G. S. Johannessen
Section for food and feed microbiology, National Veterinary Institute, P.O.Box 8156 Dep, 0033 Oslo, Norway
Abstract
In organic agriculture animal manure is commonly used as fertilizer for the production of vegetables. Since manure may harbour pathogenic bacteria such as Salmonella spp., E. coli
O157:H7 and other Shiga-toxin producing E. coli, Yersinia spp. and Campylobacter spp., questions have been raised of their bacteriological safety. A few surveys have shown that both indicator organisms and pathogens may be isolated from organic vegetables, but if such vegetables are of poorer bacteriological quality than conventionally produced vegetables remains to be seen. However, there are studies that have shown that pathogens may be transmitted from manure-amended soil to the plants and that the bacteria may be internalized into lettuce tissue. In contrast, other studies indicate that such transmission does not take place. In conclusion, there are several factors that influence the transmission of pathogens and indicators from manure to vegetables and more research is needed in this field.
Keywords: Animal manure, lettuce, E. coli, pathogens
Introduction
Manure is commonly applied as fertilizer to fields used for vegetable production in both conventional and organic agricultural systems with increased used in the latter. Application of untreated manure may represent a risk of contamination of the vegetables; as such manure may harbour pathogenic bacteria such as Salmonella spp., Listeria monocytogenes,
Escherichia coli O157:H7 and other Shiga-toxin producing bacteria that may contaminate
soil, irrigation water and the plants. However, the fact that the pathogenic bacteria often are present in low concentration makes it necessary to look for surrogates when examining food stuffs for the presence of potential pathogenic bacteria. Because E. coli is always present in faecal material and has the intestines as its only reservoir, it is normally used as an indicator of faecal contamination. Thus, the presence of E. coli on plants such as lettuce may suggest that faecal contamination has taken place and that faecal pathogens may be present.
This paper focuses on transmission of E. coli, used as an indicator, and pathogenic bacteria from manure to product.
Presence of indicator organisms and pathogenic bacteria in organic vegetables
There have only been a few studies investigating the presence of indicator organisms and pathogenic bacteria in organic vegetables. Although the results from these studies show that the bacteriological quality generally is good, indicator organisms, such as E. coli, and pathogens, such as Listeria monocytogenes, have been detected (Loncarevic et al, 2005; McMahon and Wilson 2001; Sagoo et al, 2001). There has been one outbreak of food borne disease that could be directly linked to the consumption of organically produced vegetables (Tschäpe et al, 1995). In a nursery school in Germany several people (children and adults) became ill after eating sandwiches that were made with organic parsley that were
contaminated with Shiga-toxin producing Citrobacter freundii. There is, however, no evidence that organically produced vegetables are of poorer bacteriological quality than conventionally produced vegetables.
Transmission of bacteria from manure to lettuce
In the production of lettuce, manure is commonly applied before sowing seeds or transplanting into the field. Different types of manure, such as manure-based composts, firm manure and slurry may be used and theoretically these different manure types could influence the hygienic quality of the lettuce. Several studies have investigated the potential for transmission of pathogenic bacteria and indicator organisms from manure to lettuce using different approaches (Dong et al, 2003; Islam et al, 2004a; Islam et al, 2004b; Johannessen et al, 2004; Johannessen et al, 2005; Natvig et al, 2002; Solomon et al, 2002; Warriner et al, 2003). Results from Solomon et al, (2002) indicated that E. coli O157:H7 was transmitted from the soil via plant roots to the lettuce seedling leaves when seeds were sown in contaminated soil. In contrast, an experiment by Johannessen et al, (2005) where transmission
of E. coli O157:H7 through the roots to the plants were studied, the bacterium was not
detected in any of the samples of the plant. In the last-mentioned study seedlings were transplanted into soil that was fertilized with manure that was contaminated with E. coli
O157:H7. This suggests that the time of introduction of contamination is important. Another direct transmission route from manure-amended soil to plant is through splashes of soil either from heavy rainfall or water spreaders. Application of manure after the plants have come up or after transplanting of seedlings may also lead to contamination of the plants and are not recommended. However, there are also other factors that are important in the transmission of indicator organisms and pathogens from the use of animal manure as fertilizer. The type and concentration of bacteria are important. It has been shown that different bacteria have different potential for the colonization of plants (Dong et al, 2003). The climate plays a role and also the type of vegetable is of importance. Gagliardi and Karns (2002) showed that E. coli O157:H7 persisted for a longer period of time with cover crops. The same trend was observed by Islam et al, (2004a) who found E. coli O157:H7 persisted for a longer time in soil covered with parsley plants than in soil from lettuce plots, which were bare after the lettuce were harvested.
Conclusion
The potential for lettuce to become contaminated with indicator organisms and pathogenic bacteria from the use of animal manure seems to be relatively small. However, more research is needed in the field of transmission routes of pathogens in vegetables production.
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