Thesis (M.A.)--Humboldt State University, 2007. / Includes bibliographical references (leaves 84-101). Also available via Humboldt Digital Scholar.

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Soil salinity is a widespread problem that affects crop productivity. We expect that saline soils also have altered microbial community structure, soil food webs and related soil properties. To test this, we sampled field soils across four farms in eastern North Dakota that host salinity gradients. We evaluated microbial biomass carbon, phospholipid fatty acid analysis and nematode counts in moderately saline and low saline soils. Additionally, we measured soil properties that represent potential food sources and habitat characteristics that influence microbial communities. We found higher microbial group abundance in moderately saline soils than in the lower saline soils. In contrast, we found lower nematode abundances in the moderately saline soils. We also observed increased labile carbon, nitrogen, phosphorus, and water content in the moderately saline soils. Based on our results, saline soils appear to have unique soil biological characteristics, which have implications for overall soil function along salinity gradients.

Leafy greens like lettuce and spinach are a common vehicle for foodborne illness in United States. It is unknown if native plant epiphytic bacteria may play a role in the establishment of enteric pathogens on leaf surfaces. The objective of this study was to characterize the bacterial communities of fresh and packaged spinach leaves and to explore interactions with E. coli O157:H7. We assessed the bacterial diversity present on the spinach leaf surfaces and how parameters such as spinach cultivar, field conditions, post-harvest operations and the presence of E. coli O157:H7 affected its diversity. Differences in bacterial population size and species richness were associated with differences in plant topography; flat leaves had smaller bacterial populations than savoy leaves, which correlated with larger number of stomata and trichomes in savoy leaves. During spinach growing season shifts in environmental conditions affected richness and population size of the spinach bacterial community. Decreases in the overall soil and ambient temperature and increased rainfall decreased richness and bacterial population size. Fresh spinach richness and composition assessed by parallel pyrosequencing of 16S rRNA elucidated 600 operational taxonomic units, with 11 different bacterial phyla. During postharvest operations diversity indexes and evenness tended to decrease, likely attributed to storage at low temperature and time of storage (4°C and 10°C), that promoted the dominance of g-Proteobacteria. Bacteria isolated from fresh spinach elicited growth inhibition of E. coli O157:H7 in vitro, which was associated with nutrient competition. In contrast growth enhancement produced by epiphytes was associated to low correlations in carbon source utilization and the ability of E. coli O157:H7 to rapidly utilize carbon resources. In packaged spinach, E. coli O157:H7 altered the composition of the bacterial community and its growth was promoted on packaged spinach when a disinfection and temperature abuse occurred, removal of the epiphytic bacteria resulted in significant increases in numbers of E. coli O157:H7 at 10°C and was associated with increased expression of E. coli O157:H7 virulence and stress response genes. The large diversity present on the surface of spinach leaves significantly impacted the ecology of enteric pathogens like E. coli O157:H7 on the phyllosphere. / Ph. D.

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Four strains of the fluorescent pseudomonads were studied to determine the effect of controlled atmospheres on the growth and fluorescent siderophore production at pH 6.0 and 7.8. Bacterial strains were grown in Liquid King's Medium B for 48 hr in the presence of O2/CO2 combination percentages of 21.0/0.03, 18.3/3.0, 15.0/6.0, 12.0/9.0, and 9.0/12.0. The bacterial biomass was determined after centrifugation and the siderophores were isolated, partially purified, and quantified spectrophotometrically. Results showed a steady decline in growth and in siderophore production per unit biomass with decreases in the O2/CO2 ratio at pH 7.8 and to a lesser extent at pH 6.0. The average percentage changes in siderophore production levels, relative to control were +0.8, -1.2, -18.2, and -40.6 at pH 6.0; -33.0, -50.4, -66.8, and -64.1 at pH 7.8 in the presence of O2CO2 levels of 18.0/3.0, 15.0/6.0, 12.0/9.0, and 9.0/12.0, respectively.

published_or_final_version / Ecology and Biodiversity / Doctoral / Doctor of Philosophy

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