Interactions between microbial community structure and pathogen survival in soil

Abstract

Manure and slurry are valuable resources that may enhance many soil properties. However, organic amendments can pose a significant health risk to both humans and livestock if not managed correctly due to pathogenic loads that may be carried within them. Therefore it is crucial to identify the factors that affect pathogen survival in soil, in order to gain maximum benefit from such resources, whilst minimising the threat to public and animal welfare. This research aimed to elucidate the impact of microbial community structure on pathogen decline following entry of such organisms into the soil. It was hypothesised that pathogen survival would be significantly influenced by both diversity and phenotypic configuration of the microbial community. This was experimentally investigated within three distinctly different biological contexts. Firstly, it was shown that the survival of Escherichia coli 0157 was significantly affected by the presence of an intact microbial community. Microcosms consisting of sterile and non-sterile sand and clay soils were inoculated with E. coli and destructively sampled over time. The pathogen remained stable at 4°C, irrespective of biological status. However at 18°C, the pathogen grew in sterile soil and declined in non-sterile soil. This result was attributed to microbial antagonism in non-sterile soil, which only became apparent at 18°C, due to increased metabolic activity of the native community. The next experiment was designed to investigate the impact of microbial diversity and community configuration on the survival of a suite of model pathogens. A gradient of community complexity was created by inoculation of gamma-irradiated soil mesocosms with a serial-dilution of a suspension of a field soil. Soils were incubated to allow biomass equilibration and the establishment of distinct community phenotypes. Sub-samples were then inoculated with Listeria, Salmonella and E. coli strains and survival was monitored over 160 days. Death rates were calculated and plotted as a function of dilution, which represented diversity, and of principal component (PC) scores from PLFA profiles, which represented the phenotypic community context. There was some evidence of a diversity effect as weak negative linear correlations were observed between death rate and dilution for S. Dublin and environmentally-persistent E. coli. However, a much stronger correlation was observed between death rate and certain PC scores for these organisms. No effect of diversity or phenotype was detected on either L. monocytogenes or E. coli 0157. These results suggest that pathogen survival was affected by diversity, but the phenotypic community context was apparently much more influential. Additionally, such community effects were specific to pathogen type. Pathogen survival was also investigated in the context of highly-contrasting communities within a range of naturally-derived field soils. PLFA analysis was used to determine phenotypic community structure and soils were also characterized for a range of physico-chemical properties. They were inoculated with Listeria, Salmonella and E. coli strains as above. Pathogen survival was monitored over 110 days and death rates were calculated. Physicochemical and biotic data, including PC scores derived from PLFA profiles, were used in stepwise regression analysis to determine the predominant factor influencing pathogen-specific death rates. PC scores were identified as the most significant factor in pathogen decay for all organisms tested, with the exception of an environmentally-persistent E. coli isolate. Overall, these results demonstrate the importance of soil biological quality, specifically the configuration of the microbial community, in pathogen suppression, and provide a possible means to assess the inherent potential of soils to regulate pathogen survival. This may lead to the identification of management strategies which will ultimately accelerate pathogen decay, and therefore improve the safety of agricultural practice.EThOS - Electronic Theses Online ServiceGBUnited Kingdo