Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Biology and Microbiology


Studies were conducted to reveal virulence differences between ten O157:H7 Escherichia coli strains from human disease outbreaks and ten O157:H7 strains from healthy cattle. The strains were evaluated for ability to cause clinical disease, histological lesions, Vero cell death, a latex-bead agglutination and the ability to adhere to HEp-2 and Henle 407 cell lines. Studies utilizing 100 gnotobiotic pigs compared strain virulence differences in clinical and histopathological effects. The results indicate significant (p=0.012) clinical and histological virulence differences between human and bovine sources of O157:H7 strains. Virulence may be a factor of the type or quantity of Shiga toxin produced. The same twenty strains were used to evaluate their cytotoxic activity against Vero cells. Data from my study showed a significant (p=0.001) difference in CD 50 cytotoxin titers, without regard to bacterial source of origin, human or bovine. A V-TEC Reverse Passive Latex Agglutination (RPLA) assay was used to quantitate with both Shiga toxin types to determine if virulence differences correlated with the type or amount of each toxin produced. Stx2 production, C S disease signs, and clinical virulence showed high correlation. There were significant differences (p=0.0001) between Stxl and Stx2 production but these did not consistently correlate with the in vivo virulence patterns, particularly when addressing the bovine-origin strain effects. The bovine-origin strains exhibited a range of virulence that remained less than the human-origin strains, and Stx production averaged less than the human-origin strains, indicating that there is a virulence difference between the strains of different origins. One of these mechanisms may be adherence. We tested the adherence patterns and colony forming units (CFU's) of the 2 most virulent strains and the two least virulent strains from both sources on HEp-2 and Henle 407 cell lines to evaluate the ability of the 0157 :H7 strains to adhere, but found no significant difference (p=0. 07 and p=0 .15 respectively). The results of the gnotobiotic observations, the cytotoxic vero effects, Stxl and Stx2 toxin production results, and the bacterial adherence to HEp-2 and Henle 407 cell lines were evaluated by the Student t test to assess virulence comparisons.

Library of Congress Subject Headings

Escherichia coli O157:H7 Escherichia coli infections Virulence (Microbiology)



Number of Pages



South Dakota State University