Heaton and Jones (2007) have reviewed contamination of ready to eat produce and survival of microorganisms capable of causing human illness on the phylloplane (the plant leaf and stem surfaces). The premise of the review is that if a pathogen can adapt to survive on the phylloplane, then the chances of an infectious dose of microorganisms remaining at consumption is increased. Early studies by Beuchat (1999) showed that E. coli O157:H7 contained within bovine faeces and inoculated onto lettuce could be isolated from lettuce for up to 15 days after inoculation. Later work by Fett (2000) further suggested that the basis of survival for human enteropathogenic bacteria transferred to the plant surface, is incorporation into phylloplane biofilms (biofilms are microscopic patches of slime which contain bacteria; the slime protects the bacteria contained within it and makes them difficult to kill; Figure 1). Later still, Morris and Monier (2003) estimated that between 30% and 80% of the total bacterial population on a leaf surface will be contained within a biofilm structure. Furthermore, biofilms tend to be associated with sources of plant nutrients such as leaf veins. Fett (2000) showed that bacterial biofilms were already established on the cotyledons, hypocotyls and roots of alfalfa, broccoli, sunflower and clover sprouts as quickly as two days after seed germination.
Although there are established strategies which microorganisms can adopt to prolong their survival on phylloplanes, it is, in general, a difficult place for bacteria to survive and most that find their way onto a plant surface will perish quite rapidly (Hutchison et al., 2008). The single most important factor in controlling bacterial numbers on the phylloplane is the ultra violet (UV) radiation contained within sunlight (Jacobs and Sundin 2001; Heaton and Jones 2008). Phylloplane bacterial isolates either tend to have efficient DNA damage repair mechanisms, or preferentially colonize sites that are protected from UV such as within the interior of a leaf or structures shaded from direct sunlight (Jacobs and Sundin 2001). Elasri and Miller 1999 report that containment within a biofilm helps protect bacteria against the damaging effects of UV irradiation. It is important to stress that bacteria are not static entities. Phylloplane microorganisms evolve and adapt, becoming more suited to their environment. Bacteria can reproduce quickly and under optimal conditions a new generation can be created as quickly as every 10 minutes. Thus bacterial adaptation to be better suited to the phylloplane can occur within days rather than years and phylloplane communities exhibit a marked shift towards UV tolerance as the growing season progresses (Jacobs and Sundin 2001). Of great concern, are the findings of Brandl (2006) who suggests that a period of residence in the phyllosphere may lead to increased virulence of enteropathogens as a consequence of the adaptations which must occur in order for them to survive there.
More recently, Hutchison et al. (2008) have investigated how long zoonotic agents can survive on the phylloplane in the UK using HDC funding (Figure 1). Their experiments involved artificially contaminating irrigation water with E. coli O157, Salmonella Typhimurium, Campylobacter jejuni and Listeria monocytogenes before using the water for the irrigation of newly-planted spinach and iceberg lettuce. The numbers of each pathogen were followed on a weekly basis by taking samples of each crop. The experiments were continued until either the numbers were too low to be detected or the plants reached a size suitable for harvest. Two levels of contamination were used: a high contamination treatment and a lower one. As might be expected, declines in bacterial numbers on the phylloplane was observed with time and the treatment with higher numbers of zoonotic agents showed persistence for longer. In each of the tree replicates spanning two growing seasons, the workers observed initial rapid decreases in bacterial numbers. Typically, after one month, the researchers were unable to detect any pathogens on the phylloplane. The experiments were undertaken over a range of day lengths and light intensities. As has been reported previously, intense, bright sunlight caused extremely rapid declines in pathogen numbers.
References (click a reference to read it (where it's available); some require purchase from the publisher)
Beuchat, L. R. (1999) Survival of enterohaemorrhagic Escherichia coli O157:H7 in bovine faeces applied to lettuce and the effectiveness of chlorinated water as a disinfectant. J Food Prot 62, 845-849.
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. J. Appl. Microbiol. 105,848-857.