AbstractsBiology & Animal Science

Interactions between sewage sludge and the survival of pathogenic bacteria in soil

by Stephanie Ellis

Institution: Cranfield University
Year: 2015
Record ID: 1391774
Full text PDF: http://dspace.lib.cranfield.ac.uk/handle/1826/9208


Sewage sludge is a potentially valuable resource that can enhance both the structure and fertility of soil. However, it can also harbour enteric pathogens which pose a significant socio-economic risk to society. Therefore it is important to understand the factors that govern the persistence of such pathogens in soil, when co-introduced with sewage sludge, in order to mitigate risk and to further avail of such a valuable resource. This research aimed to clarify how microbial activity and the presence of sewage sludge would influence the persistence of co-introduced enteric pathogens in soil. It was theorised that the addition of sewage sludge to soil would cause the formation of organic matter (OM) and nutrient-rich niches. Such niches, in turn, would encourage the enhanced activity of the local soil microbial community, instigating greater competition for local resources, i.e. a hot spot of microbial activity that would lead to a decline in the introduced enteric pathogens. It was also hypothesised that the interface between the soil and sewage sludge may influence such interactions, as the physicochemical characteristics could affect the extent of exposure and subsequent interactions between enteric pathogens and the soil microbial community. These theories were investigated using four different perspectives that linked closely with each other. In initial studies, two cohorts of microcosms consisting of different proportions of sewage sludge to soil were inoculated with either E. coli or S. Dublin and destructively sampled over a 42 day period. E. coli prevailed at greater numbers when inoculated directly into soil and sewage sludge, whilst it declined to the greatest extent within mixed microcosms containing 25% sludge. All treatments containing S. Dublin appeared to decline at a similar rate, which was more linear than the decline observed within treatments inoculated with E. coli. From these findings, it can be concluded that there are no direct relationships between the proportion of sludge to soil and its affect on pathogen survival. A subsequent experiment implemented a similar treatment strategy, whilst using indigenous sewage sludge E. coli. The use of this microbe provided data which was more suited to the original premise of this work, as under such scenarios it would be indigenous sewage sludge E. coli that would be of concern. Therefore, microcosms consisting of different proportions of sewage sludge, containing indigenous E. coli, were destructively sampled over a 56 day period. The indigenous sewage sludge E. coli exhibited a more consistent linear decline after the first week. However, the indigenous E. coli were again not significantly affected by different proportions of sewage sludge to soil. It was theorised that this lack of variation in response to varying proportions of sewage sludge to soil may have been associated with a lack of available substrate within the system, or some form of partitioning effect between soil and sewage sludge matrices, which prevented the microbial communities from…