Date of Award

12-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Microbiology

Advisor

Jiang, Xiuping

Committee Member

Greene , Annel K

Committee Member

Henson , J. Michael

Committee Member

Hughes , Thomas A.

Abstract

Composting has been proven to be an effective method used to inactivate pathogens in dairy manure. However, research has shown that if the compost heaps are unturned pathogens can persist, especially on the surface of the heaps for extended periods of time. As such, it is important to evaluate potential interventions that can be used to ensure that pathogens are inactivated at all locations of minimally maintained compost heaps under field conditions. The objectives of this study were to: 1) determine the impact of compost initial carbon-to-nitrogen (C:N) ratio on pathogen destruction when composting dairy manure in unturned heaps, 2) evaluate the use of common on-farm materials as coverings on the newly-formed for compost heaps in order to inactivate pathogens at the compost surface, and 3) study the use of competitive exclusion bacteria and bacteriophages as a treatment to reduce pathogen presence at the compost surface.
The survival of Escherichia coli O157:H7 and Salmonella enterica Typhimurium at initial populations of ca. 7 log CFU/g was investigated in compost heaps at low (<20:1), mid (25:1 - 30:1) and high (>35:1) initial C:N ratios. Two field trials were performed in this study, and pathogen survival was monitored at two locations (surface and bottom) of each compost heap. The temperatures in each heap were in the following order at each specified location: high C:N>mid C:N>Low C:N. E. coli O157:H7 was no longer detected at the bottom location of the heaps after 30, 14, and 7 days, and 21, 21, and 7 days in the low, mid, and high C:N ratio heaps in Trials 1 and 2, respectively. In Trial 1, Salmonella was not detected after 21, 14 and 7 days in the low, mid and high C:N ratio heaps, respectively; as compared with non-detection after days 14, 3 and 1 of
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composting in the low, mid and high C:N ration heaps, respectively, in Trial 2. Importantly, both E. coli O157:H7 and Salmonella were detectable at compost surface on day 60 in each trial. The results indicate that the high initial C:N ratio of the composting resulted in rapid pathogen inactivation if composting dairy manure in unturned heaps.
In the second objective of the study, field trials were performed to determine if E. coli O157:H7 survival on the compost surface would be affected by the application of a physical covering to the surface of the newly-formed compost heaps. Four trials, representing conditions that would be experienced during all four seasons, were conducted with compost heaps covered with finished compost (FC) at different thicknesses and initial moisture contents (MC), fresh straw or uncovered, which served as the control. Pathogen inactivation occurred most rapidly in the FC covering with 30% MC at a thickness of 30 cm, as a greater than 7 log CFU/g reduction of E. coli O157:H7 was observed by day 21, 5, 21 and 7 of composting in Trials 1, 2, 3 and 4, respectively. Importantly, E. coli O157:H7 was detected in the control samples at the end of each trial, either through direct plating or enrichment culture. Denaturing gradient gel electrophoresis analyses revealed changes in bacterial communities covered with FC as compared to the control. Our results suggest that E. coli O157:H7 can be inactivated at the compost surface using a physical covering such as FC, validating recommendations made by the U.S. EPA and California's Leafy Green Marketing Agreement.
The survival of E. coli O157:H7 at the compost surface after the application of biological control methods was investigated under greenhouse and field conditions in the final objective of the study. Under greenhouse conditions, the application of an 11-strain
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mixture of competitive exclusion (CE) bacteria was moderate effective in reducing E. coli O157:H7 populations when compared to the control. In the field study, the three biological treatments were: 1) a 5-strain cocktail of CE bacteria, 2) a two-strain cocktail of anti-E. coli O157:H7 bacteriophages at a multiplicity of infection (MOI) of 100, and 3) a two-strain cocktail of anti-E. coli O157:H7 bacteriophages at a MOI of 100 with 1% dried lime added into the compost. Compost samples without the addition of a biological treatment served as the controls. Day 1 post-treatment, E. coli O157:H7 population reductions were ca. 0.6 and 1.5 log CFU/g greater in the bacteriophage treatment and the bacteriophage with lime treatment, respectively, than those observed in the control. Overall, pathogen persistence in the treatment samples followed this trend (from high to low): CE>control>phage>phage+lime. The CE treatment allowed an initial growth of E. coli O157:H7 after it was applied to the compost. The data indicates that the application of bacteriophages can result in the inactivation of E. coli O157:H7 at the compost surface, and that the moisture content is critical in affecting its efficacy.
Overall, our studies show that use of the correct composting ingredient mixture and alternative treatment methods can be implemented to ensure that pathogens are controlled at all locations of the compost heap under field conditions.

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Microbiology Commons

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