Thermal and Nonthermal Factors Affecting Survival of Salmonella and Listeria monocytogenes in Animal Manure–Based Compost Mixtures

Abstract

Reduction of enteric pathogens in animal manures before field application is essential for mitigating the risk of foodborne illness associated with produce. Aerobic composting of manures has been advocated as an effective treatment for reducing pathogen populations, and heat is a major factor contributing to pathogen inactivation. This study was initiated to determine the potential contribution of both thermal and nonthermal (pH, volatile acids, and ammonia) factors to pathogen inactivation during aerobic composting in bioreactors for mixtures containing manure from various sources (dairy, chicken, and swine). The test mixtures were formulated with an initial moisture content of 60% and a C:N ratio of 20:1, using straw and cottonseed meal as amendments. Mixtures were then inoculated with Salmonella and Listeria monocytogenes labeled with green fluorescent protein at initial populations of ca. 10(7) CFU/g. Three replicate trials of each treatment were conducted. Temperatures within the bioreactors were recorded at 30-min intervals, and duplicate samples were withdrawn daily from two sampling locations within the bioreactor. Significant regression models were derived relating decreases in pathogen populations to the degree of heat generated in the mixture (cumulative heat) and the pH of the mixture on the day before the pathogen losses were calculated (P < 0.0002). Although pathogens in swine manure compost mixtures were inactivated by the third day of composting, very little heat was generated in these mixtures, which were characterized by significantly higher levels of volatile acids compared with the other two compost mixtures. Therefore, volatile acids could help achieve pathogen inactivation when temperatures are too low such as when heat is lost too quickly at the surface of static compost piles or during winter composting.

Assessing the inactivation of Salmonella in dairy wastewater at varying thermal conditions

“…While the application of animal manure to cropland reduces dependence on chemical fertilizer for replenishing soil nutrients, manure borne contamination such as pathogens also poses risks to water, soil and environmental health. Controlling the risk of contamination in vegetables and crops grown on land receiving manure as fertilizer (Tauxe 1997;Natvig et al 2002;Erickson et al 2014) and ambient water requires reducing pathogen and bacterial loads in manure.…”

“…Food borne pathogens such as Salmonella and E. coli are known to be present in the gastrointestinal tracts of ruminant animals (Laven et al 2003;Wells et al 2001), leading to the sporadic shedding of these pathogens in their feces (Erickson et al 2014;Rhoades et al 2009) which can cause manure borne pathogen contamination, and thus infections.…”

“…One option for controlling the transport of pathogens to food crops and water is that all manure must be treated prior to application as fertilizer (Erickson et al 2014). Untreated manure or the manure, which have not been stored for sufficient time intervals for inactivating pathogens, can facilitate transfer of pathogens to ready-toeat crops (Islam et al 2004a;Natvig et al 2002;Toth et al 2012), as well as to surface and ground water.…”

“…Methods such as drying (Amin-Nayyeri et al 2010), aerobic digestion (Dumas et al 2010), anaerobic digestion (Aitken et al 2005;Popat et al 2010;Pandey and Soupir 2011), composting (Maeda et al 2010;Erickson et al 2014), heat treatment (Shepherd et al 2010;Garcia et al 2010) and radiation treatment (Sinton et al 2007;Brandon et al 1977) are proposed for treating the manure and controlling manure borne pathogens.…”

“…Many factors, including pH, temperature, mixing, non-mixing, feedstock, C:N (carbon:nitrogen) ratio, incubation time, aerobic condition and anaerobic environment, influence pathogen survival (Toth et al 2012;Allievi et al 1994;Pandey et al 2015;Erickson et al 2014;Kim et al 2009;Harikishan and Sung 2003;Hipsey et al 2008). However, one of the most important factors regulating the survival of pathogens is temperature, particularly when pH levels lie in the neutral range (pH = ~7.0) (John and Rose 2005;Hipsey et al 2008).…”