The use of animal manure as crop fertilizer contributes to the sustainable
recycling VE822 of essential nutrients and organic matter required to maintain good soil quality. However, care must be taken to avoid soil and plant contamination with human pathogenic bacteria present in untreated animal manure as well as dissemination of the bacteria. A large part of the outbreaks caused by pathogenic bacteria is related to the consumption of raw produce contaminated with human pathogens such as Salmonella spp. (Semenov, 2008). Salmonella spp. are more persistent in soil compared with other bacterial pathogens (Guan & Holley, 2003), displaying long periods of survival (Zibilske & Weaver, 1978) and only slightly reduced cell numbers over time (Guo et al., 2002a). Salmonella has been detected in fecal cultures from the majority
of dairies (Kirk, 2003), posing a significant risk of further pathogen dissemination to soil and fresh plant produce through the application of untreated cattle manure to agricultural fields. In several cases, cows carried Salmonella asymptomatically, i.e. they did not have clear symptoms that humans infected with Salmonella show (Semenov, 2008). Salmonella cells present in cattle manure have been shown to survive for at least 60 days at 4 and 20 °C (Himathongkham et al., 1999), but were not detectable after 19 days at 37 °C. Upon application of contaminated manure to soil, Salmonella was shown to survive for up to 300 days, with higher initial bacterial inoculation doses normally resulting in extended survival periods of Salmonella in the soil (Jones, 1986; Baloda et al., 2001; Islam et check details al., 2004). Whether Salmonella can disseminate to plant roots depends on factors such as the site of colonization (Doyle & Erickson, 2008), i.e. whether bacteria colonize the root surface or exhibit endophytic colonization of roots and aboveground plant tissues. For example, Salmonella enterica has been shown to penetrate epidermal cell walls of barley
roots (Kutter et al., 2006) and has been detected in sterilized leaf samples from crops grown in soil contaminated P-type ATPase with Salmonella (Franz et al., 2007). The entry sites of the pathogens are believed to be around cracks (Wachtel et al., 2002) and lateral root junctions (Cooley et al., 2003; Dong et al., 2003; Warriner et al., 2003), which display increased exudation of nutrients (Jablasone et al., 2005). Internalized pathogens may move systemically through plants (Guo et al., 2002b), but contamination of edible plant parts has been also reported to occur via movement along the plant surface (Cooley et al., 2003). Bacteria that manage to reach leaf surfaces must contend with harsh conditions (i.e. lack of nutrients and sunlight), and the persistence of S. enterica is 30–40-fold lower in the phyllosphere compared with in the rhizosphere (Cooley et al., 2003).