, soil, inoculation through irrigation water, plants grown from seedlings

Strong association with the root system shown by cultural techniques and fluorescence microscopy

Islam et al.

Lettuce, parsley, unspecified cultivar

Nontoxigenic B6-914

Field setting, inoculated 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

Cooley et al.

Lettuce, unspecified cultivar

E. coli O157:H7 Odwalla

Laboratory setting, inoculated seeds or seedlings inoculated with cell suspensions, co-inoculation with two epiphytic bacteria

Survival and growth on seedlings and mature plants inhibited by co-inoculation with E. asburiae, supported by with W. paucula on roots and leaves of soil-grown plants

Franz et al.

Lettuce, cv. Tamburo

Nontoxigenic B6-914

Laboratory setting, hydroponic system, inoculated potting soil, grown from seed or seedlings

Internalization indicated by recovery from surface-sterilized, ground tissue samples

In terms of assessing water use risks, despite the initial rapid decline; it is apparent that human pathogens such as E. coli O157 can 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 agents that can cause human illness.

References

Barker-Reid, F., Harapas, D., Engleitner, S., Kreidl, S., Holmes, R. and Faggian, R. (2009) Persistence of Escherichia coli on Injured iceberg Lettuce in the Field, Overhead Irrigated with Contaminated Water. Journal of Food Protection 72:458-464

Cooley, M. B., D. Chao, and R. E. Mandrell. 2006. Escherichia coli O157:H7 survival and growth on lettuce is altered by the presence of epiphytic bacteria. Journal of Food Protection 69:2329–2335.

Delaquis, P., Bach, S. and Dinu, L.D. (2007) Behaviour of Escherichia coli O157:H7 in leafy vegetables. Journal of Food Protection 70:1966-1974.

Franz, E., A. D. van Diepeningen, O. J. de Vos, and A. H. C. van Bruggen. 2005. Effects of cattle feeding regimen and soil management type on the fate of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium in manure, manure amended soil and lettuce. Applied and Environmental Microbiology 71:6165–6174.

Hutchison, M.L., Avery, S.M. and Monaghan, J.M. (2008) The air-borne distribution of zoonotic agents from livestock waste spreading and microbiological risk to fresh produce from contaminated irrigation sources. Journal of Applied Microbiology 105:848-857.

Islam, M., M. P. Doyle, S. C. Phatak, P. Millner, and X. Jiang. 2004. Persistence of enterohemorrhagic Escherichia coli O157:H7 in soil and on leaf lettuce and parsley grown in fields treated with contaminated manure composts or irrigation water. Journal of Food Protection 67:1365– 1370.

Johannessen, G. S., R. B. Frøseth, L. Solemdal, J. Jarp, Y. Wasteson, and L. M. Rørvik. 2004. Influence of bovine manure as fertilizer on the bacteriological quality of organic iceberg lettuce. Journal of Applied Microbiology 96:787–794.

Solomon, E. B., S. Yaron, and K. R. Matthews. 2002. Effect of irrigation method on transmission and persistence of Escherichia coli O157:H7 on lettuce. Journal of Food Protection 66:2198–2202.

Wachtel, M. R., and A. O. Charkowski. 2002. Cross-contamination of lettuce with E. coli O157:H7. Journal of Food Protection 65:465–470.