Ursula Waack

Document Type

Thesis - University Access Only

Award Date

2011

Degree Name

Master of Science (MS)

Department / School

Biology and Microbiology

Abstract

Population dynamics of Bacillus cereus in three-member soil bacterial consortia The rich bacterial diversity present in soil is not mirrored in culture-dependent approaches. For example, Bacillus cereus sensu Lato is common in soil cultures, and grows very rapidly on laboratory media and in soil extract, but culture-independent methods indicate that it represents far less than 1% of the soil community. This phenomenon suggests that B. cereus is affected by other species in soil that would cause this apparently strong competitor species to make up only a small proportion of the community. The aim of this project was to investigate the interactions between B. cereus and other soil-derived isolates in neutral and negative interaction three-species soil microcosms using liquid soil extract with or without ground com stover as carbon source(s). B. cereus, slow-growing bacteria, and com-degrading bacteria were isolated from com field soil. Neutral and negative interaction consortia were used to examine population dynamics in co-culture. Neutral consortium members displayed low-level antagonistic effects while negative groups showed greater antagonistic effects against B. cereus. Microcosms (n=3) consisting of glass beads(¢= 0.1 mm) with or without corn stover and filter-sterilized soil extract were inoculated with a three-member negative or neutral interaction consortium, and culturable counts were determined over 21 d. B. cereus showed a decrease in fitness when grown in co-culture with other bacteria than when grown individually. B. cereus reached a greater population density in single-member consortia in the presence of soil extract and stover than with soil extract alone. B. cereus had a greater number in single-member consortia than in three-member consortia. B. cereus generated the greatest proportion of spores in the three-member negative interaction consortia with only soil extract. In both soil extract single-member consortia, neutral and negative, the B. cereus population was composed entirely of spores after day seven. B. cereus displayed a lower population density when grown with antagonistic bacteria than with bacteria with undetected antagonistic effects. The difference in the B. cereus spore production between the three member neutral and negative consortia would suggest that the antagonism in the negative consortia triggers a higher percentage of sporulation. In addition to the presence of B. cereus spores in all consortia, B. cereus vegetative cells were presence for the entire length of the experiment in four out of the six consortia which is indicative of an actively growing B. cereus community in the soil. Evaluating the effect of repeated residue removal on soil bacterial community structure as analyzed by 16SrRNA – DGGE Soil bacteria play vital roles in many soil processes such as the carbon and nitrogen cycles. The microbial biomass of a field can be affected by the type of crop management system used. This experiment aimed to test which crop management technique would have the greatest effect on the microbial biomass: no till or till, presence or absence of crop residue, and the amount of nitrogen used. Soil samples were collected from a com field. The plots had either O or 150 lbs N/acre. Each nitrogen plot was subdivided based on the presence or absence of the previous year's crop residue and whether the plot had been tilled. The DNA from each soil sample was extracted and amplified. After amplification, replicate plots were combined and samples were analyzed using DGGE. DGGE data was analyzed using PAST software and both PCA analysis and neighbor-joining trees were created. When comparing the different treatments, nitrogen supplementation had the greatest effect on the bacterial community. After nitrogen the presence or absence of residue contributed more to the community than whether the plot was tilled or not. Further work could be conducted to determine if the amount of nitrogen available to a community remains as the greatest influence to a community or if another crop management technique will have more influence.

Library of Congress Subject Headings

Bacillus cereus

Corn -- Soils

Bacteria

Soil microbiology

Gel electrophoresis

Microcosm and macrocosm

Format

application/pdf

Number of Pages

129

Publisher

South Dakota State University

Download

Share

COinS